Basic Java Questions(Core Java Faq) part 3

What is Constructor?

       A constructor is a special method whose task is to initialize the object of its class.

       It is special because its name is the same as the class name.

       They do not have return types, not even void and therefore they cannot return values.

       They cannot be inherited, though a derived class can call the base class constructor.

       Constructor is invoked whenever an object of its associated class is created.

How does the Java default constructor be provided?

If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

Can constructor be inherited?

No, constructor cannot be inherited, though a derived class can call the base class constructor.

What are the differences between Contructors and Methods?

	Constructors	Methods
Purpose	Create an instance of a class	Group Java statements
Modifiers	Cannot be abstract, final, native, static, or synchronized	Can be abstract, final, native, static, or synchronized
Return Type	No return type, not even void	void or a valid return type
Name	Same name as the class (first letter is capitalized by convention) -- usually a noun	Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
this	Refers to another constructor in the same class. If used, it must be the first line of the constructor	Refers to an instance of the owning class. Cannot be used by static methods.
super	Calls the constructor of the parent class. If used, must be the first line of the constructor	Calls an overridden method in the parent class
Inheritance	Constructors are not inherited	Methods are inherited

How are this() and super() used with constructors?

       Constructors use this to refer to another constructor in the same class with a different parameter list.

       Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

What are the differences between Class Methods and Instance Methods?

Class Methods	Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class	Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword. Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members.	Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class.	Instance methods are not declared as static.

How are this() and super() used with constructors?

       Constructors use this to refer to another constructor in the same class with a different parameter list.

       Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

What are Access Specifiers?

One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

What are Access Specifiers available in Java?

Java offers four access specifiers, listed below in decreasing accessibility:

       Public- public classes, methods, and fields can be accessed from everywhere.

       Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.

       Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.

       Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.

Situation	public	protected	default	private
Accessible to class  from same package?	yes	yes	yes	no
Accessible to class  from different package?	yes	no, unless it is a subclass	no	no

What is final modifier?

The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.

       final Classes- A final class cannot have subclasses.

       final Variables- A final variable cannot be changed once it is initialized.

       final Methods- A final method cannot be overridden by subclasses.

What are the uses of final method?

There are two reasons for marking a method as final:

       Disallowing subclasses to change the meaning of the method.

       Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

What is static block?

Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

What are static variables?

Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.

              static type  varIdentifier;

where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

What is the difference between static and non-static variables?

A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.

What are static methods?

Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:

       A static method can only call other static methods.

       A static method must only access static data.

       A static method cannot reference to the current object using keywords super or this.

       reference to the current object using keywords super or this.