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Program: How implement bounded types (extend superclass) with generics?


As of now we have seen examples for only one type parameter. What happens in case we want to access group of objects comes from same family, means extending same super class? You can restrict the generics type parameter to a certain group of objects which extends same super class. You can achieve this my specifying extends <super-class> at class definitions, look at the example, it gives you more comments to understand.


package com.java2novice.generics;

public class MyBoundedClassEx {

	public static void main(String a[]){
		//Creating object of sub class C and 
		//passing it to BoundEx as a type parameter.
		BoundEx<C> bec = new BoundEx<C>(new C());
		bec.doRunTest();
		//Creating object of sub class B and 
		//passing it to BoundEx as a type parameter.
		BoundEx<B> beb = new BoundEx<B>(new B());
		beb.doRunTest();
		//similarly passing super class A
		BoundEx<A> bea = new BoundEx<A>(new A());
		bea.doRunTest();
		//If you uncomment below code it will throw compiler error
		//becasue we restricted to only of type A and its sub classes.
		//BoundEx<String> bes = new BoundEx<String>(new String());
		//bea.doRunTest();
	}
}
/**
 * This class only accepts type parametes as any class
 * which extends class A or class A itself.
 * Passing any other type will cause compiler time error
 */
class BoundEx<T extends A>{
	
	private T objRef;
	
	public BoundEx(T obj){
		this.objRef = obj;
	}
	
	public void doRunTest(){
		this.objRef.printClass();
	}
}

class A{
	public void printClass(){
		System.out.println("I am in super class A");
	}
}

class B extends A{
	public void printClass(){
		System.out.println("I am in sub class B");
	}
}

class C extends A{
	public void printClass(){
		System.out.println("I am in sub class C");
	}
}

Output:
I am in sub class C
I am in sub class B
I am in super class A
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Java Generics Sample Code Examples

  1. Write a simple generics class example.
  2. Write a simple generics class example with two type parameters.
  3. How implement bounded types (extend superclass) with generics?
  4. How implement bounded types (implements an interface) with generics?
  5. What is generics wildcard arguments? Give an example.
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Object oriented programming organizes a program around its data, i.e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.
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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java is registered trademark of Oracle.
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