Question

The method Thread.yield:

Causes the currently executing thread object to temporarily pause and
allow other threads to execute.

So in the following code:

public class Test implements Runnable {  

    private int stopValue;  

    public Fib(int stopValue){  
        this.stopValue = stopValue;  
    }  

    @Override  
    public void run() {  

        System.out.println("In test thread");  
        for(int i = 0; i < stopValue; i++){  
            c = i + 1;  
        }  

        System.out.println("Result = "+c);        
    }  

    public static void main(String[] args){  
        int defaultStop = 1024;  
        if(args.length > 0){   
            defaultStop = Integer.parseInt(args[0]);  
        }  
        Thread a = new Thread(new Fib(defaultStop));  
        System.out.println("In main");  
        a.setDaemon(true);  
        a.start();  
        Thread.yield();       
        System.out.println("Back in main");  
    }  

}

I expect that I should see:

1. In main then
2. In test thread

and the rest would be undefined. But I don’t understand why sometimes I only see:
In main and Back in main and not any print statement from Test thread?

Answer 1

yield() is a hint to the OS scheduled but doesn’t provide any guarantees in terms of scheduling. It doesn’t always pause very long. If you call it repeatly it might only take a few micro-seconds.

Starting a thread takes time and even if you main thread pauses briefly, it may finish before the background thread starts.

---

As you can see, yield() pauses very briefly.

long start = System.nanoTime();
long runs = 20000000;
for (int i = 0; i < runs; i++)
    Thread.yield();
long time = System.nanoTime() - start;
System.out.printf("Thread.yield() took an average of %,d ns.%n", time / runs);

prints

Thread.yield() took an average of 148 ns.

by comparison, System.nanoTime take longer on my machine.

long start = System.nanoTime();
long runs = 20000000;
for (int i = 0; i < runs; i++)
    System.nanoTime();
long time = System.nanoTime() - start;
System.out.printf("System.nanoTime() took an average of %,d ns.%n", time / runs);

prints

System.nanoTime() took an average of 656 ns.

Both times will vary from OS to OS and machine to machine.