generics - Why do we use "? super T" in java? -

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• difference between <? super t> , <? extends t> in java 10 answers

for example, using when using generic comparator - comparator<? super t> c

i understand means comparator compare t , super classes, don't understand logic behind it. why not use "? extends t" ? or, example, have found similar question of asked "list<? super number>" - mean list gonna receive number objects , super classes? number's superclass object, mean list can receive object?

it doesn't mean t , super-classes. means t , sub-classes. no, list cannot receive object, can receive sub-class of number.

list<? super number> numbers = new arraylist<>(); integer = 23; double d = 3.14; object o = new object(); numbers.add(i); numbers.add(d); numbers.add(o); // compilation error 

integer , double both extend number. sub-classes of number. can added list of numbers.

number sub-class of object. if @ source code, not explicitly extend (although implement serializable). means super-class object. cannot add object numbers list.

object o2 = 300; numbers.add(o2);    // compilation error numbers.add((number) o2);   // no compilation error once cast 

in fact, can add object if number. of course, numbers objects, although not objects numbers. however, if declared object, first have cast number.