Making sense of generic Java method header

Question

I was reading up on generics in Java here and here, and while I understand the point of having them, I'm having a little trouble understanding the header of a generic Java method. I'm used to:

public returnType methodName(parameters){body}

What keeps confusing me is the specification of a generic method declaration:

public *genericType* returnType methodName(parameters){body}

or

public static <T> void fromArrayToCollection(Collection<T> c) {}

What is this type declaration (<T> between static and void) affecting? Is it just outlining the type that the method parameters can be?

It means "This method has a generic type parameter referred to as T" — khelwood
– khelwood, Commented Jan 15, 2016 at 16:26
@khelwood Why doesn't the existence of Collection<T> in the signature implicitly have that same meaning? — Matt
– Matt, Commented Jan 15, 2016 at 16:27
It lets you add a generic type parameter to just that method, as opposed to declaring it at the class level. — azurefrog
– azurefrog, Commented Jan 15, 2016 at 16:27
@Matt Because there is nothing (other than convention) which says that you name type variables like T. For example, you could write void foo(Collection<String> strs) { String bar = ""; } (which compiles), but you could also write <String> void foo(Collection<String> strs) { String bar = ""; } which does not, since String is a type variable, not java.util.String. Similarly, if it were inferrable that T might mean a type variable, there is no way of knowing whether you mean that or some class which you forgot to import, T. — Andy Turner
– Andy Turner, Commented Jan 15, 2016 at 16:35

khelwood · Accepted Answer · 2016-01-15 16:40:19Z

4

The <T> between static and void in this case means "There is a generic type parameter being referred to as T in this method."

Compare case 1:

class Foo<T> {
     public void doFoo(Collection<T> items) ...
}

with case 2:

class Foo {
    public <T> void doFoo(Collection<T> items) ...
}

and case 3:

class Foo {
    public void doFoo(Collection<T> items) ...
}

In the first case, T is a generic type for the whole class, so if you call doFoo, the <T> in the collection you pass in needs to be the same T as the generic type of the instance of Foo<T>.

In the second case, T is a generic type existing only in the method doFoo.

In case 3, T must be an actual type, because there is no declaration that it is a generic type parameter.

edited Jan 15, 2016 at 16:40

answered Jan 15, 2016 at 16:35

khelwood

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Comments

Aaron · Accepted Answer · 2016-01-15 16:39:58Z

2

The <T> declared as return type references the <T> declared in the parameter. That way you know that if you feed your method a collection of Strings, you'll get a String as a result.

So in conclusion, a generic type give you the ability to reference in your code a type without having to specify it yet.

edited Jan 15, 2016 at 16:39

answered Jan 15, 2016 at 16:35

Aaron

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2 Comments

augray Over a year ago

The types within the generic body are actually only known at compile time (rather than runtime), thanks to type erasure

Aaron Over a year ago

@augray "fixed" it, but it seems clunky. I'll see if I can find a better way to express it. Thanks for remining me generics were specified during compilation

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