SCJP: Class Declarations #Java Short Notes #SCJP

class declarations

Start with modifiers such as public, private followed by class keyword
The class name, with the initial letter capitalized
The name of the class's parent (superclass), preceded by the keyword extends (if any). A class can only extend (subclass) one parent.
list of interfaces implemented by the class, preceded by the keyword implements (if any). A class can implement more than one interface
The class body, surrounded by braces, {}.
Class member variable declarations
- Requires three components, in order:
- Zero or more modifiers, such as public or private
- The field's type
- The field's name.

Abstract Classes

A class declared with abstract keyword is an abstract class. It may or may not include abstract methods
Abstract classes cannot be instantiated
Abstract classes can be subclassed
Class containing abstract methods, must be declared to be abstract
Abstract methods are methods declared without implementation (braces)
The subclass of an abstract class must provide implementations of all the abstract methods otherwise the subclass itself needs to be declared as abstract.
An abstract class may have static and final fields and methods. Interfaces can not
If an abstract class contains only abstract method declarations with no implementations, it should better be declared as an interface
It's a good design to declare a common abstract class with common methods with implementations for several very related classes (will extend the abstract class)
An abstract class can implement an interface but are not bound to implement all interface methods

Nested Classes

Classes declared under another class are called nested classes
Two types: Static Nested: declared with static keyword, Inner Nested: declared with no static keyword
inner classes have access to other members of the outer class including private members
Static nested classes do not have access to other members of the outer class
Static nested classes are accessed using the outer class name such as: OuterClass.StaticNestedClass
To use inner classes, the outer class must be instantiated first. Then, inner object can be created as follows: OuterClass.InnerClass innerObject = outerObject.new InnerClass();
Local inner classes: Declared within the body of a method
Anonymous inner classes: Declared within the body of a method without naming it
Inner classes may have similar access modifiers like other outer class members
Why use nested classes:
- For logically grouping classes that are only used in one place
- It increases encapsulation.
- May lead to more readable and maintainable code

From: http://sitestree.com/?p=4866
Categories:Java Short Notes, SCJP
Tags:
Post Data:2012-09-13 21:25:27

উন্মুক্ত শিক্ষার আসর – Open Education