Difference between List< ? extends MyParentClass> and List< MyParentClass >

Difference between List< ? extends MyParentClass> and List< MyParentClass >

First look into the following code that is quite self explanatory.

 

  public static void main(String[] args) {

    List<Serializable> serializables = new ArrayList<Serializable>();

    serializables.add(new SomeClass1());

    serializables.add(new SomeClass2());

    process(serializables);

    process2(serializables);//no issues

    List<SomeClass1> SomeClassList = new ArrayList<SomeClass1>();

    SomeClassList.add(new SomeClass1());

    SomeClassList.add(new SomeClass1());

    process(SomeClassList);

    process2(SomeClassList);//compile time error in this line

}

private static void process(List< ? extends Serializable> list){

    Iterator< ? extends Serializable> itr = list.iterator();

    while (itr.hasNext()){

        Serializable s = itr.next();

        System.out.println("process1: " + s.getClass());

    }

}    

private static void process2(List<Serializable> list){

    Iterator<Serializable> itr = list.iterator();

    while (itr.hasNext()){

        Serializable s = itr.next();

        System.out.println("process2: " + s.getClass());

    }

}

public class SomeClass1 implements Serializable {

    private static final long serialVersionUID = 1L;

    private int a;

    private int b;

/*Getters and setters here*/

}

/***Here is my SomeClass2 that I will add it to List***/

import java.io.Serializable;

public class SomeClass2 implements Serializable {

    private static final long serialVersionUID = 1L;

    private String s;

/*Getters and setters here*/

}

Output is

process1: SomeClass1

process1: SomeClass2

process2: SomeClass1

process2: SomeClass2

process1: SomeClass1

process1: SomeClass1

Extract:

• List< MyParentClass> aList;

We can add any object that belongs to MyParentClass or below its hierarchy. But if there is any other List say List, we CAN NOT assign it to aList.

• List< ? extends MyParentClass> bList;

Here if there is any other List say List, we CAN assign it to bList.

Thanks

Manish Kumar Agarwal

Manish.java@outlook.com