Atomic integer multithreaded example

AtomicIntegerExample.java

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicIntegerExample {
    public static void main(String args[]) throws InterruptedException {
        AtomicInteger a = new AtomicInteger();
        NonAtomicInteger n = new NonAtomicInteger();

        Runnable r = () -> {
            for(int j = 0; j < 100_000; j++) {
                a.getAndIncrement();
                n.getAndIncrement();
            }
        };


        Thread t1 = new Thread(r);
        Thread t2 = new Thread(r);
        Thread t3 = new Thread(r);
        Thread t4 = new Thread(r);

        t1.start();
        t2.start();
        t3.start();
        t4.start();


        t1.join();
        t2.join();
        t3.join();
        t4.join();

        System.out.printf("The atomic integer should be 400000, it is %d\n", a.get());
        System.out.printf("The non-atomic integer should be 400000, it is %d\n", n.get());
    }

    public static class NonAtomicInteger {
        private int i = 0;
        public NonAtomicInteger() { i = 0; }
        public int getAndIncrement() { return i++; }
        public int get() { return i; }
    }
}

When I run this on my 4-core machine, I get:

$ javac AtomicIntegerExample.java && java AtomicIntegerExample
The atomic integer should be 400000, it is 400000
The non-atomic integer should be 400000, it is 295007