Java Generic with ArrayList <? extends A> add element

Question

I have classes A, B, C and D where B extends A, C extends A and

Answer 1

Look at this example and choose as per your requirement

package com.generic;

import java.util.ArrayList;

public class SuperExtendTest {

    /**
     * @param args
     */
    public static void main(String[] args) {

    }
    public void test() throws Exception {
        ArrayList parents=new ArrayList<>();
        parents.add(new Parent());
        parents.add(new Child());
        //parents.add(new GrandParent()); Not allowed

        ArrayList p=new ArrayList<>();
        ArrayList c=new ArrayList<>();
        ArrayList gp=new ArrayList<>();

        testExtendP(p);
        //testExtendP(c); Not allowed only super type allowed no child
        testExtendP(gp);

        testExtends(c);
        testExtends(p);
        //testExtends(gp); Not allowed because GrantParent does not extends Parent
    }

    /**
     * This Method allowed get operations for Parent 
     * No add 
     * 
     */
    public void testExtends(ArrayList list) throws Exception {
    /*  list.add(new Child());
        list.add(new Parent());
        list.add(new GrandParent());
        // can not add
        */
        Child c=(Child) list.get(0);
        Parent parent=list.get(0);
        GrandParent gp=list.get(0);
        /**
         * Unsafe collection way
         */
        ArrayList list2=new ArrayList();
        list.addAll(list2);
    }

    /**
     * This Method allowed add operations for Parent and it's sub types and on get it gives Object type 
     * 
     */
    public void testExtendP(ArrayList list) throws Exception {
        list.add(new Child());
        list.add(new Parent());
        //list.add(new GrandParent());
        /*can not add because GrandParent can not be converted to Parent type
         * 
         * The method add(capture#3-of ? super Parent) in the type ArrayList is not applicable for the arguments (GrandParent)
         * 
         */

        Child c=(Child) list.get(0);
        Parent parent=(Parent) list.get(0);
        GrandParent gp=(GrandParent) list.get(0);
        Object obj=list.get(0);

        /**
         * Unsafe collection way
         */
        ArrayList list2=new ArrayList();
        list.addAll(list2);
    }

    /**
     * This Method allowed all operations for Parent and it's sub types 
     * 
     */
    public void testDirect(ArrayList list) throws Exception {
        list.add(new Child());
        list.add(new Parent());
        //list.add(new GrandParent()); 
        /*
         * Can not add GrandParent (Normal generic rule)
         */
    }

}
class GrandParent{

}

class Parent extends GrandParent{

}
class Child extends Parent{

}