/*
 * CSE 332
 * SECTION Week 10
 */

class Bag {
		private String s;
		Bag() {
			
		}
		synchronized void put(String t) {
			 this.s = t;
		}
		synchronized  String get() {
			 return s;
		}
		
		void changeContents() {
			this.s = "whatever";
		}
	}
	
	
	
//  One bad interleaving might be:

// Assume all threads can access b1 and b2:
Bag b1 = new Bag();
Bag b2 = new Bag();
b2.put("rocks");

// then Thread 1 does:		
b1.put("gold");
				// but here, Thread 2 does:
				b1.put("rocks");
if(b2.get().equals("rocks")) {
	b2.put(b1.get());
}	

// The result will be that b1 gets changed to "rocks" but Thread 1 doesn't 
// know it, and will store "rocks" into b2 when it meant to put gold there.

// let's try solve it by synchronizing on b1:

// Assume all threads can access b1 and b2:
Bag b1 = new Bag();
Bag b2 = new Bag();
b2.put("rocks");

// then Thread 1 does:
synchronized(b1) {
  b1.put("gold");
                                // but here, Thread 2 does:
                                b1.put("rocks");
				// however, since put is synchronized on "this" and "this" here is b1, 
				// it blocks until Thread 1 releases, so all is good
  if(b2.get().equals("rocks")) {
				// but let's say instead, maybe Thread 2 does this here:
				b2.put("tons of stuff better than gold");
        b2.put(b1.get());
  }
}

// now what will happen is, since we didn't synchronize on b2, b2 can be altered in the middle of execution, and we
// overwrote stuff that's better than gold with just gold.

// So let's try synchronizing on both!

Bag b1 = new Bag();
Bag b2 = new Bag();
b2.put("rocks");

// thread 1
synchronized(b1) {
  synchronized(b2) {
    b1.put("gold");		
				// thread 2: b1.put("rocks"); will wait until thread 1 is done with b1
    if(b2.get().equals("rocks")) {
				// thread 2: b2.put("tons of stuff better than gold") will wait until thread 2 is done with b2
	b2.put(b1.get());
		
    }
  }
}	

// but we have another problem...


// thread 1
synchronized(b1) {
  synchronized(b2) {
    b1.put("gold");
    if(b2.get().equals("rocks")) {
                                // thread 2 can do this here:
				b1.changeContents();
				// this will NOT wait, because changeContents was not synchronized
				// on b1 (or on anything, really) so even though b1 is the lock
				// when entering changeContents's method in Thread  2, changeContents doesn't 
				// go and check or try to acquire any locks.  So it will proceed along merrily....
        b2.put(b1.get());
                        
    }
  }
}

// And at the end of this, both b1 and b2's strings are "whatever"

// so to solve this problem, synchronize changeContents inside the Bag class, e.g. 
		
synchronized void changeContents() {
	this.s = "whatever";
}