Featherweight Java

/* Featherweight Java (FJ) Example: Pair class with setfst
 * From Igarashi, Pierce, Wadler (2001)
 * "Featherweight Java: A Minimal Core Calculus for Java and GJ" */

class A extends Object {
    A() { super(); }
}

class B extends Object {
    B() { super(); }
}

class Pair extends Object {
    Object fst;
    Object snd;

    Pair(Object fst, Object snd) {
        super();
        this.fst = fst;
        this.snd = snd;
    }

    Pair setfst(Object newfst) {
        return new Pair(newfst, this.snd);
    }
}

new Pair(new A(), new B()).setfst(new B())