Java tutorial part 2

Polymorphism
• Polymorphism is the ability of an object to take on
many forms.
• The most common use of polymorphism in OOP
occurs when a parent class reference is used to
refer to a child class object.
• Any Java object that can pass more than one IS-A
test is considered to be polymorphic.
• In Java, all Java objects are polymorphic since any
object will pass the IS-A test for their own type and
for the class Object.

Polymorphism
• It is important to know that the only possible way to
access an object is through a reference variable.
• A reference variable can be of only one type.
• Once declared, the type of a reference variable
cannot be changed.
• The reference variable can be reassigned to other
objects provided that it is not declared final.
• The type of the reference variable would determine
the methods that it can invoke on the object.

Polymorphism
• A reference variable can refer to any object of its
declared type or any subtype of its declared type.
• A reference variable can be declared as a class or
interface type.
refer example from next slide.

Method Overriding Cont..
1. Sub class can override the methods defined by the super
class.
2. Overridden Methods in the sub classes should have same
name, same signature , same return type and may have
either the same or higher scope than super class method.
3. Java implements Run Time Polymorphism/ Dynamic Method
Dispatch by Method Overriding. [Late Binding]
4. Call to Overridden Methods is Resolved at Run Time.
5. Call to a overridden method is not decided by the type of
reference variable Rather by the type of the object where
reference variable is pointing.
6. While Overriding a Method, the sub class should assign
either same or higher access level than super class method.

EXAMPLE METHOD OVERRIDING
class A
{
void show()
{
System.out.println("Hello This is show() in A");
}// End of show() Method
B class overrides show() method from
} // End of class A
super class A
class B extends A
{
void show()
{
System.out.println("Hello This is show() in B");
}// End of show() Method
} // End of class B class override
{
public static void main(String args[])
Call to show()
{
of A class
// super class reference variable
// can point to sub class object
A a1 = new A();
a1.show();
Call to show()
a1 = new B();
of B class
a1.show();
}

Is this Method

class A
Overriding
{
void show(int a)
NO
{
System.out.println("Hello This is show()
in A");
}
}
class B extends A
{
void show()
{
System.out.println("Hello This is show()
in B");
}
}
OUTPUT
Hello This is show() in A
Hello This is show() in B

class override1
{
{
/*
A a1 = new B();
a1.show(); */
A a1 = new A();
a1.show(10);
B b1 = new B();
b1.show(10);
b1.show(); }

Dynamic Method Dispatch
1. Super class reference variable can refer to a sub class object.
2. Super class variable if refers to sub class object can call only
overridden methods.

3. Call to an overridden method is decided by the type of object
referred to.
A a1 = new B();
a1.show(); // call to show() of B
a1 = new C();
a1.show(); // call to show() of C
a1 = new D();
a1.show(); // call to show() of D

A

B

C

Assume show() Method is
Overridden by sub classes

D

DYNAMIC METHOD DISPATCH
class A
{
void show()
{
System.out.println("Hello This is
show() in A");
}
}
class B extends A
{
void show()
{
System.out.println("Hello This is
show() in B");
}
}

class C extends A
{
void show()
{
System.out.println("Hello This
is show() in C");
}
}
class D extends A
{
void show()
{
System.out.println("Hello This
is show() in D");
}
}
CONTINUED…..

class override2
{
public static void main(String
args[])
{
A a1 = new A();
Hello This is show() in B
a1.show();
a1 = new B();
Hello This is show() in C
a1.show();
Hello This is show() in D
a1 = new C();
a1.show();
a1 = new D();
a1.show();
}
}

class override3
{
{
A a1 = new B();
B b1 = (B) a1;
/*
A a1 = new B();
C c1 = (C) a1;
Exception in thread "main"
java.lang.ClassCastException: B
at override3.main(override3.java:39)
*/
}
}

Examples Overriding
class A
{
void show() { …. }
}
class B extends A
{
void show() { …. }
void show(int x) { … }
void print() { … }
}

A a1 = new B();
a1.show() ;
// Valid
// a1.show(10); // Invalid
//a1.print();
// Invalid

When a super class variable points to a sub class
object, then it can only call overridden methods of the sub
class.

class A
{
protected void show()
{
System.out.println("Hi");
}
}
class B extends A
{
void show()
{
}
}

D:Java1>javac AB.java
AB.java:10: show() in B cannot
override show() in A; attempting
to assign weaker
access privileges; was protected
void show()
^
1 error

class A
{
private void show()
{
}
}
class B extends A
{
int show()
{
return 10;
}
}

IS THIS METHOD
OVERRIDING

NO
CODE WILL COMPILE
& RUN SUCESSFULLY

class A
{
static int show()
{
System.out.println("class A");
return 0;
}
}
class B extends A
{
void show()
{
System.out.println("class B");
}
}

What’s Wrong Here

sample.java:12: show() in B
cannot override show() in A;
overridden method is static
void show()
^
1 error

Nested Classes

Java programming language allows you to
define a class within another class
Enclosing
class OuterClass
Class OR
{ ...
Outer Class
A nested class is a member
class NestedClass { ... of its enclosing class
}

}

Nested
Class
1.Nested has access to other members of the
enclosing class,even if they are declared private
2. Can be private, public, protected or friendly
access

Nested Class Types
Static nested classes
1. Static keyword applied for class declaration
2. Static nested class can use the instance
fields/methods of the outer class only through object
reference.
3. Static nested class can be accessed

OuterClass.StaticNestedClass
4. To create an object for the static nested class, use this
syntax:
OuterClass.StaticNestedClass nestedObject = new
OuterClass.StaticNestedClass();

Nested Class Types cont..
•

Non-Static nested classes

1. These nested classes do not have static keyword applied
2. Non-Static nested class can use the instance fields/methods of
the outer class directly.
3. To create an object for the non-static nested class, use this
syntax:

OuterClass.NestedClass nestedObject = Outerobjectreference.
new innerclass();
Inner class instance can
only exists inside
Outer class instance.

Example 1 [Non-static Nested Class]
class B
class A
{
{
int b;
private int a; Outer Class
Nested
B(int b)
A(int a)
class with
{
{
friendly
int c = b+10; access
this.a =a;
this.b = c;
}
}
void print()
void show()
{
{
System.out.println("a="+a)
print();
;
System.out.println("b="+b);
}
Call to
}
print() of
} // End of class B
outer class
} // End of class A

Example 1 [Non-static Nested Class]
class innertest1
{
{
A a1 = new A(10);
A.B b1 = a1.new B(100);
b1.show();
}
}

cont….

Inner class Name

Outer class Reference

To create an inner class instance for nonstatic classes you need an outer class
reference.
Inner class Reference
Outer class Name

If class B is Private then it is not visible in main().
A.B b1 = a1.new B(100); is WRONG/INVALID

class A
{
private int a;
private int b=10;
A(int a)
{
this.a=a;
}

Local b
B’s instance Field b
A’s instance Field b
void show()
{
B b1 = new B(30);
b1.show();
}
} // End of Outer class A

Outer class
Nested Inner class [Nonstatic Type]

class B
{
private int b;
Instance Field of B
B(int b)
{
this.b =b;
}
Outer Class A’s a
void show()
{
int b=20;
System.out.println("a="+a);
System.out.println("this.b="+this.b);
System.out.println("Outer b="+A.this.b);
}
} // End of B inner class

class innerTest
{
{
// Create an inner class B's instance
// Call show() method
// STEP 1
// Create an Outer Instance first

A a1 = new A(20);
A.B b1 = a1.new B(-30);
b1.show();

a=20
b=20
this.b=-30
Outer b=10

// inner class object instantiation thru anonymous
outer
// reference
A.B b2 = new A(30).new B(-40);
a=30
b2.show();
b=20
}
this.b=-40
}
Outer b=10

Static Inner class / Static Nested class Example
class A
{
private int a;
A(int a)
{
this.a =a;
}
void print()
{
System.out.println("a="+a
);
}
Static nested class can
refere to outer members
only through outer
reference

static class B
{
int b;
B(int b)
Static inner
{
class
int c = b+10;
this.b = c;
}
void show()
{
// print(); INVALID
A a1 = new A(10);
a1.print();
}
} // End of class B
} // End of class A

Example cont….

class innertest10
{
{
A.B b1 = new A.B(100);
b1.show();
Instance of static Inner
}
class
}

Static Nested class Example 2
class A
{
private int a;
protected static int b=10;
A(int a)
{
this.a=a;
}
public void show()
{
display();
}
public static void display()
{
}

Example 2 cont….
static class B
{
private int a;
B(int a)
{
this.a=a;
}
void show()
{
// A.this.show(); // Won't work show() is non-static in outer
display(); // Will work as method is static in outer
// System.out.println("a="+A.this.a);
// Won't work a is non-static in outer
System.out.println("b="+b); // Will refer to its own b
System.out.println("A'sb="+A.b); // will refer to outer class B
new A(40).show();
// This is how you can call non static methods of outer
}
} // End of inner class B
} // End of class A

Example 2 cont….
class innerTest1
{
public static void main(String
args[])
{
A.B b1 = new A.B(-30);
D:jdk1.3bin>java innerTest1
b1.show();
}
b=10
}
a=-30
b=100
A'sb=10
a=40
b=10

Local Inner classes [ Classes Within method body]

class A
{
private int a;
A(int a)
{
this.a=a;
1.
}
void show()
2.
{
class B
{}
3.
}
}
4.

Class declared within a
method body.
Here method is show()
Local inner classes Can not
be declared as
public,private or protected

Class B is visible only in method
show().
It can be used within this show()
method only
Local inner classes can only use final
variables from its enclosing method.
However inner classes can refer to its
fields of enclosing class.

class A
{
private int a;
A(int a)
{
this.a=a;
}
void show()
{
int x=10;
class B
{
private int b;
B(int b)
{
this.b=b;
}
void display()
{

System.out.println("x="+x);
}

} // End of class B
} // End of show() method
} // End of A class

D:jdk1.3bin>javac
innerTest2.java
innerTest2.java:23: local
variable x is accessed from
within inner class;
to be declared final
System.out.println("x="+x);
^
1 error

Reference for A’s a
Reference for B’s b
Reference is wrong / errorneous
‘x’ is local variable inside the local
method. Local classes can use only
final fields from enclosing method

class innertest
{
public static void
main(String args[])
{
final int a1=10;
new A(20).show();
print();
}// End of main
static void print()
{

/*
A a1 = new A(30);
a1.show();
*/

System.out.println("Hello"
);
}
}

class A
{
private int a;
private int b;
int c;
A(int a)
{
this.a =a;
b = a+20;
c = a+40;
}
void show()
{
System.out.println("a1="+a1);
System.out.println("c="+c);
}
} //End of A

OUTPUT

E:oop>java innertest
a1=10
a=20
b=40
c=60
Hello

Anonymous Inner classes
• Another category of local inner classes
• Classes without any name i.e classes having
no name
• Can either implements an interface or
extends a class.
• Can not have more than one instance active
at a time.
• Whole body of the class is declared in a single
statement ending with ;

Cont…
• Syntax [ If extending a class]
[variable_type_superclass =] new superclass_name() {
// properties and methods
} [;]
• Syntax [ If implementing an interface]
[variable_type_reference =] new reference_name() {
// properties and methods
} [;]

Anonymous Inner Class Example
class A
{
private int a;
A(int a)
{
this.a =a;
}
void show()
{
} // End of show()
}// End of class A

class innertest1
{
{

Anonymous inner class extending super class A

A a1 = new A(20){
public void show()
{
super.show();
System.out.println("Hello");
}
public void display()
{
}
};
a1.show();
// a1.display();
Calling show from inner class
}
}

interface X
{
int sum(int a,int b);
int mul(int x,int y);
}
class innertest2
{
{
Anonymous inner class implementing an interface

X x1 = new X()
{
public int sum(int a,int b)
{
return a+b;
}
public int mul(int a,int b)
{
return a*b;
}
};
System.out.println(x1.sum(10,20));
System.out.println(x1.mul(10,20));
}// End of main
}// End of innertest2

Java tutorial part 2

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Java tutorial part 2