Some concepts

Lecture 5

Object Oriented ProgrammingEastern University, Dhaka

Md. Raihan Kibria

Primitive types

public class PrimitiveDemo {

int i;//integer long l;//is longer than integer float fl;//float double dbl;//double; has more precision than float char[] charArray;//char array int[] intArray;//integer array boolean b;//true or false

public static void main(String[] args) { PrimitiveDemo demo = new PrimitiveDemo(); demo.charArray = new char[]{'E', 'A', 'S', 'T', 'E', 'R', 'N'}; for (int i=0; i<demo.charArray.length; i++) System.out.print(demo.charArray[i]); }}

Almost like the primitives in C:

Prints: EASTERN

Boxed typesA primitive like integer has a Boxed

equivalent:

Primitive Boxed

integer Integer

long Long

double Double

float Float

boolean Boolean

Integer I = null;//is possible because it is an objectBut int i = null;//is illegal because i is not an object

Type conversionpublic class TypeConversion {

public static void main(String[] args) { long longval = 234234234L; int intval = (int)longval; }}

Strings

Strings are objects in java. An object of String is created in the following:

public class StringDemo {

public static void main(String[] args) { String name1 = "john"; System.out.println(name1); }}

public class StringDemo { public static void main(String[] args) { String name1 = "john"; name1 = name1 + " kerry"; System.out.println(name1); }}

Strings can be concatenated:

Prints: john kerry

Methods

public class MethodDemo {

static void doSomething(){//static method System.out.println("hello"); }

void doALittle(){//instance method System.out.println("a text"); }

public static void main(String[] args) { //static method can be called without creating instance MethodDemo.doSomething();

//instance method must be called only on the instance MethodDemo demo = new MethodDemo(); demo.doALittle(); }}

Constructor

Constructor: as soon as an object is instantiated, constructor is called. Constructor is a method with the same name as the class. But there are no type modifiers (void, int, etc.) to the left of a constructor.

public class ConstructorDemo {

int i; public ConstructorDemo(){ i = 9; }

public void printValue(){ System.out.println("Value is: " + i); }

public static void main(String[] args) { new ConstructorDemo().printValue(); }}

Prints: value is: 9

public, protected, private modifiers

public definitions are accessible outside of package

protected definitions are accessible inside the package

private definitions are accessible only within the class

Example of public/privatepackage com;

public class A { public void doSomething(){ doSomethingPrivately(); }

private void doSomethingPrivately(){

}

public static void main(String[] args) {

}}

package net;

import com.A;

public class B {

public static void main(String[] args) { A a = new A(); a.doSomething(); }}

doSomethigPrivately() of A is not accessible from B