Interview FAQ's In JAVA - Part IV

[sivaji]

Interview FAQ's In JAVA

Q: What are different types of inner classes?

A: Nested top-level classes, Member classes, Local classes, Anonymous classes

    Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class
    just like any other top-level class.

    Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a
    package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner
    interfaces. All of these are of the nested top-level variety.

    Member classes - Member inner classes are just like other member methods and member variables and access to the
    member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested 
    top-level class. The primary difference between member classes and nested top-level classes is that member classes
    have access to the specific instance of the enclosing class.

    Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of
    their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a
    more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and
    static are not usable.

    Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have
    no name, you cannot provide a constructor.

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

A: 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 ABCD
     location: package io
     import java.io.ABCD;


Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import
     com.MyTest.UnitTests.*?

A: 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.


Q: What is the difference between declaring a variable and defining a variable?

A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means
    declaration + initialization.

    e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.


Q: What is the default value of an object reference declared as an instance variable?

A: null unless we define it explicitly.


Q: Can a top level class be private or protected?

A: No. A top level class can not 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.


Q: What type of parameter passing does Java support?

A: In Java the arguments are always passed by value .


Q: Primitive data types are passed by reference or pass by value?

A: Primitive data types are passed by value.


Q: Objects are passed by value or by reference?

A: 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 .


Q: What is serialization?

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


Q: How do I serialize an object to a file?

A: 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.


Q: Which methods of Serializable interface should I implement?

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


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

A: 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.


Q: What is the common usage of serialization?

A: 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.


Q: What is Externalizable interface?

A: 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.