Compile time vs Runtime errors

[Sharda Vajjhala]

Hi
I am confused as to when a piece of code will throw a compile time or runtime error.
For eg, this code there is no compiler error.Instead it gives a runtime error.
If a call to wait/notify must be within synchronized code, then why does it not complain at compile time. Is it not syntactically incorrect?

  
public class Agg{
public static void main(String argv[]){
Agg a = new Agg();
a.go();
}
public void go(){
DSRoss ds1 = new DSRoss("one");
ds1.start();
}
}
class DSRoss extends Thread{
private String sTname="";
DSRoss(String s){
sTname = s;
}
public void run(){
notwait();
System.out.println("finished");
}
public void notwait(){
while(true){
try{
 System.out.println("waiting");
 wait();
}catch(InterruptedException ie){}
System.out.println(sTname);
notifyAll();
}
}

}

I am going wrong with most of these kinds of questions.
Can someone help me understand the difference between the 2 error cases, especially in more subtle cases ( I know when the syntax is wrong for a statement)?

Thanks
Sharda

[Barkat Mardhani]

Originally posted by Sharda:
If a call to wait/notify must be within synchronized code, then why does it not complain at compile time. Is it not syntactically incorrect?

I think it compiles because it is possible that any other thread (not mentioned in current compilation unit) via a synchronized method could be calling this method. To demonstrate that I have just added one more layer of call in your code, and it compiles and runs fine...

public class Agg { 
public static void main(String argv[]) {
Agg a = new Agg();
a.go();
}
public void go() {
DSRoss ds1 = new DSRoss("one");
ds1.start();
}
}
class DSRoss extends Thread {
private String sTname="";
DSRoss(String s) {
sTname = s;
}
public void run() {
notwait1();
System.out.println("finished");
}
public synchronized void notwait1() {
notwait();
}
public void notwait() {
while(true) {
try { 
System.out.println("waiting"); 
wait();
}
catch(InterruptedException ie){}
System.out.println(sTname);
notifyAll() ;
}
}
}

[Shishio San]

Hi,

public class IllegalMonitorStateException
extends RuntimeException
Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.

The thread needs to own the object's monitor before making a call to funtions like wait, notify, notifyall as stated above. Since it's a unchecked exception you don't get a compile time error but a runtime one.

[Shishio San]

Just to put clear-cut.

In Java terminology, a monitor (or semaphore) is any object that has some synchronized code.