Questions tagged [bounded-wildcard]

A bounded wildcard is a type argument of the form ? extends T or ? super T. Bounded wildcards are a feature of generics in the java language. These type arguments represent some unknown type, with either an upper or lower bound.

• A bounded wildcard with an upper bound ? extends T requires that the original type was T or a subtype of T:

  static <T> void fill(
          Collection<T> collection,
          int count,
          Supplier<? extends T> supplier) {
  
      for (int i = 0; i < count; ++i)
          collection.add( supplier.get() );
  }
  
  List<Number> list = new ArrayList<>();
  Supplier<Double> random = Math::random;
  
  fill( list, 10, random );
  

• A bounded wildcard with a lower bound ? super T requires that the original type was T or a supertype of T:

  static <T> void forEach(
          Iterable<T> iterable,
          Consumer<? super T> consumer) {
  
      for (T element : iterable)
          consumer.accept( element );
  }
  
  List<Integer> list = Arrays.asList(1, 2, 3);
  Consumer<Object> printer = System.out::println;
  
  forEach( list, printer );
  

The bounded wildcard ? extends Object is equivalent to the unbounded-wildcard.

See also:

• Type Arguments of Parameterized Types in the Java Language Specification
• Official wildcards tutorial
• What is PECS (Producer Extends Consumer Super)?