If you declare the method as synchronized (as you're doing by typing public synchronized void addA()) you synchronize on the whole object, so two thread accessing a different variable from this same object would block each other anyway.
If you want to synchronize only on one variable at a time, so two threads won't block each other while accessing different variables, you have synchronize on them separately in synchronized () blocks. If a and b were object references you would use:
public void addA() {
synchronized( a ) {
a++;
}
}
public void addB() {
synchronized( b ) {
b++;
}
}
But since they're primitives you can't do this.
I would suggest you to use AtomicInteger instead:
import java.util.concurrent.atomic.AtomicInteger;
class X {
AtomicInteger a;
AtomicInteger b;
public void addA(){
a.incrementAndGet();
}
public void addB(){
b.incrementAndGet();
}
}