It's confusing because type declaration and usage both use the <T> syntax. I think there are only 2 places where you can declare new generic types in Java:

1. In the definition of a class or interface

class MyClass<T> ...
//           ^^^- Type declaration

interface MyInterface<T,U> ...
//                   ^^^^^- Declaring 2 types 

2. Before a static method

// The declared type must then be used as a parameter of that method...
static <T> void foo(T t...
//     ^^^

// or as the return type:
static <T> T foo(...
//     ^^^

Is that true? Am I using the right words? Should I say "introduce" a type variable instead of "declare?" Things like the following statement worry me:

It is possible for a constructor to be generic independently of whether the class the constructor is declared in is itself generic.

Source: Java Language Specification 8.8.4. Generic Constructors

I'm trying to write up the limits of Java generics, but I'm only just getting started.

  • 4
    The method doesn't have to be static. It's just that instance methods can reuse the declaration from their class, and it's usually a good idea to do so, therefore you rarely see type declarations on them. Nov 17, 2016 at 14:59
  • @KilianFoth instance methods are often limited by inheriting from a super-class or interface, but yes, you're right for instance methods that don't override any other methods. Nov 17, 2016 at 15:43

1 Answer 1


Any method including constructors can take type parameters. I have yet to see a case in Java where you'd want a generic constructor, but it's entirely legal to declare something like

class Thingy {
    private final ArrayList<String> items;

    public <A> Thingy(Collection<A> values, Function<A, String> function) {
        items = new ArrayList<>(values.size());
        for (A thing : values)

And invoke the constructor as new Thingy(items, x -> x.toString()) when relying on type inference, or as new<Integer> Thingy(items, x -> x.toString()) when specifying the type parameter explicitly. That's right, the constructor type arguments go after the new keyword.

Instance methods may also have type parameters, e.g. I could define

class DataSource<T> {
  public <R> DataSource<R> transform(Function<T, R> transformation) { ... }

When compared with other type systems with type parameters (C++, C#, Haskell, …), Java doesn't have notable restrictions regarding the places where type parameters may or may not be declared. Instead, this type-erasure based system has notable restrictions on how those type parameters may be used, e.g. regarding arrays, catch clauses, primitive types, and the lack of type aliases.

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