Why do we need Synchronization?

• The "Synchronized" keywords prevents concurrent access to a block of code or object by multiple Threads.  
• Synchronized keyword in Java provides locking, which ensures mutual exclusive access of shared resource and prevent data race.
• To prevent consistency problem and thread interference .

Producer consumer problem:

Common   class
package com.instanceofjavaforus;

public class Common {
    int x;
   
    boolean flag=true;
    //if flag is true producer thread has to produce
    // if flag is false consumer thread has to produce

    synchronized public void produce(int i){
       
        if(flag){
           
            x=i;
            System.out.println("producer thread has produced "+i);
            flag=false;
            notify();
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }      
        }
     }
   

    synchronized public int consume(){
        if(flag){
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
           
        }
        flag=true;
        notify();
        return x;
    }
   
}

package com.instanceofjavaforus;

public class ProducerThread extends Thread {
    Common c;
   
    ProducerThread(Common c){
        this.c=c;
    }
   
    public void run(){
        int i=0;
        while(true){
           
            i++;
            c.produce(i);
           
           
            try {
                Thread.sleep(600);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } 
        } 
    } 
}

ConsumerThread:

package com.instanceofjavaforus;

public class ConsumerThread extends Thread {
   
    Common c;
   
    ConsumerThread(Common c){
        this.c=c;
    }
   
    public void run(){
       
        while(true){
           
            int x=c.consume();
            System.out.println("Consumer consumes"+x);
            try {
                Thread.sleep(600);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } 
        } 
    }
}

ProducerConsumerTest:

package com.instanceofjavaforus;

public class ProducerConsumerTest {
   
    public static void main(String args[]){
        Common c= new Common();
        ProducerThread pt=new ProducerThread(c);
        ConsumerThread ct= new ConsumerThread(c);
        pt.start();
        ct.start();
    }
}

Output:

producer thread has produced 1
Consumer consumed 1
producer thread has produced 2
Consumer consumed 2
producer thread has produced 3
Consumer consumed 3
producer thread has produced 4
Consumer consumed 4
producer thread has produced 5
Consumer consumed 5
producer thread has produced 6
Consumer consumed 6