Question

as i said ,
what is the difference between
public static synchronized .. and
public static ?
any example ?

Answer 1

One point you have to be careful about (several programmers generally fall in that trap) is that there is no link between synchronized static methods and non synchronized static methods, ie:

class A {
    public static synchronized f() {...} //class level lock
    public static g() {...} //object level lock
}
public class TestA{
    public static void main(String[] args){
        A a = new A();
       //Thread 1:
       a.f();
       //Thread 2:
       a.g();
    }
}

f() and g() are not synchronized with each other and thus can execute totally concurrently.