Global Variable in Java

Global Variables in Java

Global variables are the variables that can be accessed from anywhere in the program. These are different from local variables that we define in methods or main method which can only be used in that particular method. The scope of local variables is limited to that method only, i.e. they cannot be accessed outside the method they are declared in. Whereas, global variables can be used by any of the methods in a program.

In programming languages like C, C++, Python, etc. global variables can be declared anywhere in the program using the global keyword. But Java's object-oriented programming structure makes it difficult to reference variables in multiple places. Thus, unlike other languages, Java officially doesn't have the concept of global variables.

However, we can achieve the functionality of global variables using existing properties of Java, such as static and final keywords.

Why Java doesn't Use Global Variables

Java is purely an object-oriented programming language and thus, everything in Java is wrapped in a class. All the related variables and methods are kept together, making the program structured.

Global variables are to be declared outside the methods which are against OOP principles. In Object-Oriented Programming Language, everything is an object and all the programs and data reside within classes and objects. Global variables also have other security threats, and thus in java, we don't have global variables.

Let us take an example of global variable ver as in the code below:

Now one of the team members interprets it as the vertical distance from the center and the other interprets it as a version number. To avoid such confusing situations, we use variables inside classes and functions. If the variable is related to the version then it must be stored in the about class as follows:

If it is related to the vertical distance then it is stored in Grid class.

Scope of Global Variables

The scope of a variable refers to the part of the program where we can access the variable. In its scope, we can edit, update, print, or copy the value of a variable. Global variables are the variables whose scope is the entire program. In other words, these variables are accessed from anywhere in the program.

Creating Global Variables in Java

Since Java does not have the concept of global variables, we can simulate their functionality by creating constants using static or static final keywords, referencing a class, or implementing an interface, similar to how other programming languages implement global variables.

Let us see each of the ways in detail to create global variables in Java.

Create Global Variables by Using the Reference Class in Java

Reference class is a class where we store all the global values together if they are used commonly in all the parts of the program. If multiple global variables are required, then we use this method of defining global variables.

A reference class is a class that stores all the global variables and they can be accessed simply by Reference.className anywhere in the program.

Let us take an example of a Circle class that uses the reference variable PI to calculate the area of the circle.

Output:

In the above example, a class called Reference doesn't have any other functionalities, only constants are declared. Each constant is public thus, anyone can access it. They all are final which implies their value cannot be changed. Also, we don't require the object to call the variable as it is static. The public class Circle has a method called area that takes the radius of a circle and returns the area. It uses the reference variable PI to calculate the area. In the main method, we use the area method to calculate the area of a circle of radius 23.

Create Global Variables by Using Interfaces in Java

We can define all the global variables in an interface and the class that requires to access the variables can implement this interface. All the variables in the interface are public, static and final by default.

Let us take an example of class Gravitation that has two methods. The first one calculates the force exerted by one body on another, and the second one calculates the weight using mass of a body.

Output:

We have defined an Interface Constant that holds the constant values such as the Universal Gravitation constant, the mass of earth, the mass of the moon, etc.

Gravitation class implements this interface. The method Force calculates the force exerted by Earth on Moon. The Weight method calculates the weight of the body by its mass and acceleration due to gravity.

Create Global Variables by Using the Static Keyword in Java

The simplest way to create global variables is using the static keyword and declaring the variable outside the method in the class. As we are using the static keyword, we can directly call the variable. If we don't use static keywords, the object of the class is required to access the variable.

Let us take an example of the Greet Class and Message Class to greet everyone.

Output:

In the example above, the variable firstMessage is a global variable. It can be accessed by both the classes Greet and Message as both the classes belong to the same program and firstMessage is a public(global) and static variable. As it is static, we do not require an object of the Greet class to use the variable in the Message class. We can call it using className.variableName as in the above example Greet.firstMessage. In this way, we can achieve the functionality of the global variable in Java.

Create Global variables by using static & final keywords in Java

We can also declare static variables as final, i.e. they cannot be changed later in any part of the program once declared and initialized. It is mostly used to declare constants in Java.

Let us take an example of Calculate class that calculates the amount to be paid using MRP(Maximum Retail Price).

Output:

In the example above the deliveryCharge is declared as final and static which means it cannot be changed in the program for any value of MRP i.e. it is fixed. We calculate the amount to be paid using the TotalPrice method and return it to the user.

Major Differences Between Local & Global Variables

The variables that are declared within the method are known as the local variables. They can be accessed only inside the method they are declared in. We need to pass these as a parameter to other functions if we wish to use them outside our function. Global variables need not be returned or passed, they are available to all the functions.

Major differences between Local and Global variables:

Advantages of Using Global Variables

• Global variables in Java can be accessed by all the methods in the program.
• We need to declare them only once outside the methods.
• It can be accessed multiple times from any of the methods in the program.
• For example, we can declare PI outside the methods as it is a constant value and needs to be used again and again.

Disadvantages Of Using Global Variables

• If so many variables are declared global, then they stay in the memory from the beginning till the end and the program may run out of the memory.
• Any function or method can change the value of the global variable if it is not declared as final as it is accessible to all the methods of the program. Furthur this changed value can affect the calculations done by other functions.

Conclusion

• Global variables can be accessed by all the methods in the program, whereas local variables can only be used in the method they are declared in.
• As Java is Object Oriented Programming Language, it doesn't have global variables.
• We can use static variables in a class, static final variables in a class, create a new class to store static variables only, or create an interface to store variables to give the functionality of global variables in Java.