Packages


Many times when we get a chance to work on a small project, one thing we intend to do is to put all java files into one single directory. It is quick, easy and harmless. However if our small project gets bigger, and the number of files is increasing, putting all these files into the same directory would be a problematic for us. In java we can avoid this sort of problem by using Packages. Packages are nothing more than the way we organize files into different directories according to their functionality, usability as well as category they should belong to. Packaging also help us to avoid class name collision when we use the same class name as that of others. For example, if we have a class name called "Vector", its name would crash with the Vector class from JDK. However, this never happens because JDK use java.util as a package name for the Vector class (java.util.Vector). So our Vector class can be named as "Vector" or we can put it into another package like com.mycompany.Vector without fighting with anyone. The benefits of using package reflect the ease of maintenance, organization, and increase collaboration among developers.

How to create a Package : Suppose we have a file called HelloWorld.java, and we want to put this file in a package world. First thing we have to do is to specify the keyword package with the name of the package we want to use(world in our case) on top of our source file, before the code that defines the real classes in the package, as shown in our HelloWorld class below : package world; public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } One thing you must do after creating a package for the class is to create nested subdirectories to represent package hierachy of the class. In our case, we have the world package, which requires only one directory. So, we create a directory world and put our HelloWorld.java into it.

Interfaces

 Interfaces have another very important role in the Java programming language. Interfaces are not part of the class hierarchy, although they work in combination with classes. The Java programming language does not permit multiple inheritance (inheritance is discussed later in this lesson), but interfaces provide an alternative. In Java, a class can inherit from only one class but it can implement more than one interface. Therefore, objects can have multiple types: the type of their own class and the types of all the interfaces that they implement. This means that if a variable is declared to be the type of an interface, its value can reference any object that is instantiated from any class that implements the interface.
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