A simple Java program showing example usage of the synchronized keyword.

Collision.java

import java.lang.Runnable;
import java.lang.Thread;

/**
 * The Main Class.
 **/
public class Collision {

    /**
     * Main entry point into the application.
     **/
    public static void main(String[] args) {

        Incrementer incrementer = new Incrementer();
        Thread[] threads = new Thread[100];
        
        for (int i=0; i<100; i++) {
            threads[i] = new Thread(incrementer);
            threads[i].start();
        }

        for (Thread thread : threads) {
            try{
                thread.join();
            }
            catch (java.lang.InterruptedException e){
                System.out.println("Failed to join");
            }
        }

        System.out.println(String.format("Total: %s", incrementer.count));
    }
}

/**
 * Incrementer class used to increment the Incrementer Class.
 **/
class Incrementer implements Runnable{

    public int count;

    public void increment(){
        this.count ++;
    }

    /**
     * Implemented method, runs in a thread.
     **/
    @Override
    public synchronized void run(){
        this.increment();
        System.out.println(String.format("Count: %s", this.count));
    }
}