Question

Can anyone explain this code:

public class Main implements Runnable{

private int i;
public synchronized void run() {
    System.out.print("i = "+ i +"\n");
    if (i % 5 != 0) {
        i++;
    }
    for (int x = 0; x < 5; x++, i++) {
        if (x > 1) {
            Thread.yield();
        }
    }
}

 public static void main(String[] args) {
    Main n = new Main();
    for (int x = 100; x > 0; --x) {
        new Thread(n).start();
    }
}
}

and how the output is

i= 0
i= 5
i= 10
i= 15
i= 20
.
.
.
i= 499

Answer 1

The bottom loop creates 100 threads. Each thread is run immediately, in parallel with the others. In each thread,

• at the start, the current value of shared-attribute ‘i’ is printed
• if i is not divisible by 5, it is incremented once
• then a loop is executed, up to 5 times. The last 3 times it is executed, it yields to other threads. Every time this loop executes, it increments shared-variable ‘i’

Therefore, i is incremented 5 times if it was divisible by 5, and 6 times if it was not. But, the run() method is synchronized. Therefore, no other thread is allowed to execute while one of them is already executing: your program is effectively single-threaded, yield()s are ignored (no other thread can execute), and the output identical to

for (int i=0; i<500; i+= 5) System.out.println(i);
System.out.println(499);

Remove the synchronized to see all sorts of race-conditions altering the output.