Why google-collections contains semantically equal functions and strange generics?

Why google-collections or guava contains semantically equal functions? e开发者_StackOverflow中文版xample:

static
<T> Predicate<T>
and(Predicate<? super T>... components) 

static
<T> Predicate<T>
and(Predicate<? super T> first, Predicate<? super T> second) 

I.e. all functions that can accept several arguments.

The second question why do defintion of such functions use generic <? super T> instead of <T>?

To answer the first question, the varargs version (Predicate<? super T>...) will give you a warning about the unchecked creation of a generic array when called with several generic predicates (e.g. Predicate<T>). For the common case of combining two predicates, you don't get that warning.

To answer the second question, taking Predicate<? super T> means you can pass in a Predicate<Object> (or Predicate<Number> or whatever) when calling the method to create a Predicate<Integer>. For example, if Predicates.notNull() were a Predicate<Object> (as it should be) and you wanted to combine that and some Predicate<Integer>, it would not be possible if the arguments were required to be of type Predicate<T>.

First part is for performance, I believe.

In the second part it have to be <? super T>, because Guava allows you to put in any Predicate which can be evaluated on any super-type of T and T itself. Not only T itself as it would be with just <T>. This makes sense due to the fact that a specific Predicate may be more general and is therefore defined on a more general type but you want to use it with some sub-type.

You see the same with the TreeSet Constructor: TreeSet(Comparator<? super E> c).

Also have a look at Generics Tutorial Page 18