PROGRAMMING IN JAVA - LECTURE -49- USING A PACKAGE.- BCA- S3.

Using a Package
Let us now consider some simple programs that will use classes from other packages. The listing below shows a package named package1 containing a single class ClassA_

package package1;
public class ClassA
{
public void displayA()
{
System.out .printin("Class A”);
}
}
This source file should be named ClassA.java and stored in the subdirectory package1 as stated earlier, Now compile this java file. The resultant ClassA.class will be stored in the same subdirectory.
Now consider the listing shown below:
import packagel.Class A;
class PackageTestl
{
public static void main(String args[])
{
ClassA objectA = new ClassA();
objectA.displayA();
}
}

This listing shows a simple program that imports the class ClassA fromthe package package1. The source file should be saved as PackageTest1.java and then compiled, The source file and the compiled file would be saved in the directory of which package1 was a subdirectory. Now we can run the program and obtain the results.

Now let us consider another package named package2 containing again a single class as shown below:
package package2;
public class ClassB
{
protected int m = 10;
public void displayB()
{
System.out.println(“Class B”);
System. out .println( “m = “+ m);
}
}
As usual, the source file and the compiled file of this package are located in the subdirectory package2.
imports package1 and package2. Note that we have used star instead of explicit class name in importing package2.
Importing classes from other packages
import package1.ClassA;
import package2. *;
class PackageTest2
{
public static void main(String args[ ])
{
ClassA objectA = new ClassA( );
ClassB objectB = new ClassB();
objectA.displayA();
objectB.displayB();
}
}
When we import multiple packages it is likely that two or more packages contain classes with identical names. Example:
package pack1;
public class Teacher
{………………..}
public class Student
{………………..}
package pack2;
public class Courses
{………………..}
public class Student
{………………..}
We may import and use these packages like:
import pack1.*;
import pack2.*:
Student student1; // create a student object
Since both the packages contain the class Student, compiler cannot understand which one to use and therefore generates an error. In such instance, we have to be more explicit about which one we intend to use.
import pack1.*;
import pack2.*;
pack1.Student student1;// OK
pack2. Student student2; // OK
Teacher teacher1; // No problem
Courses course1; //No problem
Note that the variable m has been declared as protected. A subclass in another package can inherit a protected member. It would not have been possible if it has been declared as either private or “default”.