CODEWITHSUNDEEP
javainheritancearraylist
Mars
Mars

Reputation: 936

2d arraylist and class inheritance in java

assume I have Class A as the parent of Class B and Class C

I initialized two ArrayList<B> bList and ArrayList<C> cList. Then I have another ArrayList<ArrayList<A>> bigList.

I want to store bList and cList into that bigList by bigList.add(bList) and bigList.add(cList), and this gives me an error. Anyone have suggestion of how I should fix this or another way of doing it?

Thank you

Upvotes: 0

Views: 539

Answers (3)

DNA
DNA

Reputation: 42597

To create a list of lists in this situation, you need to use <? extends A> to allow subclasses of A, not just A itself.

    List<B> blist = new ArrayList<B>();
    List<C> clist = new ArrayList<C>();

    List<List<? extends A>> listofalist = new ArrayList<List<? extends A>>();

    listofalist.add(blist);
    listofalist.add(clist);

If instead you want to copy the contents of the B and C lists into the A list, then you need to use ArrayList.addAll(), not add().

add(Object o) is for adding individual objects to a list.

addAll(Collection c) is for adding all the contents of one list into another list (or other Collection).

See the javadoc for ArrayList.

Upvotes: 0

Louis Wasserman
Louis Wasserman

Reputation: 198033

bigList should be ArrayList<ArrayList<? extends A>> to allow this to work.

Upvotes: 3

wvm2008
wvm2008

Reputation: 3034

Simply based on what you just wrote, your problem could be that you're trying to this:

bigList.add(aList);

When you mean to do this:

bigList.add(bList);

Upvotes: 0

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