Unit Iv Packages&interfaces

Packages & Interfaces Definition: A package is a collection of classes, interfaces and other packages. Packages resemble directories or folders. Packages are used to group the logical related classes and interfaces. Packages help in avoiding name conflicts.

Creation of package: Creation of a package is simply putting a class or interface in it. So for this we need to add a package statement in the class or the interface we define. package packageName; Ex: 1 package graphics; class Circle{ ……. } package graphics; class Rectangle{ ……. } A package statement should be written at the top of every source/ java file. If a class doesn’t have the package statement, then the class is placed in the default package, which is the current directory location.

Accessing a package: For example if we have created the above package graphics in say d:\ java\ graphics\Circle.java Then if we run the Circle.java program. It will first search for the class in the current directory or in the location where the CLASSPATH environmental variable is set. If we always run any java program from one level above the package level, then there is no need to set the classpath variable. But if we set the CLASSPATH variable, then we have to run the java programs from that path.

Importing packages: package graphics; Circle.java Rectangle.java Triangle.hava

package graphics3D ; extends

Rectangle3D.java

If a class Rectangle3D extends a class Rectangle existing in some other package, it has to first import that package into the current package in order to use the classes in it. Ex: package graphics3D; import graphics; class Rectangle3D extends Rectangle{ ….. } import is the keyword used to access the classes or interfaces of one package into another.

Interface: An interface is a way to declare a type consisting only of abstract methods and related constants (final data), enabling any implementation to be written for those methods. An interface provides a protocol binding to the classes that implement it. An interface is a collection of abstract methods and constants (final data members). An interface cannot be instantiated. An interface can contain only final type variables. Fields in an interface are always static and final. interface is the keyword used for creating interfaces. accessSpecifier interface name{ type final-variable1 = value; type final-variable2 = value; … .. . return-type method-name1(parameter list);

return-type method-name2(parameter list); .. .. } Ex 1: interface Polygon{ float PI = 3.13f; float area(); float circumference(); } All the methods in an interface are implicitly abstract. The abstract methods of an interface are implemented by a class using implements keyword.

Implementing an Interface: Ex 1 continued. class Circle implements Polygon{ public float radius: public Circle( float r){ radius= r; } public float area(){ return PI * radius * radius ; } public float circumference{ return 2*PI * radius ; } } class PolygonUse{ public static void main(String args[]){ Circlew c= new Circle(); System..out.println(“Area of circle = ”+ c.area() ); System..out.println(“Circumference of circle = ”+ c.circumference() ); } The class which implements an interface should give definitions to all its abstract methods.

If the above Polygon class doesn’t give the implementation to all the abstract methods of the interface, then the class has to be declared as abstract.

Differences between classes and interfaces: Classes 1) Classes can be instantiated 2) Classes can have all types of data members. 3) One class can extend the function of another class using the extends keyword. 4) Methods in classes are concrete/ abstract 5) Classes are defined with keyword “class”.

Interfaces 1) Interfaces cannot be instantiated 2) Interfaces can have only final variables. 3) A class implements an interface using the implements keyword. 4) Methods in an interface are by default abstract 5) Interfaces are defined using the keyword “interface”

Extending interfaces: An interface can extend another interface. Unlike classes, an interface can extend more than one interface. Ex: interface A{ ….. } interface B extends A{ ….. } class C implements B{ …… } Note: Multiple inheritance is not directly supported in java through classes, but can be achieved using interfaces.