Synchronized Block v.s. Synchronized Method

First of all to achieve Multithreading mechanism in java we should go for synchronization. And this can be done in two ways depending on the requirement.
1. Synchronized block and
2. Synchronized method.
if you go for synchronized block it will lock a specific object.
if you go for synchronized method it will lock all the objects.
Inother way Both the synchronized method and block are used to acquires the lock for an object. But the context may vary. Suppose if we want to invoke a critical method which is in a class whose access is not available then synchronized block is used. Otherwise synchronized method can be used.
Synchronized methods are used when we are sure all instance will work on the same set of data through the same function Synchronized block is used when we use code which we cannot modify ourselves like third party jars etc
For a detail clarification see the below code for example:
 
//Synchronized block
class A
{ public void method1() {...} }
class B
{
public static void main(String s[])
{ A objecta=new A();
A objectb=new A();
synchronized(objecta){objecta.method1();}
objectb.method1(); //not synchronized
}
}

//synchronized method
class A
{ public synchronized void method1() { ...}
}
class B
{
public static void main(String s[])
{
A objecta=new A();
A objectb =new A();
objecta.method1(); objectb.method2();
}
}