Access Control in Java

Modifiers fall into two categories:

• Access modifiers: public, protected, private and default (package-level access).
• Non-access modifiers: transient, synchronized, native, strictfp, final, abstract and static.

Access Modifiers

Two types of access are there:

• Whether method code in one class can access a member of another class
• Whether a subclass can inherit a member of its superclass

A default member may be accessed only if the class accessing the member belongs to the same package, whereas a protected member can be accessed by a subclass in the same package (through dot operator and inheritance) and even if it is in a different package (through inheritance only).

You cannot access a protected member using the dot (.) operator in the subclass if the subclass is in a different package from the parent class.

The following code snippet makes it clear:

package certification;
public class Parent {
    protected int x = 9; // protected access
    
    protected int getX() {
        return x;
    }
}

package other; // different package
import certification.Parent;
class Child extends Parent {
    public void testIt() {
        System.out.println("x is " + x); // No problem; Child
                                         // inherits x
        Parent p = new Parent(); // Can we access x using
                                 // p reference?
        System.out.println("X in parent is " + p.x); // Compiler
                                                     // error
        System.out.println("X in parent is " + p.getX()); // Compiler
                                                          // error
    }
}

NOTE: If Child class would have been in the same package as Parent then there would be no compiler error.

The below table gives the picture of all access modifiers:

Non-Access Modifiers

Final is the only modifier which can be applied to local variables. It can also be used in method arguments like:

// final in method arguments, can't be altered inside the method
public Record getRecord(int fileNumber, final int recordNumber) {}

Final when applied to a method prevents it to be overridden and when applied to a class makes it un-inheritable i.e, it can never be subclassed (no class can extend it).

An abstract method is a method that’s been declared (as abstract) but not implemented. In other words, the method contains no functional code. And the class which contains at least one of such methods is an abstract class and has to be declared abstract. An abstract class can never be instantiated.

A method can never, ever, ever be marked as both abstract and final, or both abstract and private or both abstract and static.

A class having even a single abstract method has to be declared abstract or if it extends an abstract class then it must implement all abstract methods of the superclass otherwise you have make it abstract as well.

Comparison of modifiers on variables vs. methods: