December 3, 2009 at 10:47 AM
An identifier is used to identify a package, class, interface, method, or variable. It allows a programmer to refer to the item from other places in the program.It may be a word beginning with a letter or underscore, and continuing with letters, numbers, and underscores.
Overloaded methods have the same names but different argument lists. The arguments may differ in type or number, or both. However, the return types of overloaded methods can be the same or different.
A class is the blueprint from which individual objects are created.
Object is an instance of a class.It has state,behaviour and identity.
Constructors are used to initialize the instance variables (fields) of an object.Constructors are similar to methods, but there are some differences:
* There is no return type given in a constructor signature (header). The value is this object itself so there is no need to indicate a return value.
* There is no return statement in the body of the constructor.
* The first line of a constructor must either be a call on another constructor in the same class (using this), or a call on the superclass constructor (using super). If the first line is neither of these, the compiler automatically inserts a call to the parameterless super class constructor.
Interface is the collection of methods with empty implementations and constants variables ( variables with static and final declarations ). All the methods in an interface are "public and abstract" by default. Since interfaces are abstract in nature so they can not be directly instantiated.
Access modifiers are used to specify the visibility and accessibility of a class, member variables and methods. Java provides some access modifiers like: public, private etc.. These can also be used with the member variables and methods to specify their accessibility.
1. public keyword specifies that the public class, the public fields and the public methods can be accessed from anywhere.
2. private: This keyword provides the accessibility only within a class i.e. private fields and methods can be accessed only within the same class.
3. protected: This modifier makes a member of the class available to all classes in the same package and all sub classes of the class.
4. default : Its not a keyword. When we don't write any access modifier then default is considered. It allows the class, fields and methods accessible within the package only.