Visibility (controlling access to members of a class)

Syntax#

• public type name[ = value];
• private type name[ = value];
• protected type name[ = value];
• type name[ = value];
• public class name{
• class name{

Remarks#

From the Java tutorial:

Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:

• At the top level—public, or package-private (no explicit modifier).
• At the member level—public, private, protected, or package-private (no explicit modifier).

A class may be declared with the modifier public, in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private), it is visible only within its own package.

At the member level, you can also use the public modifier or no modifier (package-private) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected. The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

The following table shows the access to members permitted by each modifier.

Access Levels:

Interface members

Public Visibility

Visible to the class, package, and subclass.

Let’s see an example with the class Test.

public class Test{
    public int number = 2;

    public Test(){

    }
}

Now let’s try to create an instance of the class. In this example, we can access number because it is public.

public class Other{
    
    public static void main(String[] args){
        Test t = new Test();
        System.out.println(t.number);
    }

}

Private Visibility

private visibility allows a variable to only be accessed by its class. They are often used in conjunction with public getters and setters.

class SomeClass {
    private int variable;

    public int getVariable() {
        return variable;
    }

    public void setVariable(int variable) {
        this.variable = variable;
    }
}

public class SomeOtherClass {
    public static void main(String[] args) {
        SomeClass sc = new SomeClass();
        
        // These statement won't compile because SomeClass#variable is private:
        sc.variable = 7;
        System.out.println(sc.variable);

        // Instead, you should use the public getter and setter:
        sc.setVariable(7);
        System.out.println(sc.getVariable());
    }
}

Package Visibility

With no modifier, the default is package visibility. From the Java Documentation, “[package visibility] indicates whether classes in the same package as the class (regardless of their parentage) have access to the member.” In this example from javax.swing,

package javax.swing;
public abstract class JComponent extends Container … {
    …
    static boolean DEBUG_GRAPHICS_LOADED;
    …
}

DebugGraphics is in the same package, so DEBUG_GRAPHICS_LOADED is accessible.

package javax.swing;
public class DebugGraphics extends Graphics {
    …
    static {
        JComponent.DEBUG_GRAPHICS_LOADED = true;
    }
    …
}

This article gives some background on the topic.

Protected Visibility

Protected visibility causes means that this member is visible to its package, along with any of its subclasses.

As an example:

package com.stackexchange.docs;
public class MyClass{
    protected int variable; //This is the variable that we are trying to access
    public MyClass(){
        variable = 2;
    };
}

Now we’ll extend this class and try to access one of its protected members.

package some.other.pack;
import com.stackexchange.docs.MyClass;
public class SubClass extends MyClass{
    public SubClass(){
        super();
        System.out.println(super.variable);
    }
}

You would be also able to access a protected member without extending it if you are accessing it from the same package.

Note that this modifier only works on members of a class, not on the class itself.

Summary of Class Member Access Modifiers

There was once a private protected (both keywords at once) modifier that could be applied to methods or variables to make them accessible from a subclass outside the package, but make them private to the classes in that package. However, this was removed in Java 1.0’s release.