Understanding the Role of the 'static' Keyword in Java Classes

What does the 'static' keyword do in a Java class?

package hello;

public class Hello {

    static Clock clock = new Clock();

    public static void main(String args[]) {
        clock.sayTime();
    }
}

In Java, the 'static' keyword is a modifier that alters the behavior of a class's members (variables and methods). When a member is declared as static, it is associated with the class itself rather than any specific instance of that class.

package hello;

public class Hello {

    static Clock clock = new Clock(); // static global variable

    public static void main(String args[]) {
        clock.sayTime(); // Accessing static variable directly
    }
}

Causes

• The error message 'Cannot access non-static field in static method main' occurs because the main method is static and cannot directly access instance members without an object reference.
• By declaring 'clock' as static, it becomes class-level and can be accessed directly by static methods like 'main'.
• Static members exist independently of any instances of the class, allowing them to be used without creating an object.

Solutions

• Declare members that need to be accessed in static methods as static to ensure they can be accessed directly.
• Use instance members (non-static) only when you have an instance of the class.
• If there's a need to access instance data from a static context, create an object of the class and access the instance members through that object.

Mistake: Declaring instance variables as static.

Solution: Only declare static for fields and methods that are shared across instances or do not rely on instance-specific data.

Mistake: Forget to create an instance to access non-static fields in static methods.

Solution: Always ensure to create an instance of the class before trying to access instance fields in static methods.