Question

Why google-collections or guava contains semantically equal functions?
example:

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>?

Answer 1

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>.