When InvalidMonitorStateException is thrown? why?

This exception is thrown when you try to call wait()/notify()/notifyAll() any of these methods for an Object from a point in your program where u are NOT having a lock on that object.(i.e. u r not executing any synchronized block/method of that object and still trying to call wait()/notify()/notifyAll())

wait(), notify() and notifyAll() all throw IllegalMonitorStateException. since This exception is a subclass of RuntimeException so we r not bound to catch it (although u may if u want to). and being a RuntimeException this exception is not mentioned in the signature of wait(), notify(), notifyAll() methods.

1. class IllegalMonitorStateExample  
2. {  
3.   public static void main(String a[])  
4.   {  
5.     AnObject o = new AnObject();  
6.     Thread inc = new Thread(new IncrUser(o));  
7.     Thread dec = new Thread(new DecrUser(o));  
8.     inc.start();  
9.     dec.start();  
10.   }  
11. }  
12.   
13. class IncrUser implements Runnable  
14. {  
15.   AnObject a ;  
16. IncrUser()  
17. {  
18. }  
19.     IncrUser(AnObject a)  
20. {  
21. this.a = a;  
22. }  
23. public void run()  
24. {  
25. while(true)  
26. {  
27. try  
28. {  
29. [B]//   a.wait();[/B]  
30.    a.incr();  
31.    Thread.sleep(1000);  
32. [B]  //   a.notify(); //[/B]  
33. }  
34. catch(InterruptedException ie)  
35. {  
36. ie.printStackTrace();  
37. }  
38.   
39. }  
40. }  
41. }  
42. class DecrUser implements Runnable  
43. {  
44.   AnObject a ;  
45. DecrUser()  
46. {  
47. }  
48.     DecrUser(AnObject a)  
49. {  
50. this.a = a;  
51. }  
52. public void run()  
53. {  
54. while(true)  
55. {  
56.             try  
57. {  
58.    a.decr();  
59.    Thread.sleep(1000);  
60. }  
61. catch(InterruptedException ie)  
62. {  
63. ie.printStackTrace();  
64. }  
65.     }  
66. }  
67. }  
68.   
69. class AnObject{  
70. int myNum = 0;  
71. synchronized void incr() throws InterruptedException  
72. {  
73.   if(myNum!=0)  
74.   {  
75. System.out.println("ready to incr but muNum not zero +++++++++++++++");  
76. wait();  
77.   }  
78.   myNum++;  
79.   print("produced : ");  
80.   notify();  
81. }  
82.  synchronized void decr() throws InterruptedException  
83. {  
84.   if(myNum==0)  
85.   {  
86.     System.out.println("ready to decr but myNum not 1 ----------");  
87.     wait();  
88.   }  
89.   myNum--;  
90.   print("consumed : ");  
91.   notify();  
92. }  
93.   
94. void print(String s)  
95. {  
96.     System.out.println(s + "  "+myNum);  
97. }  
98.   
99. }