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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 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.