Java keywords

Programming in Java
Lecture 9: This, Super & Final Keywords
By
Ravi Kant Sahu
Asst. Professor, LPU

Contents
• Object class
• super keyword
• final keyword
• final class
• static keyword
• this keyword
Ravi Kant Sahu, Asst. Professor @ Lovely Professional University, Punjab (India)

Object Class
 There is one special class, called Object, defined by Java.
 All other classes are subclasses of Object.
 A reference variable of type Object can refer to an object of any other
class.
 Arrays are implemented as classes, so a variable of type Object can also
refer to any array.
 In Object class, getClass( ), notify( ), notifyAll( ), and wait( ) are
declared as final.

Methods in Object class

‘final’ Keyword
• ‘final’ keyword is used to:
– declare variables that can be assigned value only once.
final type identifier = expression;
– prevent overriding of a method.
final return_type methodName (arguments if any)
{
body;
}
– prevent inheritance from a class.
final class Class_Name
{
class body;
}

‘super’ Keyword
• ‘super’ keyword is used to:
– invoke the super-class constructor from the constructor
of a sub-class.
super (arguments if any);
– invoke the method of super-class on current object
from sub-class.
super.methodName (arguments if any);
– refer the super-class data member in case of name-
conflict between super and sub-class data members.
super.memberName;

‘static’ Keyword
• used to represent class members
• Variables can be declared with the “static” keyword.
static int y = 0;
• When a variable is declared with the keyword “static”, its called
a “class variable”.
• All instances share the same copy of the variable.
• A class variable can be accessed directly with the class, without
the need to create a instance.
• Note: There's no such thing as static classs. “static” in front of
class creates compilation error.

static methods
• Methods can also be declared with the keyword “static”.
• When a method is declared static, it can be used without creating an
object.
class T2 {
static int triple (int n)
{return 3*n;}
}
class T1 {
public static void main(String[] arg)
{
System.out.println( T2.triple(4) );
T2 x1 = new T2();
System.out.println( x1.triple(5) );
}
}

• Methods declared with “static” keyword are called “class
methods”.
• Otherwise they are “instance methods”.
• Static Methods Cannot Access Non-Static Variables.
• The following gives a compilation error, unless x is also static.
class T2 {
int x = 3;
static int returnIt () { return x;}
}
class T1 {
public static void main(String[] arg) {
System.out.println( T2.returnIt() ); }
}

‘this’ Keyword
• 'this' is used for pointing the current class instance.
• Within an instance method or a constructor, this is a reference to
the current object — the object whose method or constructor is
being called.

class ThisDemo1{
int a = 0;
int b = 0;
ThisDemo1(int x, int y)
{
this.a = x;
this.b = y;
}
public static void main(String [] args)
{
ThisDemo1 td = new ThisDemo1(10,12);
ThisDemo1 td1 = new ThisDemo1(100,23);
System.out.println(td.a);
System.out.println(td.B);
System.out.println(td1.a);
System.out.println(td1.B);
}
}

Chaining of constructors using this keyword
• Chaining of constructor means calling one constructor
from other constructor.
• We can invoke the constructor of same class using
‘this()’ keyword.
• We can invoke the super class constructor from
subclass constructor using ‘super ()’

class ThisDemo{
public ThisDemo() {
this(10); System.out.println("First Constructor");
}
public ThisDemo(int a) // overloaded constructor
{
this(10,20); System.out.println("Second Constructor");
}
public ThisDemo( int a, int B) // another overloaded constructor
{
this(“Ravi Kant"); System.out.println("Third Constructor");
}
public ThisDemo(String s) // and still another
{
System.out.println("Fourth Constructor");
}
public static void main(String args[]) {
ThisDemo first = new ThisDemo();
}
}

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