Java generics grammar

Question

I couldn't understand the following statement while it is indeed compilable:

List<Integer> l = Collections.<Integer>singletonList(5);

Say, the second <Integer>, how can we put a single <Integer> in front of the method name? I suspect it's a declaration of generics but cannot find it anywhere. But I only know the definition such as List<Integer>, put <Integer> behind a generics type. Can anybody point out me a tutorial for this grammar or find the duplicated question (sorry I didn't find one during my quick search)?

Many thanks!

https://docs.oracle.com/javase/tutorial/extra/generics/methods.html — M A, Jul 19 '15 at 19:23
Also https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html — M A, Jul 19 '15 at 19:25

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

This is called a type witness, and it is referenced in the Type Inference trail:

The generic method addBox defines one type parameter named U. Generally, a Java compiler can infer the type parameters of a generic method call. Consequently, in most cases, you do not have to specify them. For example, to invoke the generic method addBox, you can specify the type parameter with a type witness as follows:
BoxDemo.<Integer>addBox(Integer.valueOf(10), listOfIntegerBoxes);

Effectively, a type witness lets the developer step in to resolve cases in which the type engine can't properly infer what type a value will result in. You'd see its usage more commonly and prevalently in Java 7, whereas Java 8 has improved its type inference capabilities.

Thank you! That's definitely helpful! – zzy Jul 19 '15 at 20:29 — zzy, Jul 19 '15 at 20:29

Java generics grammar

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