Using diamond operator in Java 7

Question

Prior to Java 7 the syntax below was used to create an ArrayList,

ArrayList<Integer> ints = new ArrayList<Integer>(Arrays.asList(1,2,3,4,5,6,7));

But since Java 7 we can omit the generic type in the constructor i.e.,

ArrayList<Integer> ints = new ArrayList<>(Arrays.asList(1,2,3,4,5,6,7));

But when I tried to do something like,

ArrayList<Number> nums = new ArrayList<>(Arrays.asList(1,2,3,4,4,5.5,6.6,7.7));

I get an error, but when I mention the generic type on right side i.e.,

ArrayList<Number> nums = new ArrayList<Number>(Arrays.asList(1,2,3,4,5.5,6.6,7.7));

The code works perfectly. What could be the reason behind this ?

Thanks in Advance.

Answer 1

Arrays.asList is a generic method, so technically, it can be invoked as:

Arrays.<Number> asList(1, 2, 3);

Even before Java 7, the language could do some limited inference to eliminate this when the left-hand side was known, as in

final List<Number> nums = Arrays.asList(1, 2, 3);

It looks like you stumbled onto an edge case where the two types of inference cannot be successfully combined, where the diamond inference and generic method inference don't get along. I'm sure the JLS sheds more light on this as well if you were to dig through it.

Answer 2

Java Generics are Invariant whereas Arrays are Covariant.

If generics in Java were covariant, if A is a subtype of B, then List[A] is a subtype of List[B]. But it is not the case in Java. ( Scala has a covariance implementation. In Scala, if B extends A, then List[B] extends List[A])

But String[] is subtype of Object[]

Hence an ArrayList<Double> cannot be cast to an ArrayList<Number> as in your case.