Unsure how to write method in Java like template in c

Question

I want to implement a function with signature

static boolean checkAnyOutOfBoundary(int[] index,ArrayList<ArrayList<T>> list)

Where I want to substitute T with other self-created class(DocScore). I tried to substitute T with Object. But when I try to instantiate the list instance, like

ArrayList<ArrayList<Object>> DocScoreList = new ArrayList<ArrayList<Object>>();
for (int i = 0; i < index.length; i++)
DocScoreList.add(root.children.get(i).docList);

The type of root.children.get(i).docList is ArrayList<DocScore>

I can not add element into DocScoreList. Does anybody know how can I implement this?

Answer 1

So you are adding an ArrayList<DocScore> to a ArrayList<ArrayList<Object>> but the add method expects an ArrayList<Object>.

Can you provide an ArrayList<DocStore> where ArrayList<Object> is required? No, because A bag of banana is not a bag of fruit

So DocScoreList has to be an ArrayList of something to which a ArrayList<DocStore> can be added. Some examples are:

• ArrayList<DocStore> actual type
• List<DocStore> because ArrayList<T> is a List<T>
• ArrayList<? extends Object> a list of some unknown type that (directly or indirectly) extends Object. Since DocStore meets the criteria ? extends Object this will work.
• List<? extends Object>
• etc. You get the idea.

Answer 2

static <T> boolean checkAnyOutOfBoundary(int[] index,ArrayList<ArrayList<T>> list)

Example of calling it:

boolean b = checkAnyOutOfBoundary( new int[]{0,1}, new ArrayList<ArrayList<String>>());

Answer 3

Because even in the case that Bar is a subclass of Foo, an ArryList is not a superclass of ArrayList.

Change

ArrayList<ArrayList<Object>> DocScoreList = new ArrayList<ArrayList<Object>>();

to

ArrayList<ArrayList<? extends Object>> DocScoreList = new ArrayList<ArrayList<? extends Object>>();