Java interface

Feb 3, 2015Download as PPTX, PDF3 likes3,953 views

An interface in Java is like a class but cannot be instantiated. It defines method signatures and constant values but not method implementations. A class implements an interface by providing method bodies for the abstract methods defined in the interface. Interfaces can extend other interfaces to inherit their methods.

 An interface is like a class. It has static constants and abstract methods.
 Interfaces are declared using the interface keyword,
 method signature
 constant declarations (variable declarations that are declared to be
both static and final).
 An interface never contains method implementations (ie function
"bodies").

access interface name
{
return-type method-name1(parameter-list);
return-type method-name2(parameter-list);
type final-varname1 = value;
type final-varname2 = value;
// ...
return-type method-nameN(parameter-list);
type final-varnameN = value;
}

 Access – either public or not used.
 Not used – default is an answer.
 i/f is only available to other member of the package which it’s
declared.
 Methods declared have no bodies and end with a semicolon after the
parameter list(abstract methods). Each class that includes an interface
must implement all of the methods.
 Variables – declared implicitly final and static, meaning they cannot be
changed by the implementing class. They must also be initialized with
a constant value.
 All methods and variables are implicitly public if the interface, itself, is
declared as public.

Understanding relationship between classes and interfaces

 To implement an interface
– include the implements clause in a class definition
access class classname [extends superclass] [implements interface
[,interface...]]
{
constant declarations ;
abstract method declarations;
}
 The methods that implement an interface must be declared public.

However, an interface is different from a class in several ways, including:
 You cannot instantiate an interface.
 An interface does not contain any constructors.
 All of the methods in an interface are abstract.
 An interface cannot contain instance fields. The only fields that can
appear in an interface must be declared both static and final.
 An interface is not extended by a class; it is implemented by a class.
 An interface can extend multiple interfaces.

interface shape
{
public String baseclass="shape";
public void Draw();
}
class circle implements shape
{
public void Draw()
{
System.out.println("Drawing Circle
here");
}}
public class inter
{
public static void main(String[] args)
{
circle s=new circle();
// shape s=new circle();
s.Draw();
} }
interface Printable
{
void print();
}
interface Showable
{
void show();
}
class A implements Printable,Showable
{
public void print()
{System.out.println("Hello");}
public void show()
{System.out.println("Welcome");
}
public static void main(String args[])
{
A obj = new A();
obj.print();
obj.show(); }}

 An interface can be declared as member of a class or another
interface – called nested interface.
 How can we define i/f inside a class and how can we access it.
class A
{
interface Message()
{
void msg();
}
}
Class test implements A.Message
{
Public void msg()
{
S.o.p(“Hello”);
}
Public static void main(String args[])
{
A.Message message=new test();
message.msg();
}
}

 Interfaces Can Be Extended:
 One interface can inherit another interface - extend.
 The syntax is the same as for inheriting classes.
// One interface an extend another.
interface A
{
void meth1();
void meth2();
}
// B now includes meth1() and meth2() -- it adds meth3().
interface B extends A
{
void meth3();
}

interface Printable
{
void print();
}
interface Showable extends Printable
{
void show();
}
class A implements Showable
{
public void print()
{System.out.println("Hello");}
public void show()
{System.out.println("Welcome");}
public static void main(String args[])
{
A obj=new A();
obj.print();
obj.show();
}}

Interfaces define methods that classes can implement. Classes implementing interfaces must define all interface methods. Interfaces can extend other interfaces, requiring implementing classes to define inherited methods as well. Interface variables are implicitly public, static, and final. A class can implement multiple interfaces and override methods with the same name across interfaces. Partial interface implementation requires the class to be abstract.

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Java interface

1. Interface in Java

2.  An interface is like a class. It has static constants and abstract methods.  Interfaces are declared using the interface keyword,  method signature  constant declarations (variable declarations that are declared to be both static and final).  An interface never contains method implementations (ie function "bodies").

3. access interface name { return-type method-name1(parameter-list); return-type method-name2(parameter-list); type final-varname1 = value; type final-varname2 = value; // ... return-type method-nameN(parameter-list); type final-varnameN = value; }

4.  Access – either public or not used.  Not used – default is an answer.  i/f is only available to other member of the package which it’s declared.  Methods declared have no bodies and end with a semicolon after the parameter list(abstract methods). Each class that includes an interface must implement all of the methods.  Variables – declared implicitly final and static, meaning they cannot be changed by the implementing class. They must also be initialized with a constant value.  All methods and variables are implicitly public if the interface, itself, is declared as public.

5. Understanding relationship between classes and interfaces

7.  To implement an interface – include the implements clause in a class definition access class classname [extends superclass] [implements interface [,interface...]] { constant declarations ; abstract method declarations; }  The methods that implement an interface must be declared public.

8. However, an interface is different from a class in several ways, including:  You cannot instantiate an interface.  An interface does not contain any constructors.  All of the methods in an interface are abstract.  An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.  An interface is not extended by a class; it is implemented by a class.  An interface can extend multiple interfaces.

9. interface shape { public String baseclass="shape"; public void Draw(); } class circle implements shape { public void Draw() { System.out.println("Drawing Circle here"); }} public class inter { public static void main(String[] args) { circle s=new circle(); // shape s=new circle(); s.Draw(); } } interface Printable { void print(); } interface Showable { void show(); } class A implements Printable,Showable { public void print() {System.out.println("Hello");} public void show() {System.out.println("Welcome"); } public static void main(String args[]) { A obj = new A(); obj.print(); obj.show(); }}

10.  An interface can be declared as member of a class or another interface – called nested interface.  How can we define i/f inside a class and how can we access it. class A { interface Message() { void msg(); } } Class test implements A.Message { Public void msg() { S.o.p(“Hello”); } Public static void main(String args[]) { A.Message message=new test(); message.msg(); } }

11.  Interfaces Can Be Extended:  One interface can inherit another interface - extend.  The syntax is the same as for inheriting classes. // One interface an extend another. interface A { void meth1(); void meth2(); } // B now includes meth1() and meth2() -- it adds meth3(). interface B extends A { void meth3(); }

12. interface Printable { void print(); } interface Showable extends Printable { void show(); } class A implements Showable { public void print() {System.out.println("Hello");} public void show() {System.out.println("Welcome");} public static void main(String args[]) { A obj=new A(); obj.print(); obj.show(); }}

Java interface

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