Solving A Java Inheritance Puzzler from https://llogiq.github.io/2015/07/14/inheritance.html

E.java

/*
 * Solving A Java Inheritance Puzzler from
 * https://llogiq.github.io/2015/07/14/inheritance.html
 */

class A {
	int x = 3;
	int a() { x++; return x + c(); } 
	int b() { return a(); }
	int c() { return x; }
}

class B extends A {
	@Override int a() { return super.a() + c(); }
}

class C extends B {
	@Override int b() { return a() * 3; }
	@Override int c() { return x + 1; }
}

public class E {

	public static void main(String[] args) {
		int x = new C().b();
		/*
		 * Because C overrides b(), x = a() * 3
		 * C inherits B, which overrides a(), so a() = A.a() + c()
		 * x = (A.a() + c()) * 3
		 * Which c() is it? A.c() or C.c()? 
		 * Without super, it is this.c(), and this == C()
		 * x = (A.a() + C.c()) * 3
		 * A.a() is x + 1 + c(), which c() is that?
		 * By the same logic, C.c()
		 * x = (x + 1 + C.c() + C.c()) * 3
		 * x = (x + 1 + x + 1 + x + 1) * 3
		 * x = (3 + 1 + 3 + 1 + 3 + 1) * 3
		 * x = 12 * 3 = 36
		 */
		System.out.println(x); // Wrong! x = 42
		/*
		 * Override analysis was correct.
		 * Mutable state analysis was wrong.
		 * Assumption that x++ translate to x+1 is wrong.
		 * (Been programming pure functional too long?)
		 * C.b() -> B.a() * 3           x = 3
		 *   B.a() -> A.a() + C.c()     x = 3
		 *     A.a() -> x + C.c()       x = 4
		 *       C.c() -> 5             x = 4
		 *     A.a() -> 9               x = 4
		 *   B.a() -> 14                x = 4
		 * C.b() -> 42                  x = 4
		 */
	}

}