Lazy Evaluation in Java (https://youtu.be/KniIeHiEzdo)

Lazy.java

package main;

public interface Lazy<R> {
	
	public R force();
	
}

Main.java

package main;

public class Main {

	@SuppressWarnings({ "rawtypes", "unchecked" })
	public static void main(String[] args) {

		Lazy a = new Singleton(1);
		Lazy b = new Singleton(2);
		Lazy c = new Thunk<Integer, Integer, Lazy<Integer>, Lazy<Integer>, Integer>((x,y) -> x+y, a, b);
		Lazy d = new Thunk<Integer, Integer, Lazy<Integer>, Lazy<Integer>, Integer>((x,y) -> x+y, a, c);
		
		System.out.println("c before force: " + c);
		System.out.println("d before force: " + d);
		d.force();
		System.out.println("c after force:  " + c);
		System.out.println("d after force:  " + d);

	}

}

Singleton.java

package main;

public class Singleton<V> implements Lazy<V> {

	private V value;
	
	public Singleton(V value) {
		this.value = value;
	}

	@Override
	public V force() {
		return value;
	}
	
}

Thunk.java

package main;

import java.util.function.BiFunction;

public class Thunk<A1, A2, LA1 extends Lazy<A1>, LA2 extends Lazy<A2>, R> implements Lazy<R> {

	private BiFunction<A1,A2,R> fun;
	private LA1 arg1;
	private LA2 arg2;
	private R result;
	
	public Thunk(BiFunction<A1,A2,R> fun, LA1 arg1, LA2 arg2) {
		this.fun = fun;
		this.arg1 = arg1;
		this.arg2 = arg2;
		this.result = null;
	}
	
	@Override
	public R force() {
		if(result == null)
			result = fun.apply(arg1.force(), arg2.force());
		return result;
	}
	
	public String toString() {
		String valString = result == null ? "_" : result.toString();
		return "Lazy(" + valString + ")";
	}
	
}