Why can I create generic object without specifying its type in any way and when type cannot be inferred?

0
votes

(nevermind:) it's illegal: HashSet<> f;

it's legal: new HashSet<>();

From type inference documentation:

You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context.

But a compiler cannot infer type argument in second example, so why does it compile?

1
javagenericstype-inference
You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters That's not what you're doing in your first snippet. – Sotirios Delimanolis
Also, it can infer. It infers Object. That rules of type inference are extremely long, but that's what new HashSet<>() reduces to. – Sotirios Delimanolis
What gives you the impression that the compiler cannot infer type argument in second example? – Software Engineer
@EngineerDollery I thought that inferring works only when there is already some code which defines generic type and then you don't have to repeat yourself in next statements. I thought that when there is no such code, then there is no context to use for inferring. But now I see inferring is a broader thing. – ctomek

1 Answers

1
votes

Generics is a compile-time construct. That is, it's used to enforce that a given variable or method can only work with a specific type or set of types. In your case the compiler can "infer" the broadest type - Object, since the HashSet is not being assigned or returned, so the types will never be used at compile time. And at runtime the types are meaningless due to type erasure.