Class members and their "visibility"

  • Review:

      • Class member = a variable or a method defined in a class

  • Visibility modifiers can be used to specify the visibility (I like to called it accessibility) of a class and its members

  • The visibility (accessibility) modifiers of Java are: 

       Already discussed: 
    public:      accessible from any class
    private:     accessible from methods in same class 

 New: 
    [empty]:     accessible from methods in same package   
    protected:   will be discussed later

The effect of public and private visibility (access) qualifiers

private members can not be accessed from outside the class:

The effect of public and private visibility (access) qualifiers

private members can not be accessed from outside the class:   demo program 

public class C1
{
    public  int x1;
    private int x2;

    public  void m1() {}
    private void m2() {}
}

public class myProg
{
    public static void main(String[] args)
    {
        C1 o = new C1();  // Make a C1 object

	o.x1 = 1; // Access  a public  variable
	o.x2 = 1; // Access  a private variable
	o.m1();   // Invokes a public  method
	o.m2();   // Invokes a private method
    }
}

Why do programming languages provide different accessibilities

  • 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 will reduce accidental update of variable(s) by a method from another class !!!

Java packages

  • Some problems are so complex that they cannot be solved by methods inside one single class

  • We may need to write multiple classes to solve such complex problems

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

  • The facilitate (= make it easier) the coorperation:

      • Java can allow methods defined in classes inside the same package to access each others members (variables and methods

  • Before we learn this accessibility modifier, I need to explain how to create packages in Java (with BleuJ)....

How to create a package in BlueJ

  • Let's make a new project in BlueJ: 

      Click on Project 

                    Name:     LearnPackage
		    Location: C:\Java\BlueJ

  • Open a File Browser and go to C:\Java\BlueJ to see what happens

  • To make a package p1 inside the LearnPackage project: 

      Click on Edit --> New Package

                    Package Name: p1

    Classes in package p1 will be stored inside a folder named p1 on the computer !

How to create a class inside a package p1 in BlueJ

  • You can open the package p1 by: 

      Double click on package p1 in LearnPackage

    A new window for LearnPackage [p1] will open

  • Open a File Browser and go to C:\Java\BlueJ\LearnPackage\p1 to see what happens

  • To make a class C1 inside the LearnPackage.p1 package: 

      Click on New Class:

                    Class Name: C1

    Classes in package p1 will be stored inside a folder named p1 on the computer !

Example of a class inside the package p1

Use BlueJ and create this new class inside package p1: 

package p1;   // Inserted by BlueJ !

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 members x2 and m2() has no access modifier
In that case, they will be assigned with the default access modifier

The default access modifier

  • If a member (= variable or method) is not assigned with a modifier:                                                        

    The member will receive the "default" access modifier

The default access modifier

  • A member with the default access can be accessed from methods in classes belong to the same package:

    Remember:   classes in the same package coorperate to perform a task !

The default access modifier

Demo: use BlueJ 

package p1;

public class C1
{
    public int x1;
    int x2;
    
    public void m1() {}
    void m2() {}
}



package p1;

public class C2
{
    void aMethod()
    {
        C1 o = new C1();
        
        o.x1 = 1;
        o.x2 = 99;  // No compile error
        
        o.m1();
        o.m2();     // No compile error
    }
}

The default access modifier

  • A member with the default access is not accessible from methods in classes belong to a different package:

    Remember:   classes in different packages do not coorperate to perform a task !

The default access modifier

Demo: use BlueJ 

package p1;

public class C1
{
    public int x1;
    int x2;
    
    public void m1() {}
    void m2() {}
}



import p1.C1;

public class myClass
{
    void aMethod()
    {
        C1 o = new C1();
        
        o.x1 = 1;
        o.x2 = 99;  // Compile error
        
        o.m1();
        o.m2();     // Compile error
    }
}

Summary

Members with the default access modifier can be accessed from methods in classes inside the same package: