Java 通用符号，<e> 放<e></e></e>

Question

The following Java code is used to create a set in Java:

    Set<String> fromUi = Set.of("val1", "val2", "val3", "val4");

Which in term calls this code:

static <E> Set<E> of(E e1, E e2, E e3, E e4) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4);
    }

What does the "double" use of the type parameter mean? ie can we not just say Set<E> instead of <E> Set<E> ?

Answer 1

can we not just say Set<E> instead of <E> Set<E> ?

No, because then the type variable E wouldn't be declared.

This isn't a "double" use:

• The first <E> is the type variable declaration
• The second <E> is part of the type of the Set<E> which is the return type of the method: it's a Set whose elements are of type E , and to which E s can be added.

Declaring one or more type variables on a method makes the method a generic method . Instance methods can access type variables from the surrounding class, or declare their own; static methods cannot access type variables from the surrounding class, and so must always declare their own.

// Generic class, E is accessible in instance methods/initializers/constructors.
class MyClass<E> {
  // Non-generic method, uses the E from the class.
  Set<E> someSet() { ... } 

  // Generic method, declares its own type variable.
  <M> Set<M> someSet1() { ... } 

  // Generic method, declares its own type variable which hides
  // the E on the class (bad idea).
  <E> Set<E> someSet2() { ... } 

  // Generic method, must declare its own type variable.
  static <E> Set<E> someStaticSet() { ... } 
}

// Non-generic classes can declare generic methods.
class MyClass {
  // Generic method, declares its own type variable.
  <M> Set<M> someSet1() { ... } 

  // Generic method, must declare its own type variable.
  static <E> Set<E> someStaticSet() { ... } 
}

Answer 2

static <E> Set<E> of(E e1, E e2, E e3, E e4) {

You can read it as: whatever 'E' is (for any type 'E'), pass 4 parameters of this type and get a Set of this type as a result.

Answer 3

Your method is static . It won't be able to access type variables declared from its class, hence it needs to declare its own <type> declaration. So its not double declaration.

The first <E> is where static method declares which type it uses
The second with Set<E> to specify the type of elements in a Set. 

you won't need to use this kind of declaration if your non static method uses same generic <type> declared by class.