Understanding the final Keyword in Java: Variable Level
Table of Contents
- 1. Introduction
- 2. The
finalKeyword in Java - 3. Usage of
finalin Variables - 4. Example Code and Explanation
- 5. Key Concepts and Considerations
- 6. Conclusion
- 7. Keywords
1. Introduction
In Java, the final keyword plays an essential role in defining constants and ensuring immutability. It can be applied to variables, methods, and classes.
This article will focus on using the final keyword with variables, particularly at the variable level, explaining its importance, use cases, and limitations.
We’ll also look at the project file to demonstrate how to apply it with practical code examples.
2. The final Keyword in Java
The final keyword is used to restrict the modification of entities. When applied to a variable, it ensures that the variable can only be assigned a value once, making it a constant.
Once a final variable is initialized, it cannot be changed.
When a variable is marked as final, it must be initialized either at the time of declaration or within the constructor of the class. If left uninitialized, it will result in a compilation error.
3. Usage of final in Variables
There are different ways to apply the final keyword in variables:
- Direct Initialization: A final variable can be initialized at the time of declaration.
- Constructor Initialization: A final variable can also be initialized inside the constructor of a class, giving you flexibility to set it based on different conditions.
- No Reassignment: Once a final variable is initialized, it cannot be reassigned.
Key Points:
- The
finalkeyword restricts any change to a variable after it has been initialized. - If a final variable is not initialized at the time of declaration, it must be initialized within a constructor.
- You cannot create a setter for a
finalvariable because the value cannot be modified once assigned.
4. Example Code and Explanation
Here’s an example program that demonstrates the use of final keyword with variables in Java:
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public class Child { public final int x; public Child() { this.x = 10; } public Child(int x) { this.x = x; } public int getX() { return x; } } |
The Main class where the Child class is used:
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public class Main { public static void main(String[] args) { Child child = new Child(); System.out.println(child.getX()); Child child2 = new Child(102); System.out.println(child2.getX()); } } |
Explanation:
- Final Variable Initialization: The
xvariable is declared asfinal, meaning it can be assigned a value only once. In this case, it is initialized to 10 in the default constructor and to the provided value in the parameterized constructor. - Using Constructors: Two constructors are provided: one default constructor that assigns a fixed value (10) and another parameterized constructor that allows passing a value for
x. - No Setter Method: Notice that there is no setter method for the
xvariable. Sincexis final, its value can only be set once, either during declaration or in the constructor. This enforces immutability for the variable.
Output:
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10 102 |
5. Key Concepts and Considerations
There are a few important points to consider when working with final variables:
- No Setters: When a variable is marked as
final, you cannot create a setter method to modify its value after it has been initialized. - Constructor Constraints: If a
finalvariable is initialized directly at the point of declaration, you cannot reassign it in the constructor. If it’s initialized within a constructor, you must ensure all constructors initialize the variable. - Immutability: A final variable promotes immutability, making your class more predictable and stable, especially in multi-threaded environments.
- Interview Tip: Understanding the differences between a constant (final variable) and mutable variables can be an essential interview question. A final variable allows only one assignment and cannot be changed, but it’s not necessarily a compile-time constant. It can be initialized at runtime using constructors.
6. Conclusion
The final keyword is a critical tool in Java programming, ensuring that variables remain constant after their initial assignment.
This promotes safe, predictable behavior, especially when working with objects in multi-threaded environments.
In this article, we’ve demonstrated how to use final at the variable level with practical examples and code explanations.
Knowing when and where to use final can enhance the design and robustness of your Java programs.