Synchronized method and threads in java Yasser Ibrahim @Jobilize

Author: Yasser Ibrahim

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Question 216 / 297: What are the possible outputs of running this program once as it is, and second with marking swim() synchronized?
Choose 2

class Swimmer implements Runnable{
	String name ;
	Swimmer(String name){
		this.name = name;
	}
	public void run() {
		new  Test().swim(name);
	}
	
}

public class Test {

	public void swim(String name) {
		System.out.print(name);
		System.out.print(name);
				
	}
	public static void main(String[] args) {
		new Thread(new Swimmer("Tom")).start();
		new Thread(new Swimmer("Hanks")).start();
	}
}

as it is, the output is always consecutive two Tom then two Hanks

or consecutive two Hanks then two Tom

as it is, the output is undetermined. It is in any order of two Tom and two Hanks

by synchronizing swim(), the output is always consecutive two Tom then two Hanks or

consecutive two Hanks then two Tom

by synchronizing swim(), the output is undetermined. It is in any order of

two Tom and two Hanks

<< First < Previous Next > Last >>

Explanation:

When running the program as it is, the two threads are invoking swim() on two different Test objects, which means they are not competing on the same object, therefore the output is undetermined. The output could be in any order of two Tom and two Hanks

When synchronizing the anon-static method method, if two threads competing to execute swim() for the same Test object, the first thread to enter the method will seize the lock of this Test object and will only release it when it finishes executing swim(). That means any other thread has to wait for the lock to be released to be able to enter swim(). But the trick in our program, that each thread is invoking swim() on a different Test object, so actually the two threads are not competing on the same object.

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