Creating two threads in Java

Question

I want to create two threads.One thread will have to print odd numbers and the other thread will have to print even numbers between 1-20 numbers. This is the code I have attempted so far but it doesn't give the expected output.

public class JavaApplication40 extends Thread {
    @Override
    public void run(){
        while (true){
            for (int i=0;i<=20;i++){
            if(i%2!=0){
                System.out.println("odd " +i);
            }
        }
        }
    }

    public static void main(String[] args) {
        Thread t =new JavaApplication40();

        for (int x=0;x<=20;x++){
            if(x%2==0){
             System.out.println("even " + x);   
            }
        }

    }

}

This code only outputs even numbers.I want odd numbers too.Someone please tell me how I can modify the above code to get the expected output.

You did not start thread. Submitting in an executor is good. — huseyin tugrul buyukisik, Apr 16 '16 at 19:50
should I call the start method after this code? Thread t =new JavaApplication40(); where should i include the start method? — Char, Apr 16 '16 at 19:51
Just after that is fine but doesnt guarantee it starts immediately — huseyin tugrul buyukisik, Apr 16 '16 at 19:54
yes but when i call start after only odd numbers are displayed and only one even number was displayed throughout its ongoing execution.why is that? — Char, Apr 16 '16 at 19:57
@Char you should include the `start()` method at the point you would like the thread start. This is usually just after creating it, assume you created it for a reason. — Peter Lawrey, Apr 16 '16 at 19:58
@PeterLawrey does that mean I have to implement runnable to get this output?Can't I stick to extending the thread class? — Char, Apr 16 '16 at 20:01
@Char You can extend Thread but it is a bad habit to get into and one day it will cause you enough pain you won't want to do it again. — Peter Lawrey, Apr 16 '16 at 20:02
http://stackoverflow.com/questions/13786083/create-two-threads-one-display-odd-other-even-numbers/36698921#36698921 — laish138, Apr 18 '16 at 16:10

Peter Lawrey · Accepted Answer · 2016-04-16T19:57:09.493

After you create a Thread you need to call start() it to start it. Try calling

t.start();

Additionally, You should extend Thread. Instead you should implement Runnable and wrap it with a Thread. Also you don't need to check if a value is odd or even if you make sure the value is always odd or even.

public class JavaApplication40 {
    public static void main(String[] args) {
        Thread t = new Thread(new Runnable() {
            public void run() {
                // starting at 1 and counting by 2 is always odd
                for (int i = 1; i < 10000; i += 2)
                    System.out.println("odd " + i);
            }
        });
        t.start();

        // starting at 0 and counting by 2 is always even
        for (int x = 0; x < 10000; x+=2)
             System.out.println("even " + x);       
    }
}

Note: the whole point of using Threads is to have independent execution and Threads take time to start. This means you could get all the even and all the odd number together. i.e. it prints them faster than a thread starts.

You might need to print much more numbers to see them printed at the same time. e.g. 10_000 instead of 20.

you have called the start method after creating the thread t right? — Char, Apr 16 '16 at 20:11
@Char that is what it does, note it will take time to start so you can see many `even` messages before the first `odd` message. — Peter Lawrey, Apr 16 '16 at 20:14

score 1 · Answer 2 · answered Apr 16 '16 at 19:50

1

You created your thread, but never started it.

Thread t =new JavaApplication40();

creates it, when started, it will call run()

Start it with t.start()

answered Apr 16 '16 at 19:50

Jaims

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score 1 · Answer 3 · answered Apr 16 '16 at 19:53

1

ExecutorService es=Executors.newFixedThreadPool(2);
es.submit(t);
for ....

es.shutdown(); // .shutdownNow() for infinite looped threads

answered Apr 16 '16 at 19:53

huseyin tugrul buyukisik

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Creating two threads in Java

3 Answers3