Threads - The Java Programming Language - page 538

Now consider this scenario, in which jareth and cory are two Friendly
objects that have become friends:
1. Thread number 1 invokes synchronized method jareth.hug . Thread
number 1 now has the lock on jareth .
2. Thread number 2 invokes synchronized method cory.hug . Thread
number 2 now has the lock on cory .
3. Now jareth.hug invokes synchronized method cory.hugBack . THRead
number 1 is now blocked waiting for the lock on cory (currently
held by thread number 2) to become available.
4. Finally, cory.hug invokes synchronized method jareth.hugBack .
Thread number 2 is now blocked waiting for the lock on jareth
(currently held by thread number 1) to become available.
We have now achieved deadlock: cory won't proceed until the lock on
jareth is released and vice versa, so the two threads are stuck in a per-
manent embrace.
We can try to set up this scenario:
public static void main(String[] args) {
final Friendly jareth = new Friendly("jareth");
final Friendly cory = new Friendly("cory");
jareth.becomeFriend(cory);
cory.becomeFriend(jareth);
new Thread(new Runnable() {
public void run() { jareth.hug(); }
}, "Thread1").start();
new Thread(new Runnable() {
public void run() { cory.hug(); }