Skip to main content

Java Package

 Introduction


    A java package is a group of similar types of classes, interfaces and sub-packages. Package in java can be categorized in two form, built-in package and user-defined package. There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc. Here, we will have the detailed learning of creating and using user-defined packages.


Simple example of java package

The package keyword is used to create a package in java.

//save as Simple.java  

package mypack;  

public class Simple{  

 public static void main(String args[]){  

    System.out.println("Welcome to package");  

   }  

}  

How to access package from another package?

There are three ways to access the package from outside the package.

1. import package.*;

2. import package.classname;

3. fully qualified name.

1. Using packagename.*

    If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages. The import keyword is used to make the classes and interface of another package accessible to the current package.

Example:

//save by A.java  

package pack;  

public class A{  

  public void msg(){System.out.println("Hello");}  

}  

//save by B.java  
package mypack;  
import pack.*;  
  
class B{  
  public static void main(String args[]){  
   A obj = new A();  
   obj.msg();  
  }  
}  

Output:
Hello

2. Using packagename.classname

If you import package.classname then only declared class of this package will be accessible.
Example:
//save by A.java  
  
package pack;  
public class A{  
  public void msg(){System.out.println("Hello");}  
}  

//save by B.java  
package mypack;  
import pack.A;  
  
class B{  
  public static void main(String args[]){  
   A obj = new A();  
   obj.msg();  
  }  
}  

Output:
Hello

3. Using fully qualified name
If you use fully qualified name then only declared class of this package will be accessible. Now there is no need to import. But you need to use fully qualified name every time when you are accessing the class or interface. It is generally used when two packages have same class name e.g. java.util and java.sql packages contain Date class.

Example:
//save by A.java  
package pack;  
public class A{  
  public void msg(){System.out.println("Hello");}  
}  

//save by B.java  
package mypack;  
class B{  
  public static void main(String args[]){  
   pack.A obj = new pack.A();//using fully qualified name  
   obj.msg();  
  }  
}  

Output:
Hello

Note:Sequence of the program must be package then import then class.


Labels:

Comments

Post a Comment

Popular posts from this blog

Decision Making and Branching Statements / Control Statements

 Introduction     Java is a set of statements which executed sequentially in the order in which they appear. This will happen when option or repetitions of certains are not necessary. However, We have a number of situation, where we may have to change the order of execution of statement on bases of certain conditions. This involves a kind of dicision making to see the particular condition occured or not and the direction of execution will execute according to it.     When program braks the sequential flow and jump on the another part of code then it's called branching . When branching is on particular condition then it's called conditional branching . If branches executes without any particular conditions then it's call unconditional branching .     Java supports such decision making capabilities and supports following statements known as Control statement or Decision making statement .     if statements     switch statement   ...
Post a Comment
Read more

Access Modifiers in Java

 Introduction There are two types of modifiers in Java: access modifiers and non-access modifiers. The access modifiers in Java specifies the accessibility or scope of a field, method, constructor, or class. We can change the access level of fields, constructors, methods, and class by applying the access modifier on it. There are four types of Java access modifiers: 1. Private: The access level of a private modifier is only within the class. It cannot be accessed from outside the class. 2. Default: The access level of a default modifier is only within the package. It cannot be accessed from outside the package. If you do not specify any access level, it will be the default. 3. Protected: The access level of a protected modifier is within the package and outside the package through child class. If you do not make the child class, it cannot be accessed from outside the package. 4. Public: The access level of a public modifier is everywhere. It can be accessed from within the clas...
Post a Comment
Read more

Static and this keyword

Static Keyword:      Most of the time static keyword in java use for memory management. We can use static keyword with variables, methods and also with block (Static block) . 1. Static with variables:     If we declare static keyword with variables then it's called static variables . Static variables are use to access or refer common properties of all objects. The static varibels gets memory only once at the time of loading class loading. It makes program more efficient i.e., It saves memory. For example,     static int length=100; 2. Static with method:     If we use static keyword with method then it's called static method . The static method is belong to class rather than object of class. Static method can invoke or call without creating instance or object of class. For example,     static void print(){          System.out.println("Welcome in Java");     } Examples of static variable, method an...
Post a Comment
Read more