数値がオブジェクトであるような体系で、同一でないオブジェクトが数値として等しくなる例

Main.java

import java.util.concurrent.Callable;

public class Main {
    public static void main(String[] args) {
        Number two1 = new Succ(new Succ(new Zero()));
        Number two2 = new Succ(new Succ(new Zero()));

        System.out.println(two1 == two2);
        System.out.println(two1.myEquals(two2));
    }
}


interface MyBoolean {
    public <T> T myIf(Callable<T> thenPart, Callable<T> elsePart) throws Exception;
    public MyBoolean not();
}

class True implements MyBoolean {
    public <T> T myIf(Callable<T> thenPart, Callable<T> elsePart) throws Exception {
        return thenPart.call();
    }

    public MyBoolean not() {
        return new False();
    }

    @Override
    public String toString() {
        return "True";
    }
}

class False implements MyBoolean {
    public <T> T myIf(Callable<T> thenPart, Callable<T> elsePart) throws Exception {
        return elsePart.call();
    }

    public MyBoolean not() {
        return new True();
    }

    @Override
    public String toString() {
        return "False";
    }
}

interface Number {
    public MyBoolean isZero();
    public Number getPred();
    public MyBoolean myEquals(Number other);
}

class Zero implements Number {
    public MyBoolean isZero() {
        return new True();
    }

    public Number getPred() {
        throw new UnsupportedOperationException();
    }

    public MyBoolean myEquals(Number other) {
        return other.isZero();
    }
}

class Succ implements Number {
    protected Number pred;

    public Succ(Number pred) {
        this.pred = pred;
    }

    public MyBoolean isZero() {
        return new False();
    }

    public Number getPred() {
        return pred;
    }

    public MyBoolean myEquals(final Number other) {
        MyBoolean isOtherZero = other.isZero();

        try {
            return isOtherZero.myIf(new Callable<MyBoolean>() {
                    public MyBoolean call() {
                        return new False();
                    }
                },
                new Callable<MyBoolean>() {
                    public MyBoolean call() {
                        return getPred().myEquals(other.getPred());
                    }
                });
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}