ADT / sum types in Java demo inspired by Philip Wadler's “Category Theory for the Working Hacker” talk

SumTypes.java

import java.util.function.Function;

interface Either<A, B> {
    public <C> C caseExpr(Function<A, C> f, Function<B, C> g);
}

class Left<A, B> implements Either<A, B> {
    private A x;

    public Left(A x) {
        this.x = x;
    }

    public <C> C caseExpr(Function<A, C> f, Function<B, C> g) {
        return f.apply(x);
    }
}

class Right<A, B> implements Either<A, B> {
    private B y;

    public Right(B y) {
        this.y = y;
    }

    public <C> C caseExpr(Function<A, C> f, Function<B, C> g) {
        return g.apply(y);
    }
}

public class SumTypes {
    private static Either<String, Integer> divide(int what, int byWhat) {
        return byWhat == 0 ? new Left<>("Division by zero :(") : new Right<>(what / byWhat);
    }

    public static void main(String[] args) {
        System.out.println(divide(196418, 10946).caseExpr(Function.identity(), x -> x.toString()));
        System.out.println(divide(-13, 0).caseExpr(Function.identity(), x -> x.toString()));
    }
}