empty generic type when creating an object

Question

Recently, I saw the following constructor call:

Set<String> s = new HashSet<>();

I was surprised that one can call the constructor of the generic class HashSet with empty parentheses < and >. What does this mean?

Read more here: http://docs.oracle.com/javase/7/docs/technotes/guides/language/type-inference-generic-instance-creation.html — vegaasen, Feb 16 '17 at 12:29

score 2 · Answer 1 · answered Feb 16 '17 at 12:29

2

Since you've already declared what type of value (String in this case) the Set will hold, repeating it is redundant.

answered Feb 16 '17 at 12:29

Zetta

Thank you. So `new HashSet<>()` is just a shortcut writing for `new HashSet()`? – John Threepwood Feb 16 '17 at 12:30
@JohnThreepwood I think it shouldn't be considered a shortcut, because repeating the generic type in the `new` is completely pointless, so there is no need for shortcuts to get to that. It would be like saying that `1 + 1` is a shortcut for `1 + 1 + 0`. Eclipse has a warning when you do that: `Redundant specification of type arguments `. – SantiBailors Feb 16 '17 at 13:27

score 1 · Answer 2 · answered Feb 16 '17 at 12:33

1

this is diamond syntax, introduced since java7

answered Feb 16 '17 at 12:33

Elias

2 Answers2