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Q9.Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA?
Null interfaces act as markers..they just tell the compiler that the objects of this class need to be treated differently..some marker interfaces are : Serializable, Remote, Cloneable

Q10. Is synchronised a modifier?indentifier??what is it??
It's a modifier. Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q11.What is singleton class?where is it used?
Singleton is a design pattern meant to provide one and only one instance of an object. Other objects can get a reference to this instance through a static method (class constructor is kept private). Why do we need one? Sometimes it is necessary, and often sufficient, to create a single instance of a given class. This has advantages in memory management, and for Java, in garbage collection. Moreover, restricting the number of instances may be necessary or desirable for technological or business reasons--for example, we may only want a single instance of a pool of database connections.

Q12.What is a compilation unit?
The smallest unit of source code that can be compiled, i.e. a .java file.