What's the difference between <T> T vs T in the return type of a method?

Question

I have a method, like this,

public <T> T doSomething(Class<T> T) {
return T.newInstance();
}

I can also do the same like this,

public T doSomething(Class<T> T) {
return T.newInstance();
}

Is there any difference between these two? Please ignore T.newInstance(), I'm basically going to create a new instance of T somehow and return it.

thanks, sam

Answer 1

What's the difference between <T> T vs T in the return type of a method?

There is no <T> T. The <T> is not part of the return type; it's a separate thing, indicating the type parameter. (You can also have public <T> void ....)

In the version with <T>, you're declaring T as a type parameter of the method, and the T in the argument and the return-type are referring to that type parameter.

In the version without <T>, you're not declaring T as a type parameter of the method, so it's most likely a type parameter of the containing class or another containing scope. (Alternatively, someone may have named an actual class or interface T, in which case you should give that person a talking-to.)

If both versions compile, then the second is probably the one you want: you probably want to be using the class's type parameter T, rather than adding a new one that hides it. (And if you really do want a new type parameter unrelated to your class's type parameter, then you should use a new name for it to avoid confusion.)