java session02

8/8/2019 JAVA Session02

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Slide 1 of 26Session 2Ver. 1.0

Java Programming Language

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 variableDeclare variables of class type

Construct an object using new

Describe default initialization

Distinguish between instance and local variables

Objectives


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Objectives (Contd.)

Describe how to initialize instance variables

Identify boolean expressions and their requirements in controlconstructs

Recognize assignment compatibility and required casts in

fundamental typesUse if, switch, for, while, and do constructions and the labeledforms of break and continue as flow control structures in aprogram


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Identifier

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 lengthExamples of valid identifiers:

identifier

userName

user_name

_sys_var1$change


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Basic Data Types in Java

Java programming language supports two basic data types:

Primitive types

Class types


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Primitive data types are simple values, not objects.

The Java programming language defines eight primitivedata types, which can be considered in four categories:

Logical boolean

Textual char

Integral byte, short, int, and long

Floating double and float

Primitive Data Types


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Class Data Type

Class types are used for more complex types, including allthe types that you declare yourself.

They are used to create objects.


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Variables, Declarations, and Assignments

Possible ways to declare and assign variables in Java areas 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 pointfloat z = 3.414f;// declare and assign boolean

boolean truth = true;// declare and assign String variableString str1 = "bye";// declare and assign value to char variablechar c = 'A'; }}


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Java Reference Type

In Java technology, beyond primitive types all other datatypes 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;

publ

ic MyDate(int day, int mo

nth, intyear) { .... }

public String toString() { . }

}


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Java 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 ofMyDate class.


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Constructing 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 operatorThe reference to the object is assigned to a variable

For example:

MyDate my_birth = new MyDate(22, 7, 1964);


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Pass-By-Value

In a single Java Virtual Machine, the Java programminglanguage only passes arguments by value.

When an object instance is passed as an argument to amethod, the value of the argument is a reference to the

object.The contents of the object can be changed in the calledmethod, but the original object reference is never changed.


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The this Reference

Two uses of the this keyword are:

To resolve ambiguity between instance variables andparameters

To pass the current object as a parameter to another method

or constructor


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Lets see how to define a reference type instance variable in aJava class, and manipulate the object referenced by thisvariable.

Demonstration


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Java 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


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Variables 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 oftwo types:

Static variables: They are created when the class is loadedand continue to exist for as long as the class is loaded.

Instance variables: They are declared without using thestatic keyword. They continue to exist for as long as the

object exists.


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Logical 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

so

mething with d}


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Right-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 2raised to the number of times specified by the secondoperand.

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


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Left-ShiftOperator


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Casting

Casting means assigning a value of one type to a variable ofanother type.

If information might be lost in an assignment, theprogrammer must confirm the assignment with a cast.

The assignment between long and int requires an explicitcast.

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


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Promotion and Casting of Expressions

Variables are promoted automatically to a longer form (suchas int to long).

Expression is assignment-compatible if the variable type isat 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


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Branching Statement

Conditional statements enable the selective execution ofportions of the program according to the value of someexpressions.

Simple if, else Statements:

The syntax is:if ( )

Complex if, else Statements:

The syntax is:

if ( )

else


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The switch Statement

The switch statement:

The syntax is:switch ( ) {

case :

*[break;]

case :

*

[break;]

default:

*[break;]}

In the switch () statement, must be expression compatible with anint type.


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Looping Statement

Looping Statements enable you to execute block ofstatements repeatedly.

Java programming language supports three types of loopconstructs:

The for loopThe while loop

The do/while loop


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Special Loop Flow Control

The following statements can be used to further control loopstatements:

The break []; command

The continue []; command

The : command, where should be a loop


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Summary

In this session, you learned that:

An identifier is a name given to a variable, class, or method. Anidentifier can not be a keyword.

Java technology keywords identify a data type name or program

construct name.The Java programming language provides class andprimitive data types.

An object can be constructed using the keyword new.

The Java programming language passes arguments only byvalue.

The this keyword is used to resolve ambiguity betweeninstance variables and parameters.

There are two types of variables, primitive type and referencetype.


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Summary (Contd.)

Casting means assigning a value of one type to a variable ofanother type.

Conditional statements enable the selective execution ofportions of the program according to the value of someexpressions:

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 ofbreak and continue help to furthercontrol the loop statements.

java session02

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