Java generics extends and super implemented in collection and its subtypes

Question

I want to understand a bit more on java generics covariance and contravariance and how extends and super together work wrt collection and individual objects

class P1 {}

class P2 implements Collection<P2> { // implement collection methods }

class Base<T> {
transient List<T> _instList = new ArrayList<>();

protected Base(final List<T> list) {
    _instList = list;
}
}

@SuppressWarnings("unchecked")
class Emp<T> extends Base<Object> {

public <C extends Collection<T>> Emp(final List<? super C> list) {
    super((List<Object>) list);
}

public <C extends Collection<T>> void salary(C c) {

}

public <C extends Collection<? super T>> void credit(C c) {

}
}

In Main method,

1 Emp<P1> emp1 = new Emp<>(new ArrayList<P1>());// works, compiles fine
2 Emp<P2> emp2 = new Emp<>(new ArrayList<P2>());// works, compiles fine
3 emp1.credit(new P1()); // doesn't compile
4 emp2.credit(new P2()); // works 

I want to understand how & why line 1 works when P1 is not a type of Collection

2 & 4 does compile, because P2 is a sub type of Collection.

Please clarify

Answer 1

Line 1 matches the typebound of the constructor.

class Emp<P1> extends Base<Object> {

    public <C extends Collection<P1>> Emp(final List<? super C> list) {
        super((List<Object>) list);
    }

replacing T with P1 and C is ArrayList. Nothing in that code requires P1 actually to implement Collection.

It is required to call credit(). Thats why you get an error in line3.