Question

class Pair<A,B> {
    final A first;
    final B sec;

    Pair(A f, B s){
        this.first = f;
        this.sec = s;
    }
}

Is there a way to ensure that A and B are immutable types ?

Answer 1

No. There is no way to identify, let alone bound, a type as being immutable.

You could create a marker interface (one without methods, like Serializable) that is used in your project to identify immutable classes, and have your team adhere to it.

For example:

interface Immutable {}

class Pair<A extends Immutable, B extends Immutable> {
}