Question

Can someone tell me what the differences between the first and second codes are?
MaxPQ stands for priority queue, which is a collection of “Key” objects that can be compared with each other.

Code 1:

public class MaxPQ<Key extends Comparable<Key>>{
...
}

Code 2:

public class MaxPQ<Key implements Comparable<Key>>{
...
}

The second code doesn’t compile, but it is not intuitive to me why we need to extend instead of implement interfaces when using a generic.

Answer 1

The difference is pretty straightforward: second code snippet does not compile and never will. With generics you always use extends, for both classes and interfaces. Also super keyword can be used there, but it has different semantics.