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  • 1. Java Programming LanguageObjectives In this session, you will learn to: – Distinguish between valid and invalid identifiers – List the eight primitive types – Define literal values for numeric and textual types – Define the terms primitive variable and reference variable – Declare variables of class type – Construct an object using new – Describe default initialization – Distinguish between instance and local variables Ver. 1.0 Session 2 Slide 1 of 26
  • 2. Java Programming LanguageObjectives (Contd.) Describe how to initialize instance variables Identify boolean expressions and their requirements in control constructs Recognize assignment compatibility and required casts in fundamental types Use if, switch, for, while, and do constructions and the labeled forms of break and continue as flow control structures in a program Ver. 1.0 Session 2 Slide 2 of 26
  • 3. Java Programming LanguageIdentifier An identifier is a name given to a variable, class, or method. Identifiers have the following characteristics: – Can start with a Unicode letter, underscore (_), or dollar sign ($) – Are case-sensitive and have no maximum length Examples of valid identifiers: identifier userName user_name _sys_var1 $change Ver. 1.0 Session 2 Slide 3 of 26
  • 4. Java Programming LanguageBasic Data Types in Java Java programming language supports two basic data types: Primitive types Class types Ver. 1.0 Session 2 Slide 4 of 26
  • 5. Java Programming LanguagePrimitive Data Types Primitive data types are simple values, not objects. The Java programming language defines eight primitive data types, which can be considered in four categories: – Logical – boolean • Textual – char • Integral – byte, short, int, and long • Floating – double and float Ver. 1.0 Session 2 Slide 5 of 26
  • 6. Java Programming LanguageClass Data Type Class types are used for more complex types, including all the types that you declare yourself. They are used to create objects. Ver. 1.0 Session 2 Slide 6 of 26
  • 7. Java Programming LanguageVariables, Declarations, and Assignments Possible ways to declare and assign variables in Java are as follows: public class Assign { public static void main (String args[]) { // declare and assign values to int integer variables int x=6, y=1000; // declare and assign floating point float z = 3.414f; // declare and assign boolean boolean truth = true; // declare and assign String variable String str1 = "bye"; // declare and assign value to char variable char c = A; }} Ver. 1.0 Session 2 Slide 7 of 26
  • 8. Java Programming LanguageJava Reference Type In Java technology, beyond primitive types all other data types are reference types. A reference variable contains a handle to an object. For example: public class MyDate { private int day = 1; private int month = 1; private int year = 2000; public MyDate(int day, int month, int year) { …….... } public String toString() { ………. } } Ver. 1.0 Session 2 Slide 8 of 26
  • 9. Java Programming LanguageJava Reference Type (Contd.) public class TestMyDate { public static void main(String[] args) { MyDate today = new MyDate(22, 7, 1964); } } Variable today is a reference variable holding one object of MyDate class. Ver. 1.0 Session 2 Slide 9 of 26
  • 10. Java Programming LanguageConstructing and Initializing Objects • Calling new xyz() performs the following actions: Memory is allocated for the object Explicit attribute initialization is performed A constructor is executed The object reference is returned by the new operator The reference to the object is assigned to a variable For example: MyDate my_birth = new MyDate(22, 7, 1964); Ver. 1.0 Session 2 Slide 10 of 26
  • 11. Java Programming LanguagePass-By-Value In a single Java Virtual Machine, the Java programming language only passes arguments by value. When an object instance is passed as an argument to a method, the value of the argument is a reference to the object. The contents of the object can be changed in the called method, but the original object reference is never changed. Ver. 1.0 Session 2 Slide 11 of 26
  • 12. Java Programming LanguageThe this Reference • Two uses of the this keyword are: To resolve ambiguity between instance variables and parameters To pass the current object as a parameter to another method or constructor Ver. 1.0 Session 2 Slide 12 of 26
  • 13. Java Programming LanguageDemonstration Lets see how to define a reference type instance variable in a Java class, and manipulate the object referenced by this variable. Ver. 1.0 Session 2 Slide 13 of 26
  • 14. Java Programming LanguageJava Programming Language Coding Conventions Examples of coding conventions are: Packages: com.example.domain; Classes, interfaces, and enum types: SavingsAccount Methods: getAccount() Variables: CurrentCustomer Constants: HEAD_COUNT Ver. 1.0 Session 2 Slide 14 of 26
  • 15. Java Programming LanguageVariables and Scope • Variables defined inside a method are called local variables, also referred to as automatic, temporary, or stack variables. Local variables must be initialized before the first use. • Variables defined outside a method are created when the object is constructed using the new xxx() call. They are of two types: – Static variables: They are created when the class is loaded and continue to exist for as long as the class is loaded. – Instance variables: They are declared without using the static keyword. They continue to exist for as long as the object exists. Ver. 1.0 Session 2 Slide 15 of 26
  • 16. Java Programming LanguageLogical Operators The boolean operators are: ! NOT & AND | OR ^ XOR The short-circuit boolean operators are: && AND || OR You can use these operators as follows: MyDate d = reservation.getDepartureDate(); if ( (d != null) && (d.day > 31) { // do something with d } Ver. 1.0 Session 2 Slide 16 of 26
  • 17. Java Programming LanguageRight-Shift Operators >> and >>> The operator >> performs an arithmetic or signed right shift. The result of this shift is that the first operand is divided by 2 raised to the number of times specified by the second operand. The sign bit is copied during the shift. This operator is used for bit patterns. The sign bit is not copied during the shift. Examples are: 128 >> 1 returns 128/21 = 64 256 >> 4 returns 256/24 = 16 -256 >> 4 returns -256/24 = -16 Ver. 1.0 Session 2 Slide 17 of 26
  • 18. Java Programming LanguageLeft-Shift Operator << The operator << performs a left shift. The result of this shift is that the first operand is multiplied by 2 raised to the number specified by the second operand. For Example: 128 << 1 returns 128 * 21 = 256 16 << 2 returns 16 * 22 = 64 Ver. 1.0 Session 2 Slide 18 of 26
  • 19. Java Programming LanguageCasting • Casting means assigning a value of one type to a variable of another type. • If information might be lost in an assignment, the programmer must confirm the assignment with a cast. • The assignment between long and int requires an explicit cast. • Examples of casting are: long bigValue = 99L; int squashed = bigValue; // Wrong, needs a cast int squashed = (int) bigValue; // OK int squashed = 99L; // Wrong, needs a cast Ver. 1.0 Session 2 Slide 19 of 26
  • 20. Java Programming LanguagePromotion and Casting of Expressions • Variables are promoted automatically to a longer form (such as int to long). • Expression is assignment-compatible if the variable type is at least as large (the same number of bits) as the expression type. • For Example: long bigval = 6; // 6 is an int type, OK int smallval = 99L; // 99L is a long, illegal double z = 12.414F; // 12.414F is float, OK float z1 = 12.414; // 12.414 is double, illegal Ver. 1.0 Session 2 Slide 20 of 26
  • 21. Java Programming LanguageBranching Statement Conditional statements enable the selective execution of portions of the program according to the value of some expressions. Simple if, else Statements: The syntax is: if ( <boolean_expression> ) <statement_or_block> Complex if, else Statements: The syntax is: if ( <boolean_expression> ) <statement_or_block> else <statement_or_block> Ver. 1.0 Session 2 Slide 21 of 26
  • 22. Java Programming LanguageThe switch Statement • The switch statement: The syntax is: switch ( <expression> ) { case <constant1>: <statement_or_block>* [break;] case <constant2>: <statement_or_block>* [break;] default: <statement_or_block>* [break;]} • In the switch (<expression>) statement, <expression> must be expression compatible with an int type. Ver. 1.0 Session 2 Slide 22 of 26
  • 23. Java Programming LanguageLooping Statement Looping Statements enable you to execute block of statements repeatedly. Java programming language supports three types of loop constructs: – The for loop – The while loop – The do/while loop Ver. 1.0 Session 2 Slide 23 of 26
  • 24. Java Programming LanguageSpecial Loop Flow Control The following statements can be used to further control loop statements: – The break [<label>]; command – The continue [<label>]; command – The <label> : <statement> command, where <statement> should be a loop Ver. 1.0 Session 2 Slide 24 of 26
  • 25. Java Programming LanguageSummary In this session, you learned that: – An identifier is a name given to a variable, class, or method. An identifier can not be a keyword. – Java technology keywords identify a data type name or program construct name. – The Java programming language provides class and primitive data types. – An object can be constructed using the keyword new. – The Java programming language passes arguments only by value. – The this keyword is used to resolve ambiguity between instance variables and parameters. – There are two types of variables, primitive type and reference type. Ver. 1.0 Session 2 Slide 25 of 26
  • 26. Java Programming LanguageSummary (Contd.) Casting means assigning a value of one type to a variable of another type. Conditional statements enable the selective execution of portions of the program according to the value of some expressions: • If and switch statements help to perform two-way and multiple-way branching. • The for and while loops test the loop condition before executing the loop body. • The do loops check the loop condition after executing the loop body. • The labeled forms of break and continue help to further control the loop statements. Ver. 1.0 Session 2 Slide 26 of 26