21. Do I need to import java.lang package any time? Why ?  

No. It is by default loaded internally by the JVM.

22. Can I have multiple main() methods in the same class?  

No the program fails to compile. The compiler says that the main() method is already defined in the class.

23. What are Checked and UnChecked Exception?  

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown.
Example: IOException thrown by's read() method•
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown.
Example: StringIndexOutOfBoundsException thrown by String's charAt() method• Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

24. What is Overriding?  

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

25. Are the imports checked for validity at compile time? Example: will the code containing an import such as java.lang.ABCD compile?  

Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying, can not resolve symbol
symbol : class lazy
location: package io

26. Does importing a package imports the subpackages as well? Example: Does importing com.MyTest.* also import com.MyTest.UnitTests.*?  

No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.

27. What is the difference between declaring a variable and defining a variable?  

In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
Example: String s; is just a declaration while String s = new String ("lazy"); Or String s = "lazy"; are both definitions.

28. What is the default value of an object reference declared as an instance variable?  

The default value will be null unless we define it explicitly.

29. Can a top level class be private or protected?  

No. A top level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.
If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.

30. What type of parameter passing does Java support?  

In Java the arguments are always passed by value.

31. Primitive data types are passed by reference or pass by value?  

Primitive data types are passed by value.

32. Objects are passed by value or by reference?  

Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object.

33. What is serialization?  

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

34. How do I serialize an object to a file?  

The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

35. Which methods of Serializable interface should I implement?  

The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

36. How can I customize the seralization process? i.e. how can one have a control over the serialization process?  

Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal.
You should implement these methods and write the logic for customizing the serialization process.

37. What is the common usage of serialization?  

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

38. What is Externalizable interface?  

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.
Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

39. When you serialize an object, what happens to the object references included in the object?  

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process.
Thus when an object is serialized, all the included objects are also serialized along with the original obect.

40. What one should take care of while serializing the object?  

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

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