importing packages

Any package can be imported with keyword import.

To import package name data_stucture

    import data_structure.*;

Above statement imports all the classes, interfaces and other types in the package, but it will not import the sub packages. So to import sub package “linear” use the below statement.

   import data_structure.linear.*;

Above diagram shows the directory structure.

1. Directory named “data_structure” contains two sub directories linear and non-linear.
2. Directory linear contains two java files named Stack.java, DataStructure.java
   
   

   package data_structure.linear;
   
   public interface DataStructure{
    int MAX_SIZE = 1000;
    
    void display();
    void insert(int data);
    void delete();
    int search(int data);
   }
   

package data_structure.linear;

public class Stack implements DataStructure{
 int top = -1;
 
 int stack[] = new int[DataStructure.MAX_SIZE];
 
 public void display(){
  for(int i=0; i<=top; i++){
   System.out.println(stack[i]);
  }
 }
 
 public void insert(int data){
  if(top < DataStructure.MAX_SIZE-1){
   top++;
   stack[top] = data;
  }
  else{
   System.out.println("Stack is not empty");
  }
 }
 
 public void delete(){
  if(top == -1){
   System.out.println("Stack is Empty");
  }
  else{
   stack[top] = 0;
   top--;
  }
 }
 
 public int search(int data){
  for(int i=0; i < top; i++)
   if(stack[i] == data)
    return 0;
  return -1;
 }
}

Lets say StackTest.java is in the package data_structure.

package data_structure;

import data_structure.linear.*;
class StackTest{
 public static void main(String args[]){
  DataStructure myStack1 = new Stack();
  
  myStack1.delete();
  
  myStack1.insert(10);
  myStack1.insert(20);
  
  myStack1.display();
 }
}

Statements to execute the StackTest.java

C:\>javac -cp . data_structure\linear\DataStructure.java

C:\>javac -cp . data_structure\linear\Stack.java

C:\>javac -cp . data_structure\StackTest.java

C:\>java -cp . data_structure.StackTest

Stack is Empty

10

20

if StackTest.java is in C drive and not in any specific package like below

import data_structure.linear.*;
class StackTest{
 public static void main(String args[]){
  DataStructure myStack1 = new Stack();
  
  myStack1.delete();
  
  myStack1.insert(10);
  myStack1.insert(20);
  
  myStack1.display();
 }
}

You can compile and run as usual like below

 

import can be used in 3 ways

1. Refer to the member by its fully qualified name 

class StackTest{
 public static void main(String args[]){
  data_structure.linear.DataStructure myStack1 = new data_structure.linear.Stack();
  
  myStack1.delete();
  
  myStack1.insert(10);
  myStack1.insert(20);
  
  myStack1.display();
 }
}

Output

Stack is Empty
10
20

As you observe the statement

data_structure.linear.DataStructure myStack1 = new data_structure.linear.Stack();

The class Stack and interface DataStructure are referred by full names. And program not imported any package explicitly.

2. Import the package member

import data_structure.linear.DataStructure;
import data_structure.linear.Stack;

class StackTest{
 public static void main(String args[]){
  DataStructure myStack1 = new Stack();
  
  myStack1.delete();
  
  myStack1.insert(10);
  myStack1.insert(20);
  
  myStack1.display();
 }
}

Output

Stack is Empty
10
20 

As you observe the statements

   import data_structure.linear.DataStructure;

   import data_structure.linear.Stack;

Program import the package members DataStructure, Stack.

3. Import the member's entire package

import data_structure.linear.*;

class StackTest{
 public static void main(String args[]){
  DataStructure myStack1 = new Stack();
  
  myStack1.delete();
  
  myStack1.insert(10);
  myStack1.insert(20);
  
  myStack1.display();
 }
}

Output

Stack is Empty
10
20

Statement “import data_structure.linear.*;” imports all the classes, interfaces and other types in the package linear, but not the sub packages.

Name Ambiguities

If a member in one package shares its name with a member in another package and both packages are imported, you must refer to each member by its qualified name. For example, the graphics package defined a class named Rectangle. The java.awt package also contains a Rectangle class. If both graphics and java.awt have been imported, the following is ambiguous.

     Rectangle rect;

In such a situation, you have to use the member's fully qualified name to indicate exactly which Rectangle class you want. For example,

     graphics.Rectangle rect;

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