转换ArrayList <String> 到ArrayList <Object>

Question

If type B can be generalised to type A , how do I convert a List<B> to a List<A> ? Do I need a loop like:

List<A> newList=new ArrayList<A>();
for(B t:oldList) newList.add(t);

Edit: I don't know the word for it, but I mean to say an object of type B can be casted to an object of type A (like B implements A ).

Edit2: It appears that the constructor also works:

List<A> newList=new ArrayList<A>(oldList);

However, I wanted a more efficient approach rather than just copying the list.

Answer 1

You shouldn't since List<B> means that it will contain only elements of type B or subtypes. List<A> would allow you to add elements of type A as well, which might be dangerous. Hence you shouldn't cast a List<B> into a List<A> .

If you create a new List<A> and put all elements of List<B> into it (eg using a loop possibly wrapped in a method) it's ok.

One simple way to fill a new list would be the addAll() method, eg

List<A> aList = ...;
List<B> bList = ...;

aList.addAll( bList );

Answer 2

As you stated in comments , If your Class B implements Class A , the below code is perfectly alight.

newList.addAll(listofBs);

Since every B is an A , you are allowed to do that.

And yes you need not to have a loop.

Answer 3

Generics does not allow you to directly up cast collection objects.

You can do couple of things:

List<B> oldList = new List<B>();
List<? super B> upcastedList = oldList; // No new object created.

If you need to cast the list to a specific type (say A), then you have no option than to loop every element and add it to new list (as you have specified in the question).

Answer 4

它可以在创建过程中添加

List<A> newList = new ArrayList<A>(oldList);

Answer 5

If you are OK with listA being unmodifiable, here's another possible answer (using Ankur's hint):

List<B> listB=new ArrayList<B>();
List<A> listA=Collections.unmodifiableList((List<? extends A>)listB);