Printing Alphabets and Numbers using multi-threading

Question

I have just started learning threads and pretty new to it.I'm trying to print alphabets and numbers one after the other.I have synchronized them using a flag but of no use.

public class Alphabets {

    public static void main(String[] args) {

              AN an= new AN(false);

              Thread t1=new Thread(new Runnable() {

                @Override
                public void run() {

                    try {

                        an.Alpha();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            });


              Thread t2= new Thread(new Runnable() {

                @Override
                public void run() {
                    try {
                        an.numbers();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }

                }
            });


              t1.start();
              t2.start();
    }

}

...

 class AN
    {

        boolean flag;

        AN(boolean flag)
        {
            this.flag=flag;
        }
        synchronized void Alpha() throws InterruptedException
        {
            if(flag==false)
            {
            for(char i='A'; i<='Z';i++)
            {
                System.out.println(+i);
                notifyAll();
                flag=true;
            }
            }
            else
            {
                wait();
            }

        }

        synchronized void numbers() throws InterruptedException
        {
            if(flag==true)
            {
            for(int i=1;i<=26;i++)
            {
                System.out.println(+i);
                notifyAll();
                flag=false;
            }
            }
            else
            {
                wait();
            }
        }


    }

My desired output is : a1b2c3d4....

My console output is : abcd...1234...

Can anybody point out the mistake since I'm unable to synchronize these two threads.

Answer 1

Change class AN to check the flag in while loop.

public class AN {
  boolean flag;

  AN(boolean flag) {
    this.flag = flag;
  }

  synchronized void Alpha() throws InterruptedException {

    for(char i = 'A'; i <= 'Z'; i++) {
      while(flag == true) {
        wait();
      }
      System.out.println(i);
      notifyAll();
      flag = true;
    }
  }

  synchronized void numbers() throws InterruptedException {

    for(int i = 1; i <= 26; i++) {
      while(flag == false) {
        wait();
      }
      System.out.println(i);
      notifyAll();
      flag = false;
    }
  }

Answer 2

Well, what you want is more or less pipelining, therefore the threads need to know when they are allowed to work.

I'd therefore use a Queue to await the input on one thread and wait for it to be set in the other thread.

Class AN:

import java.util.concurrent.BlockingQueue;

class AN
{

    BlockingQueue<Boolean> input;

    BlockingQueue<Boolean> output;

    AN(BlockingQueue<Boolean> input, BlockingQueue<Boolean> output)
    {

        this.input = input;
        this.output = output;
    }

    void Alpha() throws InterruptedException
    {
        for (char i = 'a'; i <= 'z'; i++) {
            input.take();
            System.out.print(i);
            output.put(Boolean.TRUE);

        }
    }

    void numbers() throws InterruptedException
    {
        for (int i = 1; i <= 26; i++) {
            input.take();
            System.out.print(i);
            output.put(Boolean.TRUE);
        }

    }

}

Class Test (or where your main is):

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class Test
{

    public static void main(String[] args) throws InterruptedException
    {

        BlockingQueue<Boolean> input = new LinkedBlockingQueue<>();
        BlockingQueue<Boolean> output = new LinkedBlockingQueue<>();

        AN an1 = new AN(output, input);
        AN an2 = new AN(input, output);
        output.add(Boolean.TRUE);

        Thread t1 = new Thread(new Runnable()
        {

            @Override
            public void run()
            {
                try {
                    an1.Alpha();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        });

        Thread t2 = new Thread(new Runnable()
        {

            @Override
            public void run()
            {
                try {
                    an2.numbers();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }
        });

        t1.start();
        t2.start();

    }
}

Outputs: a1b2c3d4e5f6g7h8i9j10k11l12m13n14o15p16q17r18s19t20u21v22w23x24y25z26

Answer 3

You can achieve this by using BlockingQueue and Object's wait and notify methods.

public class AlphaNum {
    public static void main(String[] args) throws InterruptedException {


    BlockingQueue<String> queue = new ArrayBlockingQueue<String>(10);

    AtomicBoolean flag = new AtomicBoolean(Boolean.TRUE);

    Object lock = new Object();

    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {

            try {
                for(int i=1;i<=26;i++){
                        synchronized (lock){
                            while (flag.get()){
                                lock.wait();
                            }
                            System.out.print(i);
                            flag.set(Boolean.TRUE);
                            lock.notify();
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    });

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                for(char c='A';c<='Z';c++){
                    synchronized (lock){
                        while (!flag.get()){
                            lock.wait();
                        }
                        System.out.print(c);
                        flag.set(Boolean.FALSE);
                        lock.notify();
                    }
                }

            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    });


    t1.start();
    t2.start();

    }
}

The above code prints a1b2c3d4.......z26

Answer 4

There are many ways to achieve this, Below one is one of them.

package interview;

public class TestAplhaAndNumberPrinterThreads {

    // created object as static so we can access print method of this class from thread's run methods
    public static TestAplhaAndNumberPrinterThreads output = new TestAplhaAndNumberPrinterThreads();

    private final Object syncer = new Object();
    private int state = 0;

    public void print(char character) throws InterruptedException {
        synchronized (syncer) {
            while (true) {
                if (state == 0) {
                    System.out.print(character + ", ");
                    state = 1;
                    syncer.notify();
                    return;
                } else {
                    syncer.wait();
                }
            }
        }
    }

    public void print(int number) throws InterruptedException {
        synchronized (syncer) {
            while (true) {
                if (state == 1) {
                    System.out.print(number + ", ");
                    state = 0;
                    syncer.notify();
                    return;
                } else {
                    syncer.wait();
                }
            }
        }
    }

    public static void main(String[] args) {
        NumberPrinter numberPrinter = new NumberPrinter();
        AlphabetsPrinter alphabetsPrinter = new AlphabetsPrinter();
        numberPrinter.start();
        alphabetsPrinter.start();
    }
}

class NumberPrinter extends Thread {

    @Override
    public void run() {
        try {
            for (int i = 1; i <= 26; i++) {
                TestAplhaAndNumberPrinterThreads.output.print(i);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

class AlphabetsPrinter extends Thread {

    @Override
    public void run() {
        try {
            for (char i = 'a'; i <= 'z'; i++) {
                TestAplhaAndNumberPrinterThreads.output.print(i);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

// output: a, 1, b, 2, c, 3, d, 4, e, 5, f, 6, g, 7, h, 8, i, 9, j, 10, k, 11, l, 12, m, 13, n, 14, o, 15, p, 16, q, 17, r, 18, s, 19, t, 20, u, 21, v, 22, w, 23, x, 24, y, 25, z, 26,

Answer 5

For you i think the big things is , you need to know how to use breakPoint to debug in ide , and than you will see why your code not work. you can see the process of your code will run i post the picture in the last , and i share you this code which will work like your wish

class AN {

boolean flag;

AN(boolean flag) {
    this.flag = flag;
}

synchronized void Alpha() throws InterruptedException {
    if (flag == false) {
        for (char i = 'A'; i <= 'Z'; i++) {
            System.out.println(i);
            flag = true;
            notify();
            wait();
        }
    }

}

synchronized void numbers() throws InterruptedException {
    if (flag == true) {
        for (int i = 1; i <= 26; i++) {
            System.out.println(+i);
            flag = false;
            notify();
            wait();
        }
    }
}

}

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