Multithread program from khalid mughal

[Jacob Sonia]

Please tell me where i am wrong in this program and how can i do it in a better wayBookmark:Question:
Create three Classes: Storage, Counter and Printer. The Storage class should store an integer. The Counter class should create a thread that starts from counting from 0(0, 1, 2, 3....) and stores each value in the Storage class. The Printer class should create a thread that keeps reading the value in the Storage class and printing it.
Create a program that create an instance of the Storage class, and sets up a Counter and a Printer object to operate on it. Modify the program to ensure that each number is printed exactly once, by adding suitable synchrnization

    public class Counter implements  Runnable{
      
      public static void main(String[] args) {
            Storage store = new Storage();
            Counter c1 = new Counter(store);
            Printer p1 = new Printer(store);
      }
      Storage st;
      public Counter(Storage store){
            st = store;
            new Thread(this, "Counter").start();
            
      }
      @Override
      public void run() {
            for(int i = 0 ; i < 10; i++){
                  st.setValue(i);
            }
            
      }

}
class Printer implements Runnable{
      Storage st;
      public Printer(Storage st){
            this.st = st;
            new Thread(this, "Printer").start();
      }
      @Override
      public void run() {
            for(int i = 0; i < 10; i++)
            System.out.println(Thread.currentThread().getName() + " " + st.getValue());
      }
      
}
class Storage{
      int  i;
      public synchronized void setValue(int i){
            this.i = i;
      }
      public synchronized int getValue(){
            return this.i;
      }
}

[Henry Wong]

Modify the program to ensure that each number is printed exactly once, by adding suitable synchrnization

For code to "ensure that each number is printed exactly once", shouldn't there be a mechanism to track whether something is printed? Meaning.... Shouldn't the counter not store another number if the previous one haven't been printed yet? Shouldn't the printer not print the number, if it is the same number that was already printed?

Henry

[Jacob Sonia]

i read the consumer producer problem and came across this solution but unfortunately it is not working exactly. Please help

    
package KhalidMughal;

public class Counter implements  Runnable{
	private  Printer p1;
	public static void main(String[] args) {
		Storage store = new Storage();
		Counter c1 = new Counter(store);
		  
	}
	Storage st;
	public Counter(Storage store){
		st = store;
		p1 = new Printer(store); 
		new Thread(this, "Counter").start();
		
	}
	@Override
	public void run() {
		Integer i = 0;
		while(true){
			if(st.getValue() == null){
				st.setValue(i);
				
				synchronized(p1){
					p1.notify();
				}
				i++;
			}
			try{
				Thread.sleep(100);
			}catch(InterruptedException e){
				
			}
		}
		
	}

}
class Printer implements Runnable{
	Storage st;
	public Printer(Storage st){
		this.st = st;
		new Thread(this, "Printer").start();
	}
	@Override
	public void run() {
		while(true){
			while(st.getValue() != null)
				synchronized(this){
					try{
						wait();
					}catch(InterruptedException e){
						
					}
				}
		System.out.println(Thread.currentThread().getName() + " " + st.getValue());
		st.i = 0;
		}
	}
	
}
class Storage{
	Integer  i;
	public Storage(){
		if(i != null && i  == 10){
			System.exit(0);
		}
	}
	public void setValue(Integer i){
		this.i = i;
	}
	public Integer getValue(){
		return this.i;
	}
}

[Himanshu Kansal]

change lines 4 and 40 to "extends Thread" instead of implements Runnable. Conceptual problem in creating threads.

Regards

[Jacob Sonia]

    package KhalidMughal;

public class Counter extends Thread{
	private  Printer p1;
	public static void main(String[] args) {
		Storage store = new Storage();
		Counter c1 = new Counter(store);
		  
	}
	Storage st;
	public Counter(Storage store){
		st = store;
		
		p1 = new Printer(store); 
		new Thread(this, "Counter").start();
		
	}
	@Override
	public void run() {
		Integer i = 0;
		while(true){
			if(st.getValue() == null){
				st.setValue(i);
				
				synchronized(p1){
					p1.notify();
				}
				i++;
			}
			try{
				Thread.sleep(100);
			}catch(InterruptedException e){
				
			}
		}
		
	}

}
class Printer extends Thread{
	Storage st;
	public Printer(Storage st){
		this.st = st;
		new Thread(this, "Printer").start();
	}
	@Override
	public void run() {
		while(true){
			while(st.getValue() != null)
				synchronized(this){
					try{
						wait();
					}catch(InterruptedException e){
						
					}
				}
		System.out.println(Thread.currentThread().getName() + " " + st.getValue());
		st.i = 0;
		}
	}
	
}
class Storage{
	Integer  i;
	public Storage(){
		if(i != null && i  == 10){
			System.exit(0);
		}
	}
	public void setValue(Integer i){
		this.i = i;
	}
	public Integer getValue(){
		return this.i;
	}
}

Done it but i am again at the same problem

[Ken Truitt]

I modified your original code as follows and it appears to work:

   
    public class Counter implements  Runnable{
      
      public static void main(String[] args) {
            Storage store = new Storage();
            Printer p1 = new Printer(store);
            Counter c1 = new Counter(store);
      }
      Storage st;
      public Counter(Storage store){
            st = store;
            new Thread(this, "Counter").start();
            
      }
      @Override
      public void run() {
            for(int i = 0 ; i < 10; i++){
        synchronized (st) {
                  st.setValue(i);
                  st.notify();
                  try {
                    st.wait();
                  }catch(InterruptedException ie) { System.err.println("Interrupted - " + ie.getMessage()); }
        }
            }
            
      }

}
class Printer implements Runnable{
      Storage st;
      public Printer(Storage st){
            this.st = st;
            new Thread(this, "Printer").start();
      }
      @Override
      public void run() {
          int counter = 0;
          synchronized (st) {
            while (counter < 9) {
            try {
              st.wait();
            }
            catch(InterruptedException ie) { System.err.println("Interrupted - " + ie.getMessage()); }
            System.out.println(Thread.currentThread().getName() + " " + (counter=st.getValue()));
            st.notify();
            }
          }
      }
      
}
class Storage{
 
      int  i;
      public synchronized void setValue(int i){   //note these method names do not follow 
            this.i = i;                           //javabean naming standard
      }
      public synchronized int getValue(){
            return this.i;
      }
}

 

[Ankit Garg]

Jacob you need to understand basically how you should have used synchronization in this program. (BTW Ken your program will halt if the run method in Counter class runs before run in Printer class). You can use flags in your program to indicate whether a number has been printed or not. This is not necessary but it will take care of spontaneous thread wake-ups. Lets see this code

public class Counter implements  Runnable{
      
      Storage st;
      public Counter(Storage store){
            st = store;
      }
      @Override
      public void run() {
            synchronized(st) {
                  for(int i = 0 ; i < 10; i++){
                        while(!st.isPrinted()) {              //loop to take care of spontaneous wake-ups
                           try {
                              st.wait();
                           } catch(Exception e) {  }
                        }
                        st.setValue(i);
                        st.setPrinted(false);
                        st.notify();
                  }
            }
      }

}
class Printer implements Runnable{
      Storage st;
      public Printer(Storage st){
            this.st = st;
      }
      @Override
      public void run() {
            synchronized(st) {
                  for(int i = 0; i < 10; i++) {
                        while(st.isPrinted()) {              //loop to take care of spontaneous wake-ups
                              try {
                                    st.wait();
                              } catch(Exception e) {  }
                        }
                        System.out.println(Thread.currentThread().getName() + " " + st.getValue());
                        st.setPrinted(true);
                        st.notify();
                  }
            }
      }
      
}
class Storage{
      int  i;
      boolean printed = true;
      public void setValue(int i){
            this.i = i;
      }
      public int getValue(){
            return this.i;
      }
      public boolean isPrinted() {
            return printed;
      }
      public void setPrinted(boolean p) {
            printed = p;
      }
}
class MainClass {
      public static void main(String[] args) {
            Storage st = new Storage();
            Counter c = new Counter(st);
            Printer p = new Printer(st);
            new Thread(c,"Counter").start();  //start the counter
            new Thread(p,"Printer").start();   //start the printer
      }
}

The flag named printed in the program will let Counter class know if the value has been printed yet. If it has not been printed, then Counter will wait till the value is printed then it will increase the value of i. For the Printer class, the flag will be used to ensure that the value is not printed more than once. The two while loops are there to take care of spontaneous wake-ups. I don't remember if you need to worry about that for SCJP or not. In spontaneous wake-up, a thread comes out of a wait call without any call to notify on the subsequent object. To decrease complexity, you can replace those loops with if conditions...