如果 Java 不支持参数化类型数组，那么 Arrays.asList() 如何使用它们？

Question

Supposedly, the method Arrays.asList looks like this:

public static <T> List<T> asList(T... a)

T... is practically equivalent to T[] , but aren't arrays of generic types disallowed in Java? For example:

// Example 1
ArrayList<Integer>[] list = new ArrayList<Integer>[10];  // This is not allowed

// Example 2
List<ArrayList<Integer> > listOfList = Arrays.asList(new ArrayList<Integer>(), new ArrayList<Integer>());  // Allowed?

In example 2, the two parameters I passed would imply that Arrays.asList is taking ArrayList<Integer>[] as parameters which contradicts example 1. Why does example 2 work when example 1 doesn't?

Answer 1

You can declare ageneric type array in Java which is perfectly legal. So this should work.

private E[] array;

But you can't instantiate a generic array like so, because arrays are reified and all the generic types are implemented as erasure. So this fictitious E is not available at runtime and you are out of luck !

array = new E[10];  // Illegal and gives compiler error.

But you have a workaround here. What you can do it just a cast.

array = (E[]) new Object[10];

This way you can create and instantiate a generic array in java. You may use @SuppressWarnings("unchecked") at the lowest possible level, may be at the site of declaration to get rid of any unchecked warnings made by the compiler.

Answer 2

Here, the T[] is in a parameter declaration, and as @Ravindra Ranwala answered, declaring a variable or parameter of type T[] is perfectly fine. It's creating an array of a type parameter type, ie new T[] that is not allowed.

In this case, by passing to the varargs argument, the compiler is actually implicitly creating an array of type ArrayList<Integer>[] to pass to the method. As you know, creating an array of a parameterized type, ie new ArrayList<Integer>[] , is also not allowed. So the compiler creates new ArrayList[] , and, normally, you would get a warning in a call such as this. However, you don't get a warning in this call because Arrays.asList() has the @SafeVarargs annotation, which indicates that they do not use the runtime component type of the array object, so it is safe.