Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Java Course 3: OOP

5,249 views

Published on

Lecture 3 from the IAG0040 Java course in TTÜ.
See the accompanying source code written during the lectures: https://github.com/angryziber/java-course

Discusses more Java basics and Object Oriented Programming.

Published in: Technology
  • Be the first to comment

Java Course 3: OOP

  1. 1. Java course - IAG0040 More Java Basics and OOPAnton Keks 2011
  2. 2. Construction ● All objects are constructed using the new keyword – MyClass fooBar = new MyClass(); ● Object creation always executes a constructor – Constructors may take parameters as any other methods ● Default constructor (without parameters) exists if there are no other constructors defined ● Constructor is a method without a return type (it returns the objects instance), the name of a constructor is the same as of the class.Java course – IAG0040 Lecture 3Anton Keks Slide 2
  3. 3. Destruction ● No need to destroy objects manually ● Garbage Collector (GC) does the job of destruction – Executed in background when memory gets low – Can actually be faster than explicit deallocation – Can be invoked manually with System.gc(); ● An object is destroyed if there are no references left to it in the program, this almost eliminates memory leaks ● Out of scope local variables are also candidates ● finalize() method may be used instead of a destructor, however, its execution is not guaranteedJava course – IAG0040 Lecture 3Anton Keks Slide 3
  4. 4. Conditions ● if-else – if (boolean-expression) a boolean-expression should return a boolean statement or Boolean value else statement ● switch an expression can return an – switch (expression) { int, short, char, byte, their case X: respective wrapper classes statement (which are unboxed) or an break; enum type default: statement a statement is a single Java break; statement or multiple } statements in curly braces { }Java course – IAG0040 Lecture 3Anton Keks Slide 4
  5. 5. While loops ● while – while (boolean-expression) statement – Executes the statement while boolean-expression evaluates to true ● do-while – do statement while (boolean-expression); – Same, but boolean-expression is evaluated after each iteration, not beforeJava course – IAG0040 Lecture 3Anton Keks Slide 5
  6. 6. For loops ● for loop – for(initialization; boolean-expression; step) statement – Example: for(int i=0, j=1; i < 5; i++, j*=2) {} ● for each loop (arrays and Iterables) – for(variable : iterable) statement – for (int a : new int[] {1,2,3}) {} – iterates over all elements of an iterable, executing the statement for each of its elements, assigning the element itself to the variableJava course – IAG0040 Lecture 3Anton Keks Slide 6
  7. 7. break and continue ● break and continue keywords can be used in any loop – break interrupts the innermost loop – continue skips to the next iteration of the innermost loop ● goto keyword is forbidden in Java, but break and continue can be used with labels – outer: while(true) { inner: while(true) { break outer; } } – Labels can be specified only right before a loop. Usage of break or continue with a label affects the labeled loop, not the innermost one.Java course – IAG0040 Lecture 3Anton Keks Slide 7
  8. 8. Arrays ● Array is a special type of Object – Arrays are actually arrays of references – Elements are automatically initialized with zeros or nulls – Automatic range checking is performed, always zero based ● ArrayIndexOutOfBoundsException is thrown otherwise – Special property is accessible: length ● Definition: type followed by [ ]: byte[] a; String s[]; ● Creation using the new keyword: byte[] a = new byte[12]; ● Access: byte b = a[0]; byte b2 = a[a.length – 1]; ● Static initialization: byte[] a = new byte[] {1,2,3}; ● Multidimensional: int[][] matrix = new int[3][3];Java course – IAG0040 Lecture 3Anton Keks Slide 8
  9. 9. Strings ● An immutable char array with additional features ● Literals: “!” same as new String(new char[] {!}) ● Any object can be converted toString() ● All primitive wrappers understand Strings: new Integer(“3”) ● Equality is checked with equals() method ● String pooling – “ab” != new String(“ab”) – “a1” == “a” + 1 ● StringBuilder is used behind the scenes for concatenationJava course – IAG0040 Lecture 3Anton Keks Slide 9
  10. 10. Classpath ● Classpath tells the JVM where to look for packages and classes – java -cp or java -classpath – $CLASSPATH environment variable ● Works like $PATH environment variable ● Every used class must be present on classpath during both compilation and runtimeJava course – IAG0040 Lecture 3Anton Keks Slide 10
  11. 11. Classpath example ● Your class net.azib.Hello ● Uses org.log4j.Logger and com.sun.World ● In filesystem: – ~/myclasses/net/azib/Hello.class – ~/log4j/org/log4j/Logger.class – ~/lib/sun.jar, which contains com/sun/World.class ● Should be run as following: (separated with “;” on Windows) – java -cp myclasses:log4j:lib/sun.jar net.azib.Hello --- classpath ---Java course – IAG0040 Lecture 3Anton Keks Slide 11
  12. 12. Java API documentation Lets examine the API documentation a bit: http://java.sun.com/javase/6/docs/api/ (http://java.azib.net -> Lirerature & Links -> API documentation)Java course – IAG0040 Lecture 3Anton Keks Slide 12
  13. 13. Javadoc comments ● Javadoc allows to document the code in place – Helps with synchronization of code and documentation ● Syntax: /** a javadoc comment */ – Documents the following source code element – Tags: start with @, have special meaning ● Examples: @author, @version, @param ● @deprecated – processed by the compiler to issue warnings – Links: {@link java.lang.Object}, use # for fields or methods ● {@link Object#equals(Object)} ● javadoc program generates HTML documentationJava course – IAG0040 Lecture 3Anton Keks Slide 13
  14. 14. Varargs● A way to pass unbounded number of parameters to methods● Is merely a convenient syntax for passing arrays● void process(String ... a) – In the method body, a can be used as an array of Strings – There can be only one varargs-type parameter, and it should always be the last one● Caller can use two variants: – process(new String[] {“a”, “b”}) - the old way – process(“a”, “b”) - the varargs (new) way● System.out.printf() uses varargsJava course – IAG0040 Lecture 3Anton Keks Slide 14
  15. 15. Autoboxing ● Automatically wraps/unwraps primitive types into corresponding wrapper classes ● Boxing conversion, e.g. boolean -> Boolean, int -> Integer ● Unboxing conversion, e.g. Character -> char, etc ● These values are cached: true, false, all bytes, chars from u0000 to u007F, ints and shorts from -128 to 127 ● Examples: Integer n = 5; // boxing char c = new Character(z); // unboxingJava course – IAG0040 Lecture 3Anton Keks Slide 15
  16. 16. Extending classes ● Root class: java.lang.Object (used implicitly) ● Unlike C++, only single inheritance is supported ● class A {} // extends Object class B extends A {} – new B() instanceof A == true – new B() instanceof Object == true ● Access current class/instance with this keyword ● Access base class with super keywordJava course – IAG0040 Lecture 3Anton Keks Slide 16
  17. 17. Extending classes ● All methods are polymorphic (aka virtual in C++) – final methods cannot be overridden – super.method() calls the super implementation (optional) ● Constructors always call super constructors – if no constructors are defined, default one is assumed ● class A { } // implies public A() {} – default constructor is called implicitly if possible ● class B extends A { public B(int i) { } // super() is called public B(byte b) { super(); } // same here }Java course – IAG0040 Lecture 3Anton Keks Slide 17
  18. 18. Abstract classes ● Defined with the abstract keyword ● Not a concrete class, cannot be instantiated ● Can contain abstract methods without implementation ● abstract class Animal { // new Animal() is not allowed private String name; String getName() { return name; } abstract void makeSound(); } ● class Dog extends Animal { void makeSound() { System.out.println(“Woof!”); } }Java course – IAG0040 Lecture 3Anton Keks Slide 18
  19. 19. Interfaces ● Substitute for Javas lack of multiple inheritance – a class can implement many interfaces ● Interface are extreme abstract classes without implementation at all – all methods are public and abstract by default – can contain static final constants – marker interfaces dont define any methods at all, eg Cloneable ● public interface Eatable { Taste getTaste(); void eat(); } ● public class Apple implements Eatable, Comparable { /* implementation */ }Java course – IAG0040 Lecture 3Anton Keks Slide 19

×