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  1. 1. A COURSE IN JAVACutajar & Cutajar © 2012
  2. 2. Concepts of OOPThe Introduction starts with giving a short definition of theterms used in object oriented programming. This isfollowed by a brief history and some good properties ofJava.The chapter then introduces the concept of Javaprogramming, by explaining the various steps used towrite a simple Java class. The concept of a class isdescribed in detail.
  3. 3. Objects & Classes To distinguish between an object and a class one may use the analogy of a mobile phone. When we speak of a class we speak of a type of object for example a class of a mobile, maybe a Nokia 370. A Nokia 370 is not a distinct object – it has no actual colour or number. On the other hand when we speak of my mobile of type Nokia 370 we are specifying a distinct object. It‘s colour is black and the number is 7942816. This is an object - a specific mobile. You can have many objects of the same class and each object we can call an instance of that class. Tom, Jerry, and Mary can each have a Nokia 370; each an instance of a Nokia 370. The designer didnt design my mobile but designed the Nokia 370 of which my mobile is an instance. So in programming we design a class and then create new instances of that class.
  4. 4. Abstraction The essence of abstraction is to extract essential properties while omitting inessential details. To keep to our analogy, an abstraction can be considered as a mobile without any details of how it functions. All that I am concerned, is that my mobile behaves exactly like any other mobile of its type, and knowing how to use one enables me to use any mobile of that type since all I am concerned is that it provide all facilities offered by a my mobile.
  5. 5. Information Hiding Information hiding is the principle of segregation of design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed. The protection involves providing a stable interface which protects the remainder of the program from the implementation (the details that are most likely to change). The user of the mobile is not concerned with the inner workings of the mobile, but only to how to switch on the mobile and the functions to use it.
  6. 6. Encapsulation The concept of encapsulation refers to building a capsule, in this case a conceptual barrier, around some collection of things. All functions of the mobile are encapsulated within the case. This makes the object much more easy to handle as identical functions are grouped together in a single mobile.
  7. 7. Methods & Data Values An object is composed of data values and methods. Methods are ways to use an object and they normally receive or return a value to or from the object. In our mobile analogy, a subscriber uses the mobile by choosing the function and pass to it, or retrieves from it, the required data Data values are values contained within the object. They are the properties of that object. These can be instance data values which belong to only one particular instance, or a class data value belonging to all objects of the same class. Methods act on these data values. Methods which access data values are called accessor methods whilst those which change it are called mutator methods, just like the functions of your mobile.
  8. 8. Java’s Past and Present The Java language was originally created to solve the problems surrounding personal digital assistants (PDAs) and consumer electronic products, such as microwaves and toaster ovens. The language had to be robust, small, and portable. However, the Java team discovered that Java‘s design made it ideal for another purpose: programming on the Internet. In 1993, the World Wide Web was gaining popularity, with its myriad platforms and operating systems, and it desperately needed a platform-independent language—so Java found a new home.
  9. 9. Java Is Platform-Independent The Java language was designed specifically to be platform-independent. This means that programs written in Java can be compiled once and run on any machine that supports Java. So, what exactly does platform- independence mean, and how does it differ from what was available before Java? Traditional compiled programming languages are compiled to machine- level binary code that is, of course, specific to the machine or platform on which it is compiled. The advantage is that the compiled binary runs quickly because it is running in the machine‘s native language. The disadvantage is that a program written in a traditional language has to be recompiled to binary code for each different hardware platform before it can be run on that machine. On the other hand, traditional interpreted programming languages are machine-neutral and can be run on various platforms, they generally run much slower than compiled applications.
  10. 10. The Java Virtual Machine (JVM) Java has incorporated the best of both worlds. The Java team created a platform-specific layer, called the Java Virtual Machine (Java VM), which interfaces between the hardware and the Java program. When you install a Java-capable browser on your computer, for example, a copy of the Java interpreter is also installed, which is what enables your browser to execute Java applets and your system to run standalone Java programs. Java interpreter and Java runtime are alternative terms for the Java Virtual Machine (VM). The Java VM presents the same interface to any applets that attempt to run on the system, so to applets, all machines look the same. The applet itself is compiled into a form of interpretable code called Java bytecodes. This special form of code can be run on any platform that has the Java VM installed.
  11. 11. The Bytecode Bytecodes are a set of instructions that are similar to machine code but are not processor-specific. They are interpreted by the Java VM. Sun‘s trademark phrase ―write once, run anywhere‖ is the promise that is fulfilled by Java. This, above all other features of the language, is what makes it so essential for interactive Web programming. It makes code much safer as it interacts only with the JVM.
  12. 12. Java Is Object-Oriented Java is a real object-oriented language, which enables you to create flexible, modular programs. It includes a set of class libraries that provide basic data types, system input and output capabilities, and other utility functions. It also provides networking, common Internet protocol, image handling, and user-interface toolkit functions. Java‘s object classes provide a rich set of functionality, and they can be extended and modified to create new classes specific to your programming needs.
  13. 13. Java Is Easy to Learn Among the original design goals of the Java team members was to create a language that was small and robust. Because they started from scratch, they were able to avoid some of the pitfalls of other more complex languages. By eliminating things such as pointers, pointer arithmetic, and the need to manage memory explicitly, the Java development team made Java one of the easiest languages to learn. However, you can still do anything in Java that you can do in traditional languages. If you have previously used a high-level object-oriented programming language, such as C++, much of Java will look familiar to you.
  14. 14. My First Java Program import javax.swing.*; import required package /* My First Java Program: comment Displaying a Window */ filename must be saved as public class MyFirstProgram { class definition starts here main method starts public static void main (String[ ] args) {indent JFrame tieqa; declare object of given class tieqa = new JFrame( ); create a new object block tieqa.setSize(300, 200); call methods of tieqa.setTitle(―Hello Cutajar‖); class JFrame tieqa.setVisible(true); } main method ends use methods } class definition ends on object
  15. 15. /* Comments *//* This is a multi-line comment and ends with  */// This is a single line comment and ends with a return Everything enclosed within (/*) and (*/) is a comment and is ignored by the compiler. Another way to write a single line comment is by starting the line with a double slash (//), everything until the end of line is ignored. Special comments enclosed in (/**) and (*/) are used in documentation. These are called JavaDoc comments
  16. 16. The Concept of a Class The example is a class which comprises: one or more import statements to import libraries (packages) containing classes of objects already defineda class a method called main() having some parameters a method has a header conveying the method‘s name (main) a method a block a block a block {enclosed in braces} comprises: a set of declarations (what you need) a set of statements (what to do) each import, declaration or statement end with a semicolon (;)
  17. 17. Object Declaration When declaring an object we are actually giving a name to an object of that class. class must be already defined JFrame is defined in JFrame tieqa; javax.swing object names start with lowercase class names start with uppercase tieqa null
  18. 18. Object CreationWhen we create an object Example: tieqa = new JFrame(); we are effectively reserving memory space for an object. This is when the object really comes in existence. tieqa = new JFrame( ); calling the creator of JFrame tieqa JFrame
  19. 19. Methods - Calling Methods In Java we call methods on objects. So we use the name of the object and the method we want to use of that object separated by a dot. Note that since we call the method of an object we are only effecting that particular instance. ( We will see later some exceptions on static classes). the dot name of parameters to send to method object tieqa.setVisible (true); name of account.deposit( 200.0 ); method student.setName(“john”); myMobile.addContact(“maria”);
  20. 20. Method Declaration When declaring a class the following statements must be included. This is the main method of which one and only one is needed for a program to execute. public static void main ( String args[ ] ) { access scope return method modifier parameters block start modifier type name optional public void deposit ( float amount) { balance = balance + amount; }
  21. 21. Naming In the example class illustrates several names invented by the programmer to identify classes, objects and variables in general. Such names are also called identifiers These identifiers :  cannot be Java keywords:  cannot contain punctuation marks or spaces  must not start with a numeral. My Window 1stVariable case
  22. 22. Naming Conventions It is a common convention to name objects starting with a lower case letters tieqa We use an uppercase letter to separate words in the identifier. myFirstWindow On the other hand the name of the classes normally start with an Uppercase letter. JFrame Java is case-sensitive, meaning that lowercase letters and upper-case letters are totally different letters. JPanel tieqa; Number and number are different JFrame tieqa; You cannot name two different things with the same name:
  23. 23. JAVA ComponentsThis section introduces the building blocks of a program.It introduces the concepts of primitive and non-primitive(object) variables, their properties and their behaviour.The operations among these variables are analysedhere, together with some new related pre-definedclasses.The concept of constants is also discussed
  24. 24. Variables A variable is an entities that can hold a value. Before it can be used it must first be declared. A variable has four properties: An Address, A memory location to store the value, whose storage capacity depends on the type of data to be stored, The type of data stored in the memory location, and The name used to refer to the memory location. (following the same naming rules already discussed).
  25. 25. Primitive Data Types byte boolean short Primitive char Integers Data Types int Numeric long float Reals double
  26. 26. Sizes and Ranges of Variables Declare the type of a variable only once, as long as its scope hasnt ended Variables can be declared anywhere in the program int i, j, k; float numberOne, numberTwo; long bigInteger; double bigNumber; int count = 10, height = 34; numberOne = 3.45F; i = 45;
  27. 27. Assignment Operators The assignment operator in Java is denoted by a single equals sign: The variable on the left of the equals sign takes the resulting value of the evaluation of the expression on the right. The syntax is: <variable name or identifier> = <expression> ; Normally on the left of the expression is the name of a variable and on the right evaluates to a value which is assigned to the variable. The left hand is called l-value. In Java the overall result of the RHS expression is returned, allowing the LHS to be assigned too as follows:
  28. 28. Other Assignment Operators There are also other assignment operators:
  29. 29. Mixed Types Expressions When terms of an expression are of different types, the processor ‗coerces‘ values to a consistent type. Coercion may cause a short to become long. (promotion) or a long into a short (demotion). If you wish to override coercion you may include a cast to specify the precise promotion or demotion required. This expression is called a mixed expression: float x; int y; x*y
  30. 30. Automatic Promotion The data types of the operands in mixed expressions are converted, based on the promotion rules. The promotion rules ensure that the data type of the expression will be the same as the data type of an operand whose type has the highest precision.
  31. 31. Explicit Type Casting Instead of relying on the promotion rules, we can make an explicit type cast by prefixing the operand with the data type using the following syntax: (<data type> ) <expression> Convert x and result to float (float) x / 3 (int) (x / y * 3.0) Convert result to int
  32. 32. Implicit Type Casting Consider the following expression: double x = 3 + 5; The result of 3 + 5 is of type int. However, since the variable x is double, the value 8 (type int) is promoted to 8.0 (type double) before being assigned to x. int x = 3.5; Demotion is not allowed higher precision
  33. 33. boolean Another primitive data types in Java is boolean which has only two possible values: true or false. Use this data type for simple flags that track true/false conditions. This data type represents one bit of information, but its "size" isnt something thats precisely defined. boolean found = false;
  34. 34. Enumerated Data Types An enum type is a kind of class definition. The possible enum values are listed in the curly braces, separated by commas. By convention the value names are in upper case as are actually constants. public enum Shape { RECTANGLE, CIRCLE, LINE }
  35. 35. chars The char data type is a single 16-bit Unicode character. Note the difference The code of „1‟ is 49 and not 1 between 1 and „1‟ Note that the single apostrophes delimit a character. The double quotes (―) delimit a string of characters. Addition (+) between characters and strings produces a concatenation and is stored in a string String s = „A‟ + „B‟
  36. 36. Character order Characters in java have an ordinal value, thus there is an order between them: This means that you can even compare characters between them.
  37. 37. Strings A string constant consists of a sequence of characters separated by double quotes. There are close to 50 methods defined in the String class. String name; name = new String(“Johnny Java”); name null name J o h n n y J a v a pointer to where index starts string starts at 0 Note that a String is not a primitive data type but a class, and likewise behaves like one. We say that Strings are immutable (cannot change value). Notice the difference between primitive data types like strings. In primitive data type the contents of the variable is the value, while in referenced data type the contents is a pointer (address) of the location where the object starts.
  38. 38. The String Class A very commonly used class of the java.lang package is the Sting class. Note that the java.lang package doesnt need to be imported as it is imported directly by default by java. There are various methods defined in the String class, to mention a few, for an object str of class String:  str.substring(i,j) will return a new string by extracting characters of str from position i to j-1 where 0 <= i < length of str, 0 < j <= length of str, and i <=j.  str.length() will return the number of characters in str.  str.indexOf(substr) will return the first position substr occurs in str.
  39. 39. Primitive Data Types Numerical data , boolean and chars are called primitive data types. For Primitive Data Types: int firstNumber; int secondNumber;  1. Allocate memory for variables. firstNumber firstNumber = 234; secondNumber = 87; secondNumber firstNumber = 187;  2. Values are placed in the memory locations of the variable firstNumber 234 secondNumber 87  3. If one of them say firstNumber gets reassigned the new value will overwrite the old one. The value of 234 is overwritten by 187 firstNumber 187
  40. 40. Referenced Data Types  Objects (including Strings) are called reference data types, because their contents is an addresses that refers to a memory locations where the objects are actually stored. Customer customer, client;  For Objects: customer = new Customer(“John”);  1. Allocate memory for variables. customer = new Customer(“Mary”); client = customer; customer client M a r y  2. An object is created and its address is stored in the variable to point at it.customer J o h n customer J o h n  3.The reference to another object overwrites the reference in customer.  4. The reference in customer is assigned to client. So both point at the same object customer M a r y client
  41. 41. Wrapper Classes A wrapper is a class that contains data or an object of a specific type and has the ability of Primitive Wrapper performing special operations on the data or boolean Boolean object. byte Byte Most common wrapper classes are for the char Character primitives. double Double Note that since wrapper classes are classes in all float Float effects, by convention they start with uppercase. int Integer Reasons for wrapper around the primitives: long Long  Converting from character strings to numbers short Short and then to other primitive data types. void Void  A way to store primitives in an object.  A way of passing primitive data types by reference. Double myDouble = new Double("10.5");
  42. 42. Autoboxing Autoboxing, introduced in Java 5, is the automatic conversion the Java compiler makes between the primitive (basic) types and their corresponding object wrapper classes (eg, int and Integer, double and Double, etc). The underlying code that is generated is the same, but autoboxing provides a sugar coating that avoids the tedious and hard-to-read casting typically required by Java Collections, which can not be used with primitive Double myDouble = 10.5;
  43. 43. Converting Back to Primitives Here are some examples to convert back to basic types Wrapper class Method Example (for converting back to primitives) Integer parseInt Integer.parseInt(“25”) gives 25 Integer.parseInt(“25.3”) gives an error Long parseLong Long.parseLong(“25”) gives 25L Long.parseLong(“25.3”) gives an error Float parseFloat Float.parseFloat(“25.3”) gives 25.3F Float.parseFloat(“abc”) gives an error Double parseDouble Double.parseDouble(“25”) gives 25.0 Double.parseDouble(“abc”) gives an error
  44. 44. Arithmetic Operators We have already used and seen some of these as they are the basic arithmetic operations whose function is intuitive.
  45. 45. Increment and Decrement Java has two special operators for incrementing or decrementing a variable by one. These may either be prefix (before the variable) or postfix (after the variable).  if you include i++ as a term of a larger expression, the original value of i is used and afterwards incremented or decremented in case of i--  if you include ++i or --i as a term of a larger expression, the new value of i is used.
  46. 46. Comparison Each comparison using one of the six infix operators below will produce a boolean result of true or false. Evaluation is from left to right and is short circuit. Short circuit means that if the operation is an ―and‖ and the first clause evaluates to false, the rest of the clauses are not evaluated. Similarly if the operation is an ―or‖ and the first clause is true the rest are omitted.
  47. 47. Chaining Logical Conditions  NOT The logical not is a prefix operator.  AND & OR  The truth tables of the && (and) and the || (or) operations shown below indicate that the two operations are commutative. For example (i && j) produces the same result as (j && i)int i = 10, j = 28;boolean valid;valid = ((i < 12) && (j <15));
  48. 48. Conditional Assignment Operator The conditional assignment (ternary) operator provides an in- line if/then/else.  If the condition is true, the expression after the ―?‖ is evaluated and returned as a result of the statement.  If the condition is false, the expression after the ―:‖ is evaluated and returned as a result of the statement.
  49. 49. Bitwise Operators Bitwise operators are used only with operands of integral types
  50. 50. Example of Bitwise Operators
  51. 51. Precedence and Associativity Precedence of Operators  Java treats operators with different importance, known as precedence. There are in all 15 levels of precedence. In general, the unary operators have a higher precedence over the binary operators and parentheses can always be used to improve clarity in the expression. Associativity of Operators  For two operators of equal precedence a second rule, ―associativity‖ applies. Associativity is either ―left to right‖ (left operator first) or ―right to left‖ (right operator first):
  52. 52. Constants We can change the value of a variable. If we want the value to remain the same, we use a constant. To specify the exact type of literal constants, one can use the L,F and D or l,f and d to define the precision for numerals. We then use the reserved keyword final to lock it. final double PI = 3.14159D; As a convention we final int MONTH_IN_YEAR = 12; use uppercase letters final short FARADAY_CONSTANT = 23060; separated by underscores for constants
  53. 53. The Math Class The Math class in the Method Description java.lang package abs(a) Absolute value of a contains class methods for exp(a) Natural number e raised to the power of a. commonly used log(a) Natural logarithm (base e) of a. floor(a) The largest whole number less than or equal to a. mathematical functions. max(a,b) The larger of a and b. Most methods of the Math min(a,b) The smaller of a and b class are static and pow(a,b) The number a raised to the power of b. therefore there is no need sqrt(a) The square root of a. sin(a) The sine of a. (Note: all trigonometric functions are to instantiate any new computed in radians) Similarly cos and tan object of the Math class random() Returns a double value with a positive sign, greater before using it. than or equal to 0.0 and less than 1.0. round(a) Returns the closest integer value, if a is double it Most Math class methods returns long, if float it returns int generate a double type, so toRadians(a) Converts the value of a in degrees to radians pa attention to precision errors //Generate a random number between 1 and 100 int r = (int)Math.round(1+Math.random()*99);
  54. 54. Op Summary Here is a summary of the operators used in Java. B.E.D.M.A.S  In most simple cases it easier to remember the precedence of operators using the slightly modified BEDMAS (Brackets Exponent Division Multiplcation Addition and Subtraction). As a general rule, instead of relying on memory, in case of doubt, use the brackets to make the precedence you desire.
  55. 55. Example public class Matematika{ public static int random(int startRange, int endRange){ int answer = (int) (startRange+Math.random()*(endRange-1)); return answer; } public static int power(int base, int exponent){ int answer = (int)Math.round(Math.exp(Math.log((double)base)*exponent)); return answer; } public static boolean isEven(int n){ return (n%2) == 0; } public static double sin(double degrees){ double radians = degrees*2*Math.PI/360D; double answer = Math.sin(radians); return answer; } }
  56. 56. Example main program and Outputpublic class UseMatematika{ public static void main (String args []){ int number1 = 1; int number2 = 6; int number3 = 2; int dice = Matematika.random(number1,number2); System.out.println("The first roll of the die gave: "+dice); double result = Matematika.power(number3, number2); System.out.println(number3+" to the power of "+number2+" = "+result); System.out.println("Sine of 30 degrees is: "+Matematika.sin(30D)); System.out.println(number1+" is even ? "+Matematika.isEven(number1)); }}
  57. 57. Programming ElementsIn this section we start constructing our own classes.First we discuss the standard input and standard outputwhich we need throughout the course to make ourprograms interactive.Secondly we deal with how classes are defined. Variousaspects of the data and methods will be described,among which the most important is the constructor.
  58. 58. The Standard Input – The Keyboard  The technique of using to input data is called standard input. We can only input a single byte using directly, so to input primitive data values, we use the Scanner class. Method Exampleimport java.util.*; nextByte( ) byte b=kb.nextByte( );… nextDouble( ) double d=kb.nextDouble( );Scanner kb;kb = new Scanner(; nextFloat( ) float f=kb.nextFloat( );int num = kb.nextInt(); nextInt( ) int i=kb.nextInt( ); nextLong( ) long l=kb.nextLong( ); nextShort( ) short s=kb.nextShort( ); next() String; useDelimiter(String) kb.useDelimiter(“n”);
  59. 59. The Standard Output – The Monitor Using System.out, we can output multiple lines of text to the standard output window. We use the print method to output a value to the standard output window. The print method will continue printing from the end of the currently displayed output. We use println instead of print to skip a line. int x = 123, y = x + x; System.out.println(" Hi John”); System.out.print( " x = “ ); System.out.println( x ); System.out.print( " x + x = “ ); System.out.print( y ); System.out.print(“n”); System.out.println( " THE END“ );
  60. 60. Defining a New Class Learning how to define our own classes is the first step toward mastering the skills necessary in building large programs. Classes we define ourselves are called programmer-defined classes. imports comments public class { class Name Data Members (fields) Member Methods }
  61. 61. Class Example importsimport java.util.*;/** * Account Class comments */public class Account{ class Name private String name; private String idCard; Data Members private float balance; (fields) public Account(String n, String id, float b){ name =n; special method (The idCard = id; Constructor ) which balance = b; builds the object on } instantiation public void deposit(float amount){ balance += amount; Member } ......... Methods}
  62. 62. Organizing Classes into a Package For a class A to use class B, their bytecode files must be located in the same directory. This is not practical if we want to reuse programmer-defined classes in many different programs The correct way to reuse programmer-defined classes from many different programs is to place reusable classes in a package. A package is a Java class library.import PackageA.*;public class UsePackage{ public static void main (String args[]){ MyFrame newFrame = new MyFrame(); newFrame.setVisible(true); }}
  63. 63. Compiling and CLASSPATH CLASSPATH  Specifying where to search for additional libraries in Windows is easily done by setting the environment variable CLASSPATH, which Java uses to see where it should find Java programs and libraries. Compilation into bytecode BankManager.class + Account.class to JVM
  64. 64. Parameters An argument or actual parameter is a value we pass to a method. A formal parameter is a placeholder in the called method to hold the value of the passed argument. An actual parameter is the actual value passed by the caller program actual parameter formal parameter public void deposit(float amount){ a1234.deposit(500); balance += amount; } in calling method Account
  65. 65. Matching Actual and Formal Parameters  The number of arguments and the number of parameters must be equal  Formal and actual parameters are paired left to right, and the names of the formal and actual parameters are by no means important for matching.  Each matched pair must be assignment-compatible (e.g. you cannot pass a double argument to a int parameter)  When we are passing primitive data values, the parameter passing is by value, so the receiving side cannot alter the values for the passing side.  When more than one parameter is passed, they are separated by a comma (,)
  66. 66. Pass-By-Value (by Copy)  The actual parameter (or argument expression) is fully evaluated and the resulting value is copied into a location being used to hold the formal parameters value during method execution. That location is typically a chunk of memory on the runtime stack for the application (which is how Java handles it), float x = 25.5F public float add(float a, int b){ Item.add(x, 25); return a+b; in } object in main x 25.5 25.5 a memory spacememory space of of object 25 25 b main no name matched literal constant by position
  67. 67. Pass-By-Reference As we can pass int and double values, we can also pass an object to a method. When we pass an object, we are actually passing the reference (address) of an object. This means a duplicate of an object is NOT created in the called method, so object parameters are passed by reference. public void add(Student a){ Student s = new Student(); a.getName(); klassi.add(s); } a s Student memory space of main
  68. 68. Note on Primitive Parameter Passing  Primitive data arguments are passed to a method by using the pass-by- value scheme.  Arguments are matched to the parameters from left to right. The data type of an argument must be assignment-compatible with the data type of the matching parameter.  The number of arguments in the method call must match the number of parameters in the method definition.  Parameters and arguments need not have to have the same name.  Local copies of values passed by reference as parameters, which are distinct from arguments, are created even if the parameters and arguments share the same name.  Parameters are input to a method, and they are local to the method. Changes made to the parameters will not affect the value of corresponding arguments
  69. 69. Access Modifiers The access modifiers public and private designate the accessibility of data members and methods.  If a class component (data member or method) is declared private, client classes cannot access it.  If a class component is declared public, client classes can access it. Internal details of a class are declared private and hidden from the clients. This is information hiding. If none of this is declared, the method or data member becomes package friendly and can be accessed only from classes in the same package. … class Service { Service obj = new Service(); public int memberOne; obj.memberOne = 10; private int memberTwo; obj.memberTwo = 20; public void doOne() { … } obj.doOne(); private void doTwo() { … } obj.doTwo(); }
  70. 70. Guideline for Access Modifiers Declare the class and instance variables private. Declare the class and instance methods private if they are used only by the other methods in the Class Diagram same class. Service Declare the class constants public if you want to make their values directly readable by the client +memberOne -memberTwo programs. If the class constants are used for internal purposes only, then declare them +doOne() -doTwo() private. Another access modifier is protected. We will private gets (-) see this one after treating inheritance, as it is public gets (+) visible by subclasses which inherit the class.
  71. 71. Static or Dynamic Instance methods or variables are associated with an object and methods use the instance variables of that object. This is the default. Static methods use no instance variables of any object of the class they are defined in. If you define a method to be static, you will be given a rude message by the compiler if you try to access any instance variables. You can access static variables, but except for constants, this is unusual. Static methods typically take all their data from parameters and compute something from those parameters, with no reference to variables.
  72. 72. Typical Static Methods Static methods typically do some kind of generic calculation. A good example of this are the many utility methods in the predefined Math class. In any program there is only one version of a static method or variable whilst in dynamic ones there is a new version every instance created. Thus on static methods there is no need to call the new statement in the client class as in dynamic classes. public class Maths{ ... double result = Maths.cube(3); public static double cube(int x) { return (x*x*x); } no need to create a new object but ... use the class directly since method is static
  73. 73. Class Variables and Constants Class Constants  provide a meaningful description of what the values stand for: number = UNDEFINED; is more meaningful than number = -1;  provides easier program maintenance. We only need to change the value in the constant declaration instead of locating all occurrences of the same value in the program code. caps for constants private static final int MAX_NUMBER=6; Class Variables  Class variables are there to keep a overall copy of a value shared among all objects of the same class, say the total number of objects created, or the sum of money gained from all objects of a vending machine. lower for variables private static int minimumBalance;
  74. 74. Class Example – Complex Number We are used to real numbers, but when we get to the square root of a negative number I we go in another dimension called the imaginary numbers. Simply said we denote sqrt(-1) = i. Thus sqrt(-9) becomes 3i. R A complex number consists of two parts a real part and an imaginary part. To keep things simple we shall represent these as integers. Suppose we want to create a new data type in the form of a class to represent complex numbers.
  75. 75. The Data Members First of all we need to define the data required for the real class, namely two integers, one for the real part and another for the imaginary part. img It is good practice to declare these as private, so that they are not directly accessible from outside the object. member methods Methods on the other hand are declared as public so that the users of this class can communicate with them. So we begin by declaring the two required fields Since only methods within the object have access to private int real; these, any bug within their value can be attributed only private int img; to the member methods.
  76. 76. The Constructor The constructor is a special member method which initialises an object when it is created. We limit, for the time being to set all fields to zero. The constructor has always the same name as the class and has no return type. Normally be implement this method as the first method in the class. If the constructor is not implemented specifically by the programmer, will be implemented by Java, resetting all data members. – The default Constructor public Complex(){ real = 0; img = 0; }
  77. 77. The Member Methods  These are methods that communicate with the external environment and act upon the data members of the object.  Note that the constructor does not have a return type. This is the only method not to have a return type.  Nearly all classes have setter (mutator) and getter (accessor) methods to set the values of their private data members.Setter (mutator) methods Getter (accessor) methodspublic void setReal(int r){ public void getImaginary(){ real = r; return img; } } Generic methods public void setValue(int r, int i){public void setImaginary(){ public int getReal(){ real = r; img = i; return real; img = i; } } }
  78. 78. Implementation /** * Class to represent a complex number/** */* Main Class for complex numbers public class Complex*/ { // The Real Part of the Complex Numberpublic class Roots{ private int real; // The Imaginary Part public static void main (String args[]){ private int img; // Declare and instantiate Complex number1; // The Constructor (Initialiser) 1 public Complex(){ 1 number1 = new Complex(); real = 0; // Declare and instantiate again img = 0; 1 } Complex number2 = new Complex(); // Set their value // Set the value of the complex number number1.setValue(2,4); public void setValue(int r, int i){ 2 2 real = r; number2.setValue(5,3); img = i; // Display the two numbers } System.out.println("Number 1 = " + // Get the real value 3 number1.getReal() + " + " + public int getReal(){ 3 number1.getImaginary()+"i"); return real; } System.out.println("Number 2 = " + number2.getReal() + " + " + 4 // Get the imaginary part number2.getImaginary()+"i"); 4 public int getImaginary(){ } return img;} } }
  79. 79. Local Variables Local variables are declared within a method declaration and used for temporary services, such as storing intermediate computation results. Global Variables are declared outside all methods and can be seen anywhere within the class. It is good practice to use local variables as much as possible. public double convert(int num) { double result; result = Math.sqrt(num * num); return result; } local variable
  80. 80. Locals, Parameters & Data Members An identifier appearing inside a method class CD { can be a local variable, a parameter, or a private int n; 1 private String artist; 2 data member. The rules are: 3 4 public CD(String n1, int n){  If there‘s a matching local variable declaration or a parameter, then the String ident; 5 identifier refers to the local variable or the parameter. 2 3 artist = n1;  Otherwise, if there‘s a matching data 4 1 this.n = n; member declaration, then the identifier refers to the data member. 5 ident = artist.substring(0,2)  Otherwise, it is an error because there‘s no matching declaration. } ...  Local variables and parameters cannot have } the same name.
  81. 81. “this” Keyword If a parameter or local variable have the same name as a data member, they hide the global variable. public Circle(){ this(0D); Thus a reference to that name } will involve only the local variable or parameter. To overcome this, the keyword :Student ―this‖ is used to refer to the global variable. this ―this‖ alone refers to the constructor of the class.
  82. 82. Method Overloading The Java programming language supports overloading methods, and Java can distinguish between methods with different method signatures. This means that methods within a class can have the same name if they have different parameter lists. Overloaded methods are differentiated by the number or the type of the arguments passed to the method. You cannot declare more than one method with the same name and the same number and type of arguments, because the compiler cannot tell them apart. The compiler does not consider return type when differentiating methods, so you cannot declare two methods with the same signature even if they have a different return type.
  83. 83. Overloading Example In this code sample, draw(String s) and draw(int i) are distinct public class DataArtist { and unique methods because ... public void draw(String s) { they require different argument ... types. } public void draw(int i) { Note that a method with a ... } signature of: public void draw(double f) { public int draw(int i){ ... } is not permitted because it public void draw(int i, double f) { ... distinguishes itself from another } method only on the return type }
  84. 84. Constructor Overloading The constructor of the class can be overloaded as all other 1 Circle circle1 = new Circle(3.5d); methods, and it is normal Circle circle2 = new Circle(); 2 practice to provide more than public class Circle{ one constructor method. private double radius; ... this can also be used to call a public Circle(){ 2 different overloaded radius = 0; } constructor of the same object.. public Circle(double r){ 1 Pay Attention: The call to this in radius = r; this case must be the first } } statement in the constructor.
  85. 85. Overloading Complex Constructor  Now that we have other tools at hand we can consider some modifications to the Complex Numbers class implementation. First we use the conditional assignment to display properly the numbers in case the imaginary part is negative and we provide an additional constructor to initialise a complex number to a given value. public class Complex{ ...... public Complex(int r, int i){Complex number1; real = r;number1 = new Complex(3,5); img = i;... }System.out.println("Number 1 = " + ... number1.getReal() + } Overloaded ((number1.getImaginary()>0)?"+":"") + constructor to set number1.getImaginary()+"i"); values too
  86. 86. The toString() Method The toString() returns a string representation of the object. In general, the toString() method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that public class Complex{ all subclasses override ... // Overiding the default toString this method. public String toString(){ So for our Complex return(real+((img>0)?"+":"")+img+"i"); } number class the ... ... toString() method would } Complex number1; look aside. number1 = new Complex(3,5); This method is the ... System.out.println("Number 1 = "+number1); method called when you try to print the object Solution is now much more directly. elegant than previous slide
  87. 87. Text Formatting The formatter class is used to present a formatted output. Instead of using the Formatter class on its own it is much more convenient to use it a a method in the Printstream (System.out) or in the String classes. System.out.printf("%6d", 498); is equivalent to: Formatter fmt = new Formatter(System.out); fmt.format("%6d", 498); int n1=34, n2=9; int n3=n1+n2; System.out.printf("%3d + %3d = %5d", n1 , n2 , n3);
  88. 88. Formatting Options Integers: % <field width> d Real Numbers : % <field width> . <decimal places> f When the string Strings: %s needs to be Hex: %x or %X These are just formatted without Octal: %o some of the displaying it, the control strings Character %c possible in the String class provides Scientific %e or %E printf a static method format which is similar to the above. „-‟: left justified „+‟: always include a sign Flags: The optional „(„: negative number in brackets flags is a set of characters that modify the output always show A better toString for Complex sign format. public String toString(){ return(String.format("%d %+di",real,img)); }
  89. 89. Returning Objects As we can return a primitive data value from a method, we can return an object from a method too. We return an object from a method, we are actually returning a reference (or an address) of an object. This means we are not returning a copy of an object, but only the reference of this object public class Roots { in main public static void main (String args[]){ class Complex n1 = new Complex(5,-5); Complex n2 = new Complex(2,-3); result is Complex n3 = n1.add(n2); System.out.println("Number 1 = "+n1); assigned to n3 System.out.println("Number 2 = "+n2); System.out.printf("(%s) + (%s) = %s",n1,n2,n3); } }
  90. 90. Implementation of add in Complex returns apointer to a class Complex public Complex add( Complex num){result = method in class + this Complex return result} public Complex add(Complex num){ Complex result = new Complex(); result.real = num.real+this.real; n3 result.img = num.img+this.img; return result; }
  91. 91. Program Control StructuresHere we first consider the selection constructs in java,namely the if and the switch statements, followed by thethree iterative constructs, namely, the for loop, while loopand the do while loop.We also investigate the alternative recursive procedureto obtain the same things in a more elegant, sometimesmore inefficient way.
  92. 92. The Student public class Student This example will be used { throughout the rest of the private String name; chapters so pay attention private int mark; Class public Student(){ name = ""; Student mark = 0; name } public Student(String n, int m){ mark :Student name = n; mark = m; member methods-name: String }-mark: int public void setName(String n){ name = n; } public void setMark(int m){+Student() mark = m; }+Student(String,int) public String getName(){+setName(String); return name; Note: This is }+setMark(int) public int getMark(){ overloading not overriding!+getMark():int return mark; }+getName():String public boolean equals(Student s){+equals(Student):boolean return this.getName().equals(s.getName()); }+compareTo(String):int public int compareTo(Student s){+toString():String return; } public String toString(){ return "Name: "+name+" Mark: "+mark; } }
  93. 93. Selection Statements The first selection statement is the if statement. The else part is optional in this statement
  94. 94. The if Statement In this syntax, if the boolean expression evaluates to true, the following statement or block of statements enclosed within braces is performed, otherwise nothing is executed. if ( <boolean expression> ) <statement>; no “then” keyword, we use brackets instead if ( mark < 45 ) System.out.println(“You failed”);
  95. 95. The if-else statement Here, if the boolean expression evaluates to true, the statement1 or block of statements enclosed within braces is performed, otherwise Statement2 is performed. use semicolon No semicolon here! before else if no braces are used if ( <boolean expression> ) <statement 1>; if ( mark < 45 ) else System.out.println(“You failed”); <statement 2>; else { System.out.println(“Lucky !! “); mark++; } use braces to enclose more than one statement
  96. 96. Boolean Operators && || ! Use boolean operators to join expressions  && is the logical and operator  || is the logical or operator P Q P && Q P || Q !P  ! is the not operator false false false false true false true false true true true false false true false true true true true falseif ( wage > 45000 && holiday == true && !married ) System.out.println(“You can go on holiday to Hawaii”);else { System.out.println(“Go work!!“);}
  97. 97. Nested if‘s The else associates with the nearest if. Thus braces must be used to impose the required association. Use of nested if‘s can be a bit confusing at times and it is sometimes preferred to use the switch statement.
  98. 98. Comparing Objects With primitive data types, we have only one way to compare them, but with objects (reference data type), we have two ways to compare them  We can test whether two variables point to the same object (use ==), or  We can test whether two distinct objects have the same contents. Proper way to compare the contents‖ String str1 = new String("Java"); same sequence of String str2 = new String("Java"); characters if (str1.equals(str2)) { System.out.println("They are equal"); } else { System.out.println("They are not equal"); }
  99. 99. Comparing Objects with == String str1 = new String("Java"); str1 String str2 = str1; J a v a if (str1 == str2) { System.out.println("They are equal"); } str2 else { System.out.println("They are not equal"); } str1String str1 = new String("Java"); J a v aString str2 = new String("Java");if (str1 == str2) { str2 System.out.println("They are equal"); J a v a } else { System.out.println("They are not equal"); }
  100. 100. equals Implementation in Complexpublic class Roots{ public static void main (String args[]){ Complex number1 = new Complex(2,-5); Complex number2 = new Complex(2,-5); in main System.out.println("Number 1 = "+number1); class System.out.println("Number 2 = "+number2); method in class if(number1.equals(number2)) Complex System.out.println("They are equal"); } public boolean equals(Complex other){} return ((this.real==other.real) && this.img==other.img)); }if ((this.real==other.real)&&(this.img==other.img)) return true; We will see a useless betterelse code implementation return false; later
  101. 101. The switch Statement The switch statement avoids a lot of nested if‘s Note:  Only integral constants may be tested  If no condition matches, the default is executed  If no default, nothing is done (not an error). There can be only one default  The break is a must! otherwise all statements from the matched case onwards are executed
  102. 102. Syntax of the switch Statement
  103. 103. Example of the switch Statement float f switch (f) { case System.out.print("Input number of legs: "); 2: int legs = keyboard.nextInt(); switch(legs){ switch (i) { case 0: System.out.println("Theres something fishy!"); break; case 2*j: case 1: case 3: case 5: System.out.println("Thats odd!"); break; case 6: System.out.println("There must be a bug!"); System.out.println("Get it debugged"); break; default:System.out.println("Its a sort of creature"); }
  104. 104. Iterations (Repetitive Statements) Repetition statements control a block of code to be executed for a fixed number of times or until a certain condition is met. Count-controlled repetitions terminate the execution of the block after it is executed for a fixed number of times. Sentinel-controlled repetitions terminate the execution of the block after one of the designated values called a sentinel is encountered. Repetition statements are also called loop statements.
  105. 105. The while statement (A Pretested Loop) int sum = 0, number = 1; while ( number <= 100 ) { sum += number; number++; } while ( number <= 100 ); {
  106. 106. The do-while Statement (Post-tested) Use this when you want to loop at least once. Ideal for keyboard entry validation. int sum = 0, number = 1; do { sum += number; number++; } while ( sum <= 1000000 );
  107. 107. for loops (Another Pretested loop)
  108. 108. for Loops Examplesfor (int c = 1; c <=3, c++){ for (int r= 1, r <=3, r++){ for (int j = 2; j < 40; j *= 2) int product = r * c; System.out.print (“ “ + product); } //finished one row; move on to next row for(int i=0; i < 3 ; i++) System.out.println(“”); int count =2;} actually a declaration and an assignment. so use braces or separate
  109. 109. Watch out! Watch out for the off-by-one error (OBOE). Make sure the loop body contains a statement that will eventually cause the loop to terminate. Make sure the loop repeats exactly the correct number of times. If you want to execute the loop body N times, then initialize the counter to 0 and use the test condition counter < N or initialize the counter to 1 and use the test condition counter <= N. Avoid a semicolon after the boolean expression cause it will make it an empty loop.
  110. 110. Escape the Block ! ‗break‘ takes you out of the present block, and ‗continue‘ will take you at the end of the end of the body of the loop ie at the next iteration of the body. You can also escape from a whole nested blocks by using the ‗goto label‘. All these are interruptions to the normal sequence of operations and should be very rarely used where the application strictly requires it like the switch statement. Abuse of these leads to ―spaghetti programming‖
  111. 111. Recursive Algorithms A recursive method is a method that contains a statement (or statements) that makes a call to itself. Example - Factorial  The factorial of N is the product of the first N positive integers:  N! = N * (N – 1) * (N – 2 ) * . . . * 2 * 1  for example 5! = 5 * 4 * 3 * 2 * 1  The factorial of N can be defined recursively as 1 if n =1 factorial (n) = n * factorial (n-1) otherwise
  112. 112. Factorial Recursive Implementation  Implementing the factorial of N recursively will result in the following method. public static long factorial (long m){ long k = factorial(4); if (m == 1) return 1; else in calling program return m* factorial(m-1); } public static long fac(long m ){ return (m==1)? 1 : m*fac(m-1); shorthand }
  113. 113. Power Recursive Implementation Similarly x to the power of y: xy = x * x(y-1) x if y =1 power(x,y) = x* power(x,(y-1)) otherwisepublic static int power (int x, int y){ int k = power(2,10); if (y == 1) return x; in main else return ( x* power(x, y-1)); }
  114. 114. Fibonacci series WHEN NOT TO USE RECURSION  The well known series of Fibonacci goes: 1, 1, 2, 3, 5, 8, 13, etc ..  Where except for the first two terms, each term is the sum of the previous two terms. The first two terms are 1. 1 n <= 2 fibonacci(n) = fibonaccci(n-1)+fibonacci(n-2) otherwisepublic static long fibonacci (long n){ long k = fibonacci(5); if (n <=2) return 1; in main else return (fibonacci(n-1)+fibonacci(n-2)); most of the terms are } recalculated
  115. 115. Arrays Strings and PatternsOne of the most important structures in computer scienceare arrays which are variables which can hold a numberof values. These are indispensable for making variableschange in loops and for not using too much variablenames.Other important structure in Java is the String which is aseries (an array) of characters and Patterns which areregular expressions used for matching strings and aremuch helpful in the validation of input data.
  116. 116. Introduction to Arrays Array, what is it? An array is a group of variables of the same data type and referred to by a common name. An array is contiguous block of memory locations referred by a common name. For example to store the marks of 5000 students, you can declare an array, marks, of size 5000 and can store the marks of as many students. int marks[] = new int[5000];
  117. 117. Why are Arrays Needed? You might come across a situation where you need to store similar type of values for a large number of data items. For example to store the marks of all the students of a university, you need to declare thousands of variables. In addition, each variable name needs to be unique. To avoid such situations, you can use arrays. An array consists of a name and the number of elements of the array. You can refer to a specific array element by the array name and the element number, which is known as the index. Note: - Array index element number always starts with 0(zero) in Java and ends at one less its length.
  118. 118. Creating Arrays The length of an array is fixed at the time of its creation. An array represents related entities having the same data type in contiguous or adjacent memory locations. The related data having data items form a group and are referred to by the same name. String employee = new String[5]; Here, the employee is the name of the array and of size 5. The complete set of values is known as an array and the individual entities are called as elements of the array. A specific value in an array is accessed by placing the index value of the desired element in a square bracket. String bestEmployee = employee[3];
  119. 119. Arrays of Primitive Data Types Array declaration  <data type> [] <variable name>; or  <data type> <Variable name>[]; float [] marks; float marks []; Array Creation  <variable> = new <data type> [<size>]; marks = new float[12]; arrays are objects
  120. 120. Array Initialization Like other data types, it is possible to declare and initialize an array at the same time.
  121. 121. Accessing Individual Elements  To access an individual element of an array we use the following notation: m1 = marks [1]; marks[2] = 97;  <variable name> [<index>]  The size of an array is given by a public data member inside the array class named length;public static void main (String args[]){ Scanner keyboard = new Scanner(; int assessments[] = new int[3]; //calculate total //get marks int total = 0; for(int i=0;i<assessments.length; i++){ for(int i=0;i<assessments.length;i++){ System.out.print("Enter mark: "); total += assessments[i]; assessments[i] = keyboard.nextInt(); } } float average = (float)total/3; System.out.println("The average mark is: "+average);(float)(total/3); cast number } not result
  122. 122. Variable-Size Declaration In Java, we are not limited to fixed-size array declaration at compilation time unlike some other languages. The following declares an array of size chosen at runtime by the user:public static void main (String args[]){ Scanner keyboard = new Scanner(; int dimension; System.out.print("Enter dimension of array: "); dimension = keyboard.nextInt(); float myArray [] = new float [dimension];} size declared at runtime
  123. 123. Two-Dimensional Arrays Two-dimensional arrays are useful in representing tabular information.  Declaration:  <data type> [][] <variable name> or <data type> <variable name> [][]; int [ ][ ] coefs [ ][ ]; int coefs [ ][ ];  Creation:  <variable name> = new <data type> [<size>][<size>]; coefr = new int[3][5];  All together: Access elements effectively a 2D array coefs[4][1]=5; is an array of arrays int x = coefs[3][2]=5;
  124. 124. Matrix (Array) Multiplication
  125. 125. Matrix Multiplication Implementation public static void main (String args[]){ int a[][] = {{5, 2, 0, 10},{3, 5, 2, 5}, {20, 0, 0, 0}}; double b[][] = {{1.50, 0.20},{2.80, 0.40},{5.00, 1.00}, {2.00, 0.50}}; float c [][] = new float [3][2]; for(int i = 0;i< b[0].length; i++){ for(int j = 0;j <a.length;j++){ c[j][i] = 0.0F; for(int k = 0;k<a[0].length;k++){ c[j][i] += (float)(a[j][k] * b[k][i]); for( int i =0; i< c.length;i++){ } for(int j = 0; j < c[0].length;j++){ } System.out.printf(" %6.2f ",c[i][j]); } } System.out.println(""); } }
  126. 126. Ragged and Multi-Dimensional In fact in Java, a two dimensional array is an array of arrays and so the 2D array is not necessarily rectangular An array in java is stored row-first.double[][] tri = new double[4][];for(int i=0;i<4;i++) tri[i] = new double[i+1]; In fact in Java you can have 3D arrays, 4D arrays to any dimension you desire. It is hard to visualise but a 3D array is nothing but an array of 2D arrays, etc. double[][] [] threeD = new double[4][4][4];
  127. 127. Arrays of Objects In Java, in addition to arrays of primitive data types, we can declare arrays of objects. An array of primitive data is a powerful tool, but an array of objects is even more powerful. The use of an array of objects allows us to model the application more cleanly and logically. 1 2 3 1 Student [] klassi; klassi = new Studnet[6]; 2 klassi[0] = new Student(); 3 klassi[1] = new Student();
  128. 128. Example of 2D Arraysimport java.util.*; int bestpos = 0; // assume first to be the bestpublic class BestWorst int worstpos = 0; // assume first to be the worst{ for(int i = 0; i< klassi.length; i++){ if(klassi[i].getMark()>klassi[bestpos].getMark()) public static final int SIZE = 3; bestpos = i; if(klassi[i].getMark()<klassi[worstpos].getMark()) public static void main (String args[]){ worstpos = i; } Student [] klassi = new Student[SIZE]; System.out.println(klassi[bestpos]); System.out.println(klassi[worstpos]); Scanner kb = new Scanner(; Uses kb.useDelimiter("n"); toString in } Student } for(int i = 0; i<klassi.length; i++){ System.out.print("Enter Name & Surname: "); String name =; kb.useDelimiter("n"); System.out.print("Enter Mark: "); int mark = kb.nextInt(); to read space klassi[i] = new Student(name,mark); between name } and surname
  129. 129. Command-Line Arguments A Java application can accept any number of arguments from the command line. This allows the user to specify configuration information when the application is launched. When an application is launched, the runtime system passes the command-line arguments to the applications main method via an array of Strings. public class HaveMore{ public static void main (String args[]){ if (args.length > 0){ System.out.println("You got some extra stuff: "); for(int i = 0; i <args.length;i++){ System.out.println(args[i]); } } } }
  130. 130. Strings A string is a sequence of characters that is treated as a single value. Instances of the String class are used to represent strings in Java. In Java a String object is immutable This means that once a String object is created, it cannot be changed, such as replacing a character with another character or removing a character The String methods we have used so far do not change the original string. They created a new string from the original. For example, substring creates a new string from a given string. Single characters can be accessed using the charAt() method String r = "Top 20"; r.charAt(1)
  131. 131. Some Useful methods in String Note: there are far too many methods in class String. See String documentation for a complete list
  132. 132. Different String Creations Depending on the methods used for creation, Sting pointers can point either to the same object or different objects: String word1, word2; String word1, word2; word1= "Java"; word1= new String("Java"); word2= "Java"; word2= new String("Java"); word1 word1 J a v a J a v a word2 word2 J a v a
  133. 133. StringBuffers In many string processing applications, we would like to change the contents of a string. In other words, we want it to be mutable. Manipulating the content of a string, such as replacing a character, appending a string with another string, deleting a portion of a string, and so on, may be accomplished by using the StringBuffer class. StringBuffer word= new StringBuffer(“Java"); word.setCharAt(0,L); word word J a v a L a v a
  134. 134. Patterns - Regular Expressions A Pattern is called a regular expression. Rules  The brackets [ ] represent choices  The asterisk symbol * means zero or more occurrences.  The plus symbol + means one or more occurrences.  The hat symbol ^ means negation.  The hyphen – means ranges.  The parentheses ( ) and the vertical bar | mean a range of choices for multiple characters. if(entry.matches("(21|27|79|99)[0-9]{6}")) System.out.println("Valid Telephone Number "); else System.out.println("Invalid Telephone Number");
  135. 135. Some ExamplesExpression Description[013] A single digit 0,1 or 3[0-9][0-9] Any two digit number 00 to 99[0-9&&[^4567]] A single digit 0,1,2,3,8 or 9[a-z0-9] Single lower case character or digit[a-zA-Z][a-zA-Z0-9_$] A valid Java identifier consisting of alphanumeric characters, underscores and dollar sign, with the first charcter being an alphabetic character[wb][qd|eed] Matches wad weed and beed(AZ|CA|CD)[0-9][0-9] Matches AZxx, CAxx and COxx, where x is a single digit
  136. 136. Garbage Collection Garbage collection is the process of automatically finding memory blocks that are no longer being used ("garbage"), and making them available again. In contrast to manual deallocation that is used by many languages, eg C and C++, Java automates this error-prone process and avoids two major problems:  Dangling references. When memory is deallocated, but not all pointers to it are removed, the pointers are called dangling references -- they point to memory that is no longer valid and which will be reallocated when there is a new memory request, but the pointers will be used as tho they still pointed to the original memory.  Memory leaks. When there is no longer a way to reach an allocated memory block, but it was never deallocated, this memory will sit there. If this error of not deleting the block occurs many times, eg, in a loop, the program may actually crash from running out of memory.
  137. 137. Javadoc Many of the programmer-defined classes we design are intended to be used by other programmers. It is, therefore, very important to provide meaningful documentation to the client programmers so they can understand how to use our classes correctly. By adding javadoc comments to the classes we design, we can provide a consistent style of documenting the classes. Once the javadoc comments are added to a class, we can generate HTML files for documentation by using the javadoc command. Javadoc comments begins with /** and ends with */ Special information such as the authors, parameters, return values, and others are indicated by the @ marker  @param @author @return @version see:
  138. 138. Stacks A programmer‘s stack is a simple concept with wide application. Stacks can be found in all kinds of programs, at system level and in every field of application. To maintain the analogy of a physical stack (of bricks or trays) we draw a programmer‘s stack (of numbers, chars or even objects) upside down. A stack may be created, as depicted, from an array of stackable objects and an integer variable for storing the number of objects currently stacked (top of stack) Three functions needed to manage such a stack:  push: Place a new object on the top of the stack  pop: Take the item that is on top of the stack  peep: Take a copy of the item on top of the stack without removing it.