List class's toArray in Java- Why can't I

问题:

I defined List<Integer> stack = new ArrayList<Integer>();

When I'm trying to convert it to an array in the following way:

Integer[] array= stack.toArray();

I get this exception:

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
Type mismatch: cannot convert from Object[] to Integer[].

Why? It is exactly the same type- Integer to Integer. It's not like in this generic case when the classes are father-and-son relation

I tried to do casting:

    Integer[] array= (Integer[]) stack.toArray();

But here I get this error:

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;

What is the problem?

回答1:

Because of type erasure, the ArrayList does not know its generic type at runtime, so it can only give you the most general Object[]. You need to use the other toArray method which allows you to specify the type of the array that you want.

Integer[] array= stack.toArray(new Integer[stack.size()]);

回答2:

The way to do it is this:

Integer[] array = stack.toArray(new Integer[stack.size()]);

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For the record, the reason that your code doesn't compile is not just type erasure. The problem is that List<T>.toArray() returns an Object[] and it has done this before generics were introduced.

回答3:

Do this instead:

Integer[] array = stack.toArray(new Integer[stack.size()]);

We need to pass the "seed" array as an argument to the toArray method.