Introduction:
If you've worked with Java, you know about static, but what about final? How do you create a variable that can never be changed? That’s where the Final Keyword in Java comes in, a fundamental concept for writing robust and secure object-oriented code.
This tutorial will not only cover the specifics of using the final keyword for variables, methods, and classes but also clarify the important difference between static and final keyword in java. By the end, you'll understand how to use this powerful modifier to improve your programs.
So, let’s start by understanding first what is final keyword in java.
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What is the Final Keyword in Java?
The final keyword in Java is like a "lock" that makes something unchangeable. When applied to a variable, its value becomes a constant. If used on a method, it cannot be overridden by a subclass, and a final class cannot be inherited.
By using the final keyword in Java, you can generate constant and immutable elements. This provides several benefits for your code. It ensures that the value or behavior of the element remains constant throughout the execution of the program. This predictability facilitates comprehension and maintenance of the code, as accidental modifications that could introduce bugs or unexpected behavior are eliminated.
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In addition, the final keyword provides developers and the Java compiler with clarity. When you mark an element as final, it indicates that you intend to make it permanent. This enhances the legibility of the code and makes it simpler for other developers to comprehend the element's purpose and constraints.
From the perspective of the compiler, the final keyword permits certain optimizations. Final variables, for instance, can be evaluated at compile time, allowing the program to replace references to those variables with their actual values. This can result in enhanced performance, as the runtime variable lookup is abolished.
The final keyword in Java enhances the security and dependability of your code. It ensures that essential elements remain unchanged by preventing unintended changes and improves code readability and maintainability. By judiciously employing the final keyword, you can generate code that is secure, predictable, and easier to comprehend and maintain.
Also Read: Difference Between Variable and Constant
Characteristics of the Final Keyword in Java:
1. Immutability: When an element is marked as final, it guarantees that neither its value nor its implementation can be changed in the future.
2. A constraint on inheritance: It states that a final class cannot be inherited from it, which eliminates the possibility of further extension.
3. A final method in Java cannot have its behavior overridden by any subclass; hence, this is a constraint on methods.
4. Evaluation of constants at compile time: The performance of the program can be improved by evaluating final variables when the program is being compiled.
5. Initialization: All final variables must have their initial values specified either when declared or within the class's constructor. This guarantees that the value of the variable will not be altered after it has been set.
6. Safety of threads: In multi-threaded systems, it is possible to access final variables in a secure manner without having to worry about synchronization issues.
Also Read: Life Cycle of Thread in Java
Where We Can Use the Final Keyword in Java?
This non-access modifier can be applied to variables, methods, and classes. Let's explore the use of the final keyword in Java with examples.
Final Variables:
When you mark a variable as final, it becomes a constant whose value cannot be changed once assigned. For example:
Here, MAX_VALUE is a final variable with a value of 100. Attempting to modify its value will result in a compilation error.
Initializing a final variable:
Final variables can be initialized in three ways: at the time of declaration, within the constructor, or in an instance initialization block. Consider the following example:
In this case, the final variable MAX_SIZE is initialized within the constructor.
Blank Final Variable:
A blank final variable is one that is declared as final but not initialized during the declaration. It must be assigned a value either in the constructor or within an instance initialization block. Here's an example:
Here, we have a class named MyClass with a blank final variable MAX_VALUE. It is declared without an initial value. However, it is assigned a value of 10 within the constructor. When we create an instance of MyClass and access the MAX_VALUE variable, it will give the output as Max value: 10. Once the MAX_VALUE variable is assigned a value, it cannot be changed further, ensuring its immutability throughout the program execution.
Also Read: Java Classes and Objects
Final Reference Variable:
When a reference variable is marked as final, the reference itself cannot be changed. However, the object it refers to can still be modified. For instance:
In this example, the reference variable obj is marked as final, ensuring that it always points to the same object. However, the object's internal state can still be modified.
Final Fields:
Final fields, also known as constant fields, are marked as final and initialized with a value that remains constant throughout the program's execution. These are typically declared using the static and final modifiers.
For example:
Here, MAX_SIZE is a final field that can be accessed by all instances of the class and holds a constant value.
When to Use Java Final Variable?
When you want to define constants that are going to remain the same all the way through your code, the final keyword is an exceptionally handy tool. It makes the program instructions easier to read and maintain and lowers the danger of accidental modification.
Final Classes:
A class is considered to be final when any other class can no longer inherit it. When it is given the "final" modifier, it ensures that its implementation cannot be changed or expanded in any way. It is common practice to do this for design or safety concerns, with the goal of guaranteeing immutability to specific classes.
For example:
Final Methods:
A final method is one that no subclass can override. By marking a method as final, you guarantee that its behavior remains consistent across all derived classes. This can be particularly useful when you want to prevent subclasses from altering or extending critical functionality.
For example:
Purpose of Java Final Methods:
Final methods provide a level of control and security within behavior class hierarchies. By preventing method overriding, you ensure that certain critical operations are consistently executed across all subclasses. This can be particularly valuable in scenarios where you want to enforce specific behavior or maintain code integrity.
Also Read: Explore Abstract Class and Method in Java: Learn Rules to Streamline Your Code
Java Final Parameter:
In Java, a final parameter is one that cannot be reassigned within the method body. It acts as a constant placeholder for the argument passed to the method. This feature often ensures the method does not modify the original parameter value.
Final and Immutable Class in Java:
While the final keyword restricts the modification of elements, an immutable class takes it a step further. It is one whose instances cannot be changed once created. All fields within an immutable class are marked as final, ensuring their permanence.
Advantages and Disadvantages of Final Variable in Java:
The pros and cons of final variables in Java are detailed below.
Advantages
1. Clarity and Readability: Final variables indicate their immutability, making code more readable and understandable.
2. Immutability: When a variable or reference is marked as final, its value cannot be altered after being assigned. This makes sure that data is unchangeable and cannot be intentionally or accidentally altered.
3. Compile-time Optimization: Final variables can be evaluated at compile-time, leading to potential performance improvements.
4. Thread Safety: Final variables can be safely shared among multiple threads without synchronization concerns.
Disadvantages
1. Limited Flexibility: Once marked as final, variables cannot be modified, which may restrict their usage in certain scenarios.
2. Code Duplication: The use of final variables may lead to code duplication if multiple values need to be assigned.
Conclusion:
The Final Keyword in Java is more than just a modifier; it's a powerful tool for creating robust, predictable, and secure code. By making variables, methods, and classes final, you are creating clear contracts that prevent unintended changes and strengthen your application's architecture.
Understanding its uses, as well as the crucial difference between static and final keyword in java, is a hallmark of a proficient developer. Applying these concepts will not only improve your code quality but also elevate your skills in object-oriented design.
FAQs
1. How does a final variable differ from a regular variable in Java?
A final variable in Java is fundamentally different from a regular variable because it can only be assigned a value once. Once initialized, its value is locked and cannot be changed for the lifetime of the variable, effectively making it a constant. A regular variable, on the other hand, can be reassigned a new value at any time. This single-assignment rule is the core of the Final Keyword in Java and is used to create immutable values that ensure data consistency and predictability in your code.
2. What are the three main uses of the final keyword in Java?
The Final Keyword in Java has three distinct applications based on where it is used:
- Final Variable: When applied to a variable, it makes the variable a constant, meaning its value cannot be changed after initialization.
- Final Method: When applied to a method, it prevents any subclass from overriding that method. This is used to enforce a specific, unchangeable behavior in an inheritance hierarchy.
- Final Class: When applied to a class, it prevents the class from being extended or subclassed. This is often done for security reasons or to ensure the implementation of a class is not altered.
3. What is a blank final variable in Java?
A blank final variable, also known as an uninitialized final variable, is a final instance variable that is declared without being assigned a value at the point of declaration. The Java compiler enforces a strict rule that this blank final variable must be initialized exactly once, either in every constructor of the class or in an instance initializer block. This allows you to create constants that are specific to each instance of an object.
4. What happens when a parameter is declared as final in a method?
When a method parameter is declared as final, it means that the reference or value of that parameter cannot be changed within the scope of that method. For a primitive type, you cannot reassign its value. For an object reference, you cannot make the parameter refer to a different object. This is often used to prevent accidental modification of a parameter and can improve code clarity, especially in anonymous inner classes that access local variables.
5. What is the difference between a final variable and a static final variable?
A final variable is an instance-level constant, meaning each object of the class will have its own copy of that constant, and it can be initialized in the constructor. A static final variable is a class-level constant. This means there is only one copy of the variable that is shared among all instances of the class. It must be initialized either at the time of declaration or in a static block. static final variables are the standard way to define true constants in Java (e.g., public static final int MAX_USERS = 100;).
6. How are final methods different from static methods?
The difference between static and final keyword in java when applied to methods is significant. A final method is an instance method that belongs to an object of the class, but it cannot be overridden by any subclasses. It is used to lock down an implementation. A static method, on the other hand, belongs to the class itself, not to any specific instance. It is called using the class name and cannot be overridden at all (it can only be "hidden" by a subclass).
7. Why would you declare a class as final?
Declaring a class as final is a crucial design decision, usually made for security and immutability. By preventing inheritance, you ensure that the behavior of your class cannot be altered or subverted by a malicious or poorly written subclass. Many of the core library classes in Java, such as String and Integer, are declared as final for this very reason—to guarantee that their behavior is always consistent and secure.
8. What is the difference between final, finally, and finalize?
These three keywords sound similar but have completely different meanings in Java:
- final: Is a keyword used to make a variable, method, or class unchangeable.
- finally: Is a block used in a try-catch statement to execute cleanup code (like closing a file) that must run, regardless of whether an exception was thrown.
- finalize: Is a method that the garbage collector calls on an object just before it is destroyed. Its use is now strongly discouraged in modern Java.
9. What does it mean for a final reference variable? Can the object it points to be changed?
This is a very important concept. When you declare a reference variable as final, it means the reference itself cannot be changed—it must always point to the same object. However, the state of the object it points to can be changed, provided the object itself is mutable. For example, if you have a final ArrayList, you cannot reassign the variable to a new ArrayList, but you can still add or remove elements from the original list.
10. How can you create a truly immutable object in Java?
To create a truly immutable object, you must combine several techniques related to the Final Keyword in Java. You should declare the class as final to prevent subclassing. All of its fields should be declared as private and final. You should not provide any "setter" methods. If the class contains any mutable object references (like a Date or ArrayList), you must return defensive copies of those objects in your getter methods to prevent external code from modifying the internal state.
11. Can an interface be declared as final?
No, an interface cannot be declared as final. The entire purpose of an interface is to be implemented by other classes. The final keyword, which prevents extension or inheritance, is logically contradictory to the purpose of an interface. The Java compiler will produce an error if you try to use the final modifier on an interface.
12. Can an abstract method be declared as final?
No, an abstract method cannot be declared as final. This is another logical contradiction that the Java compiler will prevent. An abstract method is a method with no implementation that must be overridden by a subclass. A final method is one that cannot be overridden. Therefore, a method cannot be both abstract and final.
13. Can a final variable be part of an interface?
Yes. In fact, all variables declared in an interface are implicitly public, static, and final, even if you do not explicitly write these keywords. This means that any variable you declare in an interface is automatically a constant that is shared across all implementing classes. This is a key part of how interfaces are designed in Java.
14. What happens if you try to override a final method?
If you attempt to override a final method in a subclass, your code will not compile. The Java compiler will generate a compile-time error, stating that the final method cannot be overridden. This is the mechanism by which the Final Keyword in Java enforces a strict implementation and prevents subclasses from changing a core behavior defined in the parent class.
15. Does using the final keyword improve performance?
In the early days of Java, using the final keyword on a method could sometimes lead to a minor performance improvement because it allowed the compiler to "inline" the method call. However, modern Just-In-Time (JIT) compilers are extremely sophisticated and can often perform this inlining automatically, even for non-final methods. Therefore, in modern Java, you should use the Final Keyword in Java for design and safety reasons, not for performance optimization.
16. Can a constructor be declared as final?
No, a constructor cannot be declared as final. Constructors are not inherited by subclasses, so the concept of overriding (and therefore preventing overriding with final) does not apply to them. The Java compiler will produce a compile-time error if you try to use the final modifier on a constructor.
17. What is the difference between a final variable and a constant variable in other languages?
In Java, a final variable is the primary way to create a constant. A static final variable is the closest equivalent to a compile-time constant in languages like C++ (using const or #define). The key concept is that the Final Keyword in Java enforces single assignment, ensuring the value cannot be changed after it is initialized.
18. When should I use final in my code?
You should use the Final Keyword in Java whenever you want to enforce immutability and create a clear contract in your code. Use it for variables that should not be changed after initialization (like configuration settings). Use it for methods when you want to ensure that their behavior is consistent across all subclasses. Use it for classes when you want to prevent them from being extended, often for security reasons.
19. How can I learn more about the Final Keyword in Java and other OOPS concepts?
The best way to master these concepts is through a combination of structured learning and hands-on practice. A comprehensive program, like the software development courses offered by upGrad, can provide a strong foundation in object-oriented principles, including the nuances of the Final Keyword in Java. You should then apply this knowledge by building your own projects and paying close attention to good design principles.
20. What is the main takeaway about the final keyword?
The main takeaway is that the Final Keyword in Java is a powerful tool for writing code that is more robust, secure, and maintainable. It is a clear way to communicate your design intentions to other developers (and to the compiler). By understanding the difference between static and final keyword in java and using final appropriately, you can prevent bugs and create more predictable and reliable applications.
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