Lecture 3 Packages

Based on Objects and Classes, this lecture extends the idea and introduces Packages and how it works in Java.

• Why do programming languages provide different accessibilities (i.e. private and public qualifiers):
  • A large computer program has many methods stored in different classes
  • Methods defined inside a class are used to solve the same problem
  • Methods defined inside different classes usually do not solve the same problem
  • A common cause for errors is accidental update of variable(s) by a method in a different class
  • Limiting access to variables (with the private qualifiers) will reduce accidental update of variable(s) by a method from another class
• Java packages:
  • Some problems are too complex that they cannot be solved by methods inside one single class
    • We may need to write multiple classes to solve such complex problems
      • Programs will run more efficiently if they can access variables directly
      • I.e., Getter (getRadius()) and setter (setRadius()) will slow down a program

Note 1: Package

Package in Java is used to organize multiple classes that cooperate and perform similar tasks

• It facilitates (=make it easier) the cooperation: - Java can allow methods defined in classes inside the same package to access each other members (variables and methods) directly
• Example on using a package

package p1;
public class C1{
    public int x1;
           int x2; // Default (= package) access modifier
           // 
    private int x3;

    public void m1() { }
           void m2() { } // Default (= package) access modifier
    private void m3() { }
}

• The numbers x2 and m2() has no access modifier. In that case, they will be assigned with the default access modifier.
  • A member with the default access can be accessed from methods in classes belong to the same package. - Class in a same package cooperate to perform a task
  • A member with the default access is not accessible from methods in classes belong to a different package. - Class in different packages do not cooperate to perform a task
  • See Demo.java