java - IllegalMonitorStateException while calling wait() in run() -

i have created java thread , passed stack reference it's constructor, initialize thread stack reference. in run method have created synchronized block stack object, moment calling wait in run inside synchronized block , getting illegalmonitorstateexception.

thread class :

public class producer extends thread {  stack<string> stack=null;  public producer(stack<string> stack) {     this.stack=stack; }  @override public void run() {      synchronized (stack) {         if(stack.isempty()){             try {                 wait();             } catch (interruptedexception e) {                 e.printstacktrace();             }         }     }        } } 

main class:

public class mainclass {  public static void main(string[] args) {      stack<string> stack=new stack<string>();      producer p=new producer(stack);     p.start(); } } 

output :

exception in thread "thread-0" java.lang.illegalmonitorstateexception @ java.lang.object.wait(native method) @ java.lang.object.wait(object.java:485) @ demo.producer.run(producer.java:20) 

for wait() (or notify()) work, must call on same object. have same

 synchronized (stack) {     if(stack.isempty()){         try {             this.wait();         } catch (interruptedexception e) {             e.printstacktrace();         }     }  }   

instead should do

 synchronized (stack) {     if(stack.isempty()){         try {             stack.wait(); // wait on same object synchronized.         } catch (interruptedexception e) {             e.printstacktrace();         }     }  }   

note: wait can wake spuriously have in loop or method return prematurely.

 synchronized (stack) {     while (stack.isempty()){         try {             stack.wait();          } catch (interruptedexception e) {             e.printstacktrace();         }     }  }