Java Class, Method and Variable Modifiers

The Java programming language has 50 reserved words, which have predefined meaning in the language. Several of these keywords can be prepended to the declaration of a class, method or variable and are known as modifiers.

public class HelloWorld
{
    public static void main(String[] args)
    {
        System.out.print("Hello, World");
        System.out.println();
    }
}

This “Hello world” example uses four reserved words (i.e. public, class, static, void). Of those reserved words, only public and static act as modifiers. Although, every Java application must contain a main method, whose signature is shown in the above example, the word main is not a reserved word.

Learning how and when to use each modifier can be especially difficult because their definitions are often littered throughout introductory texts. Both as a reference, and in order to facilitate learning, this article will cover each modifier on one page.

There is a total of 11 modifiers in the Java programming language. The most common modifiers in an introductory context are abstract, final, private, protected, public, and static. More advanced modifiers include native, synchronized, transient, volatile and strictfp.

Modifiers by type

Access modifiers help set the visibility and accessibility of classes, methods and variables. The keywords public, protected, and private are access modifiers. A private member is the most restrictive access level and as such is only accessible in the class in which it is defined. Therefore, private members are not inherited by a subclass

Extension modifiers indicate whether a class can or should be extended with a subclass. The keywords abstract and final are extension modifiers.

Modifiers by element

Class modifiers

Any class may be declared with the abstract, final, public¹ and strictfp modifiers. An abstract class is to be considered incomplete, such that subclasses can be created to complete the implementation.² A class can be declared final if its definition is complete and no subclasses are desired or required, since the class can not be extended by another class using the extend keyword.

The remaining access and extension modifiers (i.e. protected, private, and static), may only be declared in nested classes (i.e. classes that are enclosed within another class declaration).

Certain modifiers can not be declared together. For example, a class that is declared as abstract can only be declared with public or protected modifiers.

Method modifiers

A method may be declared with the modifier names used with classes and the native and synchronized modifiers. That includes all modifiers except for volatile and strictfp.

The abstract modifier plays a similar role when declared with methods. It means that the method is declared, but not yet implemented. Abstract methods may only be composed inside of abstract classes.

Variable modifiers

Modifier order

A compile-time error occurs if the same modifier appears more than once in a class declaration. Although, the modifiers public and static can be written in either order, convention dictates that the access modifier be written first (i.e. public static). After a access modifier, the convention dictates the following order: abstract static final strictfp.

References

• List of Java keywords at Wikipedia
• Why is the Java main method static? at Stack Overflow
• Modifiers in Java
• Java Modifier Summary and All Combinations of Features and Modifiers at Javacamp.org