ADT Sum Type in Java

EitherExample.java

import java.util.function.Function;

public class EitherExample {
    public static void main(String[] args) {
        Either<String, Integer> x = new Left<>("hello");
        Either<String, Integer> y = new Right<>(123);
        System.out.println("hello");
        System.out.println(x);
        System.out.println(y);
    }
}

abstract class Either<L, R> {
    public abstract boolean isLeft();
    public abstract boolean isRight();
    public abstract L fromLeft(L dft);
    public abstract R fromRight(R dft);
    public abstract <V> V either(Function<L, V> fl, Function<R, V> fr); //pattern matching
}

class Left<L, R> extends Either<L, R> {
    private L value;

    public Left(L x){ value = x; }
    public boolean isLeft() { return true; }
    public boolean isRight() { return false; }
    public L fromLeft(L dft) { return value; }
    public R fromRight(R dft) { return dft; }
    public <V> V either(Function<L, V> fl, Function<R, V> fr) { return fl.apply(value); }
}

class Right<L, R> extends Either<L, R> {
    private R value;

    public Right(R x){ value = x; }
    public boolean isLeft() { return false; }
    public boolean isRight() { return true; }
    public L fromLeft(L dft) { return dft; }
    public R fromRight(R dft) { return value; }
    public <V> V either(Function<L, V> fl, Function<R, V> fr) { return fr.apply(value); }
}

Adding some static methods help Either ease to use.

abstract class Either<L, R> {

    public abstract boolean isLeft();
    public abstract boolean isRight();
    public abstract L fromLeft(L dft);
    public abstract R fromRight(R dft);
    public abstract <V> V either(Function<L, V> fl, Function<R, V> fr); //pattern matching

    public static <L,R> L fromLeft(L dft, Either<L,R> either) {
        return either.fromLeft(dft);
    }
    public static <L,R> R fromRight(R dft, Either<L,R> either) {
        return either.fromRight(dft);
    }
    public static <L,R> boolean isLeft(Either<L,R> either) {
        return either.isLeft();
    }
    public static <L,R> boolean isRight(Either<L,R> either) {
        return either.isRight();
    }
    public static <L,R,V> V either(Either<L,R> either, Function<L, V> fl, Function<R, V> fr) {
        return either.either(fl, fr);
    }
}