Question

I have two very similar programs each trying to run two threads OddThread and EvenThread and trying to print the odd and even numbers in sequence . While the first one works , the second one hangs . Can anyone please pinpoint the bug in the second program ?

The first one which works :

public class ThreadTest {

public static void main(String[] args) {
    System.out.println("Odd Even test");
    NumHolder objNumHolder = new NumHolder();
    Odd o1 = new Odd(objNumHolder, "Odd Number Thread");
    Even e1 = new Even(objNumHolder, "Even Number Thread");
    o1.start();
    e1.start();
}
}

class NumHolder {
private int intCurrNum;
private boolean isEven = false;

public synchronized void printOddNumber(String tname) {
    while (isEven == true){
        try {
            wait();
        }catch (InterruptedException e) {
        }
    }
    isEven = true;
    System.out.println("Thread Name="+tname + "===Number="+intCurrNum);
    intCurrNum += 1;
    notifyAll();
}

public synchronized void printEvenNumber(String tname) {
    while (isEven == false) {
        try {
            wait();
        } catch (InterruptedException e) {
        }
    }
    isEven = false;
    System.out.println("Thread Name="+tname + "===Number="+intCurrNum);
    intCurrNum += 1;
    notifyAll();
}
}

class Even extends Thread {
private NumHolder objNumHolder;

public Even(NumHolder p_objNumHolder, String name) {
    super(name);
    objNumHolder=p_objNumHolder;
}

public void run() {
    for (int i = 0; i < 10; i++) {
        objNumHolder.printEvenNumber(getName());
    }
}
}
class Odd extends Thread {
private NumHolder objNumHolder;

public Odd(NumHolder p_objNumHolder,String name) {
    super(name);
    objNumHolder = p_objNumHolder;
}

public void run() {
    for (int i = 0; i < 10; i++) {
        objNumHolder.printOddNumber(getName());
    }
}
}

The second code which hangs :

class PrintClass {
int intCurrNum;
private boolean isEven = false;

synchronized void printOdd(){
    while(isEven){
        try{
            wait();
        }catch(InterruptedException ie){
            System.out.println("Interrupted exception in printOdd()");
            ie.printStackTrace();
        }
        isEven = true;
        System.out.println("Thread Name="+Thread.currentThread().getName() + "===Number="+intCurrNum);
        intCurrNum += 1;
        notifyAll();
    }
}

synchronized void printEven(){
    while(!isEven){
        try{
            wait();
        }catch(InterruptedException ie){
            System.out.println("Interrupted exception in printEven()");
            ie.printStackTrace();
        }
        isEven = false;
        System.out.println("Thread Name="+Thread.currentThread().getName() + "===Number="+intCurrNum);
        intCurrNum += 1;
        notifyAll();
    }
}
}
class ThreadOdd extends Thread {
PrintClass pc = null;

ThreadOdd(PrintClass pc , String name){
    super(name);
    this.pc = pc;
}

public void run(){
    for (int i = 0; i < 10; i++) {
        pc.printOdd();
    }
}
}
class ThreadEven extends Thread {
PrintClass pc = null;

ThreadEven(PrintClass pc,String name){
    super(name);
    this.pc = pc;
}

public void run(){
    for (int i = 0; i < 10; i++) {
        pc.printEven();
    }
}
}
public class EvenOddPrintClass {
public static void main(String[] args){
    PrintClass pc = new PrintClass();
    Thread to = new ThreadOdd(pc,"ThreadOdd");
    Thread te = new ThreadEven(pc,"ThreadEven");
    to.start();
    te.start();
}
}

Thanks.

Answer 1

I suggest you run your code in the debugger and step through both threads. It’s very educational. You will see exactly where the error is.