为什么Java 7和Eclipse 3.8编译器不能用新的Java 7菱形运算符编译JDK代码？

Question

import java.util.*;

public class SimpleArrays
{
  @SafeVarargs
  public static <T> List<T> asList( T... a )
  {
    return new ArrayList<>( a );
  }
}

asList() is taken from Oracles JDK implementation of java.util.Arrays.

The error is

error: cannot infer type arguments for ArrayList<>
    return new ArrayList<>( a );
1 error

How can this work? Oracle uses the same compiler that we do.

Answer 1

Attention: The ArrayList used in the java.util.Arrays class is not java.util.ArrayList , but a nested class java.util.Arrays.ArrayList .

In particular, this class has an constructor which takes a T[] as argument, which java.util.ArrayList does not have.

Copy this class, too, and it will work.

Answer 2

From what I can gather, Eclipse wants to find a specific type to infer into the templated ArrayList . For example, if your method's signature was:

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

Eclipse would have no problem inferring the type of ArrayList<>( a ) , and would infer that its type is Integer . I believe the diamond operator is meant to operate that way: to infer a specific type, not a templated one.

Fortunately, you have templated the entire method, so that you could form your statement thus:

      return new ArrayList<T>( a );

And everything would work:).