what is interface

interface

Definition:

Interface is a pure abstract class that contains

public static final variables,

public Abstract methods

It provides only forcibility (specification) to subclass.

Need of interface:

Interface is invented in java for below two reasons

1) For releasing specification

2) For implementing multiple inheritance

Syntax:

[public/] interface { public static final data-type variable-name=variable value; ---------- -----------------------------------------------------------------; ----------------------------------------------------------------------------; public abstract return-type method-name(arguments list…); ---------- -----------------------------------------------------------------; ----------------------------------------------------------------------------; }

Creating interface

Using keyword “interface” programmers can develop a class of type interface.

We can create public and default interface, but we can’t create private and protected interfaces

The default access specifier of interface is

Before interface-name we can place “abstract” keyword in our .java file but in compilation phase compiler removes abstract keyword and generates .class file

Variables in interfaces

In interface by default all variables are public static final variables.

Public – because it as to access from ay where.

Static – because we can’t instance interface.

Final – because it is common value for all subclases

Since interface variables are static final, it must be initialized else it leads to

CE symbol “=” expected

Methods in interfaces

In interface by default all methods are public and abstract methods

since methods has to access from anywhere outside the class.

By default interface members access specifier is public

/*this is our written code means InterfaceDemo1.java file*/ interface InterfaceDemo1 { public static final String shape = "Quadrilateral"; int sides = 4; public abstract double areaOfQuadrilateral(); double perimeterOfQuadrilateral(); } /*this is the compiler changed code means InterfaceDemo1.class file*/ interface InterfaceDemo1 { public abstract double areaOfQuadrilateral(); public abstract double perimeterOfQuadrilateral(); public static final String shape = "Quadrilateral"; public static final int sides = 4; } /* here we can see in variable declaration missing keywords are automatically placed by compiler. and interface access specifier "public" keyword not placed, interface default access specifier is */ /* We can compile interface but we can’t execute because it doesn’t contain main method. */

Rules on interface

Do

Don’t

For interface either directly or indirectly can't be instantiated (creating objects). because it doesn't have a constructor. Violation leads to CompiletimeError CE:InterfaceDemo1 is abstract can't be instantiated.

But we can create reference variables of interface to store either null or its subclass object reference

In interface we can’t write concrete methods, including constructors and static and non static blocks. Violation leads to CE: interface method can’t have a body.

Inheritance with interface

We must use “implement” keyword to establish inheritance relation between class and interface

Rule on interface subclass

Subclass must implement all abstract methods of interface, else subclass should be declare as abstract otherwise it leads to CompiletimeError

These implementing methods access specifier in subclass should also be public, because interface methods are public methods.

Developing inheritance with two interface

we should use “extends” keyword to establish inheritance relation between two interfaces

Multiple inheritance using interfaces

Extending super class and implements interfaces

Extending interfaces

Difference between Interface and Abstract class