java synchronized issue

Question

I'm having issues with Synchronized not behaving the way i expect, i tried using volatile keyword also:

Shared Object:

public class ThreadValue {
private String caller;
private String value;
public ThreadValue( String caller, String value ) {
    this.value = value;
    this.caller = caller;
}

public synchronized String getValue() {
    return this.caller + "     "  + this.value;
}
public synchronized void setValue( String caller, String value ) {
    this.caller = caller;
    this.value = value;
}
}

Thread 1:

class CongoThread implements Runnable {
    private ThreadValue v;
    public CongoThread(ThreadValue v) {
    this.v = v;

    }
    public void run() {
    for (int i = 0; i  10; i++) {
    v.setValue( "congo", "cool" );
    v.getValue();
    }
    }
}

Thread 2:

class LibyaThread implements Runnable {
    private ThreadValue v;
    public LibyaThread(ThreadValue v) {
    this.v = v;

    }
    public void run() {
    for (int i = 0; i  10; i++) {
       v.setValue( "libya", "awesome" );
       System.out.println("In Libya Thread " + v.getValue() );

    }
    }
}

Calling Class:

class TwoThreadsTest {
    public static void main (String args[]) {

    ThreadValue v = new ThreadValue("", "");
        Thread congo = new Thread( new CongoThread( v ) );
        Thread libya = new Thread( new LibyaThread( v ) );

    libya.start();
        congo.start();

    }
}

Occasionally i get "In Libya Thread congo cool" which should never happen. I expect only: "In Libya Thread libya awesome" "In Congo Thread congo cool"

I dont expect them to be mixed.

Answer 1

The calls can interleave like the following:

Thread 1 : v.setValue()
Thread 2 : v.setValue()
Thread 1 : v.getValue() // thread 1 sees thread 2's value
Thread 2 : v.getValue() // thread 2 sees thread 2's value