Where can we use ArrayList <? extends My_Class>

Question

Where we can ArrayList<? extends My_Class>, because it will not allow you to add any new item if you declare it as follows:

ArrayList<? extends My_Class> obj=new ArrayList<? extends My_Class>();
list.add(new Derived_My_Class()); //this is compilation error

+5

user1554627 Jul 26 '12 at 12:28

3 answers

Bozho · Answer 1 · 2012-07-26T12:30:14+0000

A commonly used abbreviation for describing these keywords: PECS: producer extends - consumer super.

So, if you use "extends", your collection produces elements of this type. So you cannot add to it, you can only take it.

A usage example would be to provide the client with your list API:

public List<? extends CharSequence> getFoo() {
    List<String> list = new ArrayList<String>();
    list.add("foo");
    return list;
}

dasblinkenlight · Answer 2 · 2012-07-26T12:32:25+0000

, ArrayList , : , .

My_Class ArrayList<My_Class>; My_Class DerivedClass .

Ahmad · Answer 3 · 2012-07-26T12:33:45+0000

... ArrayList , ,

ArrayList <Supertype> obj = new ArrayList <Supertype> ();

, , ( , ).

public getAnimals(List< ? extends Animal> obj){

obj.add(something);    //not allowed

}

, , .. (). classCastException .

That is why this is not allowed in this case. read Josh Bloch's Effective Java. he explained it well with the consumer goods industry analogy (PECS)