Learning Java, use of synchronized keyword

Question

so i was testing with synchronized keyword. Here is an example that I tried:

public class MyTest {     static int i = 0;     public static void main(String[] args) {         new Thread(t1).start();         new Thread(t2).start();     }      private static void countMe(String name){         i++;         System.out.println("Current Counter is: " + i + ", updated by: " + name);     }      private static Runnable t1 = new Runnable() {         public void run() {             try{                 for(int i=0; i<5; i++){                     countMe("t1");                 }             } catch (Exception e){}          }     };      private static Runnable t2 = new Runnable() {         public void run() {             try{                 for(int i=0; i<5; i++){                     countMe("t2");                 }             } catch (Exception e){}        }     }; }   

When I run it, the output of calling countMe() method from two threads generates this output:

Current Counter is: 1 Current Counter is: 2 Current Counter is: 4 Current Counter is: 5 Current Counter is: 6 Current Counter is: 7 Current Counter is: 3 Current Counter is: 8 Current Counter is: 9 Current Counter is: 10   

And when I change the method countMe() to:

private synchronized static void countMe(){         i++;         System.out.println("Current Counter is: " + i); }   

I get this output:

Current Counter is: 1 Current Counter is: 2 Current Counter is: 3 Current Counter is: 4 Current Counter is: 5 Current Counter is: 6 Current Counter is: 7 Current Counter is: 8 Current Counter is: 9 Current Counter is: 10   

Although this gives me clear understanding the purpose of synchronized, I want to know is there any other reason as well, that we can use synchronized. Or what I have done here, is the only eason why we need the use of this synchronized keyword?

Thanks.

EDIT: Another thing that I am confused with is that in first output why the counter went to 3 after 7. It seems a bit impossible to me, but similar results do happen every time I try, is this normal?

Answer 1

Two things:

First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

Synchronized methods enable a simple strategy for preventing thread interference and memory consistency errors: if an object is visible to more than one thread, all reads or writes to that object's variables are done through synchronized methods. (An important exception: final fields, which cannot be modified after the object is constructed, can be safely read through non-synchronized methods, once the object is constructed).

source: http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html