Why non-static variables cannot be referenced from a static context in Java?

The error non static variable cannot be referenced from a static context in Java is mostly faced by the beginners at the time of compilation of Java program. The reason to occur this error is that they use a non-static member variable in the main() method. Because the main() method in Java is a static method and it is invoked automatically, we need not to create an object to invoke it. To understand the error, first we should understand the static and non-static method in Java.

Static Method

In Java, public methods belong to an instance of class but if we talk about static method, they belong to a class not to an instance of a class. There is no need of creating an instance of the class to invoke a static method. The static member can access only the static data member and can change its value.

It's crucial to remember that utility functions and other actions that don't rely on the state of any specific object are frequently performed using static methods. They can therefore be used for activities like generic operations that apply to all instances of a class or mathematical calculations.

Non-static Method

All methods without a static keyword before their name are referred to as Non-static methods. There is no need to create an instance of the class to access the static method and static variable. The non-static methods are used dynamically or through runtime binding. Unlike the static method, we can override the non-static method.

When performing operations that need access to an object's particular state, non-static methods are usually employed. They can carry out operations specific to each instance of the class and work with instance variables. Since non-static methods capture the functionality linked to specific instances, they are crucial to defining object behaviour in object-oriented programming.

Let's create a Java program and generate the same error.

In the following example, we have declared a private variable number of type int before the main() method. In the main() method, we are trying to increment the number by 1. It is to be noted that the main() method is a static method and the variable is not static. When we compile the above program, we get the same error, as we have shown below.

StaticExample1.java

Output:

Now, let's declare the variable number as static and compile the code. Note that, variable and the main() method both are static.

StaticExample2.java

Output:

Each instance of a non-static variable holds a distinct value and is instantiated when the new() operator initializes an object. Conversely, static variables are created or initialized when the class loads into the Java Virtual Machine (JVM) and are shared across all instances of the class.

Accessing non-static variables requires an instance of the class, as each instance encapsulates its own unique state. By creating multiple objects and assigning different values to non-static variables, we can observe how each object maintains its separate state.

In contrast, static variables are accessed using the class name and can be manipulated without requiring an instance, as they are associated with the class rather than specific instances. Understanding these distinctions is essential for proper utilisation of static and non-static variables in Java programming.

StaticExample3.java

Output:

Explanation

Two classes are defined in the provided code: StaticExample and Variable. The static function increment() and instance variable number are both part of the variable class. Since the number variable is not specified as static, the increment() method runs into an error while attempting to increase it.

The three instances of the variable class, var1, var2, and var3, are generated in the main function of the StaticExample class. The number variable for each instance has a distinct value, which are 12, 13, and 14, correspondingly.Though number is an instance variable and cannot be directly accessed in a static environment, variable.increment() attempts to increment the number variable when it is called.

Solution to an Error

There is one simple way of solving the non-static variable that cannot be referenced from a static context error. In the above code, we have to address the non-static variable with the object name. In a simple way, we have to create an object of the class to refer to a non-static variable from a static context. A new copy of all the non-static variables is created when a new instance of a variable is created. So, we can access these variables by using the reference of the new instance of the class. for example, consider the following Java program.

StaticExample4.java

Output:

Explanation

A class called staticExample with an instance variable number is defined in the code that is provided. The non-static variable number can be accessed by creating a new instance of staticExample within the main function. After that, the number's value is increased by one and displayed on the console. In order to fix the "non-static variable cannot be referenced from a static context" problem, this shows how to use a non-static variable inside a static context by instantiating the class that contains the variable.