During my interview, interviewer started his question with singleton pattern. I wrote below. Then, he asked Shouldn't we check for Nullity inside getInstance method?
I replied with, It is NOT necessary, since member is static type and is being initialized at the same time. But, seems like he was not satisfied with my answer.Am I correct or not ?
class Single {
private final static Single sing = new Single();
private Single() {
}
public static Single getInstance() {
return sing;
}
}
Now, next question he ask to write singleton class for multi-threaded environment. Then, I wrote double check singleton class.
class MultithreadedSingle {
private static MultithreadedSingle single;
private MultithreadedSingle() {
}
public static MultithreadedSingle getInstance() {
if(single==null){
synchronized(MultithreadedSingle.class){
if(single==null){
single= new MultithreadedSingle();
}
}
}
return single;
}
}
Then, he had an objection with using synchronized and double check and said It is useless. Why are you checking twice and why are you using synchronized ? I tried to convince him with multiple scenario. But, he didn't.
Later, at home I tried below code where I'm using simple singleton class with multiple thread.
public class Test {
public static void main(String ar[]) {
Test1 t = new Test1();
Test1 t2 = new Test1();
Test1 t3 = new Test1();
Thread tt = new Thread(t);
Thread tt2 = new Thread(t2);
Thread tt3 = new Thread(t3);
Thread tt4 = new Thread(t);
Thread tt5 = new Thread(t);
tt.start();
tt2.start();
tt3.start();
tt4.start();
tt5.start();
}
}
final class Test1 implements Runnable {
@Override
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + " : " + Single.getInstance().hashCode());
}
}
}
class Single {
private final static Single sing = new Single();
private Single() {
}
public static Single getInstance() {
return sing;
}
}
Below is the output :
Thread-0 : 1153093538
Thread-0 : 1153093538
Thread-0 : 1153093538
Thread-0 : 1153093538
Thread-0 : 1153093538
Thread-4 : 1153093538
Thread-1 : 1153093538
Thread-2 : 1153093538
Thread-3 : 1153093538
Thread-3 : 1153093538
Thread-3 : 1153093538
Thread-3 : 1153093538
Thread-3 : 1153093538
Thread-2 : 1153093538
Thread-2 : 1153093538
Thread-2 : 1153093538
Thread-2 : 1153093538
Thread-1 : 1153093538
Thread-1 : 1153093538
Thread-1 : 1153093538
Thread-1 : 1153093538
Thread-4 : 1153093538
Thread-4 : 1153093538
Thread-4 : 1153093538
Thread-4 : 1153093538
So, question is, Is it necessary to use synchronize or/and double check method in multi-threaded environment ? It seems like my first code itself (without adding any extra line of code) was the answer for both question. Any correction and knowledge share will be appreciated.