Dynamic Dispatch

The process of assigning subclass object to superclass reference variable is called as dynamic dispatch.

Which of the following are valid?

A ob=new A();// Valid
A ob=new B();// Valid
A ob=new D();// Valid
B ob=new B();// Valid
B ob=new A();// Invalid
B ob=new C();// Invalid
B ob=new D();// Valid
D ob=new A();// Invalid
A ob=new E();// Valid

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3.10 Polymorphism

• Polymorphism (One name - Many forms).
• Polymorphism is the ability of an object to behave differently at different situations.

Types of Polymorphism

1. Static Polymorphism
2. Dynamic Polymorphism

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1) Static Polymorphism

Static polymorphism can be achieved using method overloading.

• It can be achieved with both instance and static methods.
• Static Polymorphism is also called as compile time polymorphism or early binding because Java compiler is responsible to bind the method calls with actual method at compile time.
• Java compiler will verify the method name first then method parameters to verify the matching method.
• Return type will not be considered.

class Arithmetic{
void sum(double d){ ... }

void sum(int a,boolean b){ ... }
}

Arithmetic ar=new Arithmetic();

1.ar.sum(10);
2.ar.sum(10,20);
3.ar.sum(10.0);

Case 1 : ar.sum10()

Compiler will do the following tasks:

• sum10() method will be verified in the class.
• sum10() method is not available, so compile time error will be given.

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Case 2 : ar.sum(10, 20)

Compiler will do the following tasks:

• sum() method will be verified in the class.
• sum() method is available.
• Now method parameters will be verified.
• Method with given parameters is not available. So compile time error will be given.

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Case 3 : ar.sum(10.0);

Compiler will do the following tasks:

• sum() method will be verified in the class.
• sum() method is available.
• Now method parameters will be verified.
• Method with given parameters is available.
• Compiler will bind this method call to actual matched method and it will go to next line.

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2) Dynamic Polymorphism

• Dynamic Polymorphism can be achieved using both method overriding and dynamic dispatch.
• It is also called as runtime polymorphism or late binding because JVM is responsible to decide which method will be invoked based on actual object available in reference variable.
• It can be achieved only with instance method, can't be achieved using static method.

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Person.java

class Person {

  void eating() {
    System.out.println("Person -> eating()");
  }

  void walking() {
    System.out.println("Person -> walking()");
  }

  static void sleeping() {
    System.out.println("Person -> sleeping()");
  }
}

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Employee.java

class Employee extends Person {

  void walking() {
    System.out.println("Employee -> walking()");
  }

  static void sleeping() {
    System.out.println("Employee -> sleeping()");
  }

  void working() {
    System.out.println("Employee -> working()");
  }
}

Student.java

class Student extends Person {

  void walking() {
    System.out.println("Student -> walking()");
  }

  static void sleeping() {
    System.out.println("Student -> sleeping()");
  }

  void reading() {
    System.out.println("Student -> reading()");
  }
}

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Lab518.java

class Lab518 {

  public static void main(String args[]) {
    Person pob = null;

    pob = new Student();
    pob.eating();

    pob = new Employee();
    pob.eating();
  }
}

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Lab519.java

class Lab519 {

  public static void main(String args[]) {
    Person pob = null;

    pob = new Student();
    pob.walking();

    pob = new Employee();
    pob.walking();
  }
}

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Lab520.java

class Lab520 {

  public static void main(String args[]) {
    Person pob = null;

    pob = new Student();
    pob.sleeping();

    pob = new Employee();
    pob.sleeping();
  }
}

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Lab521.java

class Lab521 {

  public static void main(String args[]) {
    Person pob = null;

    pob = new Student();
    pob.reading();

    pob = new Employee();
    pob.working();
  }
}

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Lab522.java

class Lab522 {

  public static void main(String args[]) {
    Person pob = null;

    pob = new Student();

    Student stu = (Student) pob;
    stu.reading();

    pob = new Employee();

    Employee emp = (Employee) pob;
    emp.working();
  }
}

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Lab523.java

class Lab523 {

  public static void main(String args[]) {
    A aobj = new B();

    System.out.println(aobj.x);
  }
}

class A {

  int x = 10;
}

class B extends A {

  String x = "JavaWorld";
}

Output

10

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Lab524.java

class Lab524 {

  public static void main(String args[]) {
    A aobj = new B();

    aobj.x = 99;

    System.out.println(aobj.x);
  }
}

class A {

  int x = 10;
}

class B extends A {

  String x = "JavaWorld";
}

Output

99

Lab525.java

class Lab525 {

  public static void main(String args[]) {
    B bobj = new B();
    bobj.show();
  }
}

class A {

  int x = 10;
}

class B extends A {

  String x = "JavaWorld";

  void show() {
    System.out.println(x);
    System.out.println(super.x);
  }
}

Output

JavaWorld
10

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Lab526.java

class Lab526 {

  public static void main(String args[]) {
    B bobj = new B();
    bobj.show();
  }
}

class A {

  int x = 10;
}

class B extends A {

  String x = "JavaWorld";

  void show() {
    B ref1 = this;
    A ref2 = this;

    System.out.println(ref1.x + "\t" + ref2.x);
  }
}

Output

JavaWorld    10

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Lab527.java

class Lab527 {

  public static void main(String args[]) {
    B bobj = new B();

    bobj.x = "SRI";

    A aobj = bobj;
    aobj.x = 88;

    bobj.show();
  }
}

class A {

  int x = 10;
}

class B extends A {

  String x = "JavaWorld";

  void show() {
    System.out.println(x);
    System.out.println(super.x);
  }
}

Output

SRI
88

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