Concurrent Thread Increment: Java Program to Create and Start Multiple Threads

Java Multithreading: Exercise-1 with Solution

Write a Java program to create and start multiple threads that increment a shared counter variable concurrently.

Sample Solution:

Java Code:

//IncrementThread.java
public class IncrementThread extends Thread {
  private Counter counter;
  private int incrementsPerThread;
  public IncrementThread(Counter counter, int incrementsPerThread) {
    this.counter = counter;
    this.incrementsPerThread = incrementsPerThread;
  }
  @Override
  public void run() {
    for (int i = 0; i < incrementsPerThread; i++) {
      counter.increment();
    }
  }
}

//Counter.java
public class Counter {
  private int count = 0;

  public synchronized void increment() {
    count++;
  }
  public int getCount() {
    return count;
  }
}

//Main.java
public class Main {
  public static void main(String[] args) {
    Counter counter = new Counter();
    int numThreads = 6;
    int incrementsPerThread = 10000;
    IncrementThread[] threads = new IncrementThread[numThreads];

    // Create and start the threads
    for (int i = 0; i < numThreads; i++) {
      threads[i] = new IncrementThread(counter, incrementsPerThread);
      threads[i].start();
    }

    // Wait for all threads to finish
    try {
      for (IncrementThread thread: threads) {
        thread.join();
      }
    } catch (InterruptedException e) {
      e.printStackTrace();
    }

    // Print the final count
    System.out.println("Final count: " + counter.getCount());
  }
}

Sample Output:

Final count: 60000

Explanation:

In the above exercise -

• First, we define a "Counter" class that represents a shared counter variable. It has a synchronized increment() method that increments the counter variable by one.
• Next we define an "IncrementThread" class that extends Thread. Each IncrementThread instance increments the shared counter by a specified number of increments.
• In the Main class, we create a 'Counter' object, specify the number of threads and increments per thread, and create an array of 'IncrementThread' objects. We then iterate over the array, creating and starting each thread.
• After starting all the threads, we use the join() method to wait for each thread to finish before proceeding. After all threads have finished, we print the shared counter's final count.

Flowchart:

Java Code Editor:

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