Thursday, December 29, 2011

What is final in Java? Final variable , Method and Class Example

Final in java is very important keyword and can be applied to class, method, and variables in Java. In this java final tutorial we will see what is final keyword in Java, what does it mean by making final variable, final method and final class in java and what are primary benefits of using final keywords in Java and finally some examples of final in Java. Final is often used along with static keyword in Java to make static final constant and you will see how final in Java can increase performance of Java application.

Example of Final variable, Final method and Class in Java

What is final keyword in Java?

Final keyword, final variable, final method and class in java exampleFinal is a keyword or reserved word in java and can be applied to member variables, methods, class and local variables in Java. Once you make a reference final you are not allowed to change that reference and compiler will verify this and raise compilation error if you try to re-initialized final variables in java.

What is final variable in Java?
Any variable either member variable or local variable (declared inside method or block) modified by final keyword is called final variable. Final variables are often declare with static keyword in java and treated as constant. Here is an example of final variable in Java

public static final String LOAN = "loan";
LOAN = new String("loan") //invalid compilation error

Final variables are by default read-only.

What is final method in Java
Final keyword in java can also be applied to methods. A java method with final keyword is called final method and it can not be overridden in sub-class. You should make a method final in java if you think it’s complete and its behavior should remain constant in sub-classes. Final methods are faster than non-final methods because they are not required to be resolved during run-time and they are bonded on compile time. Here is an example of final method in Java:

class PersonalLoan{
 public final String getName(){
     return "personal loan";
 }
}
class CheapPersonalLoan extends PersonalLoan{
    @Override
    public final String getName(){
        return "cheap personal loan"; //compilation error: overridden method is final
    }
}

What is final Class in Java

Java class with final modifier is called final class in Java. Final class is complete in nature and can not be sub-classed or inherited. Several classes in Java are final e.g. String, Integer and other wrapper classes. Here is an example of final class in java

final class PersonalLoan{

}

class CheapPersonalLoan extends PersonalLoan{  //compilation error: cannot inherit from final class
 
}

Benefits of final keyword in Java

Here are few benefits or advantage of using final keyword in Java:

1. Final keyword improves performance. Not just JVM can cache final variable but also application can cache frequently use final variables.

2. Final variables are safe to share in multi-threading environment without additional synchronization overhead.

3. Final keyword allows JVM to optimized method, variable or class.


Final and Immutable Class in Java

Final keyword helps to write immutable class. Immutable classes are the one which can not be modified once it gets created and String is primary example of immutable and final class which I have discussed in detail on Why String is final or immutable in Java. Immutable classes offer several benefits one of them is that they are effectively read-only and can be safely shared in between multiple threads without any synchronization overhead. You can not make a class immutable without making it final and hence final keyword is required to make a class immutable in java.

Example of Final in Java

Java has several system classes in JDK which are final, some example of final classes are String, Integer, Double and other wrapper classes. You can also use final keyword to make your code better whenever it required. See relevant section of java final tutorial for example of final variable, final method and final class in Java.

Important points on final in Java

1. Final keyword can be applied to member variable, local variable, method or class in Java.
2. Final member variable must be initialized at the time of declaration or inside constructor, failure to do so will result in compilation error.

3. You can not reassign value to final variable in Java.

4. Local final variable must be initializing during declaration.

5. Only final variable is accessible inside anonymous class in Java.

6. Final method can not be overridden in Java.

7. Final class can not be inheritable in Java.

8. Final is different than finally keyword which is used on Exception handling in Java.

9. Final should not be confused with finalize() method which is declared in object class and called before an object is garbage collected by JVM.

10. All variable declared inside java interface are implicitly final.

11. Final and abstract are two opposite keyword and a final class can not be abstract in java.

12. Final methods are bonded during compile time also called static binding.

13. Final variables which is not initialized during declaration are called blank final variable and must be initialized on all constructor either explicitly or by calling this(). Failure to do so compiler will complain as "final variable (name) might not be initialized".

14. Making a class, method or variable final in Java helps to improve performance because JVM gets an opportunity to make assumption and optimization.

15. As per Java code convention final variables are treated as constant and written in all Caps e.g.

private final int COUNT=10;

16. Making a collection reference variable final means only reference can not be changed but you can add, remove or change object inside collection. For example:

private final List Loans = new ArrayList();
list.add(“home loan”);  //valid
list.add("personal loan"); //valid
loans = new Vector();  //not valid

That’s all on final in Java. We have seen what final variable, final method is and final class in Java and what does those mean. In Summary whenever possible start using final in java it would result in better and faster code.


Java Tutorial you may like:

15 comments :

michee said...

what's the difference between final and immutable?

Great blog!
Happy New Year!

Anand VijayaKumar said...

@ Michee - Immutable approximately means the same as a final. Something that cant be changed. But, in Java, the term immutable is usually used along with String objects when we say Java String objects are Immutable.

Anand
Inheriting Java

Santu said...

Can final variables cloned along with object ?

ravi said...

Race conditions occurs when two thread operate on same object without proper synchronization and there operation interleaves on each other. Classical example of Race condition

Read more: http://javarevisited.blogspot.com/2012/02/what-is-race-condition-in.html#ixzz1xRNIKEnV

POJO said...

Making a class final is double edged sword, Some Java programmer argue that final class severely limits client's ability to inherit and extend while Some Java programmer are in opinion that final class, which is Immutable are best way to design robust System.It's trade-off. James Gosling suggest that Making a class final in Java for security reason e.g. String can be justified.

Anonymous said...

What is blank final variable in Java ? is it mandatory to initialize blank final variable in Constructor ? What is my class have multiple constructor, do I need to initialized blank final variable in all constructor or is there any alternative of that ?

Shirin said...

A blank final, means that it's not initialized explicitly at it's declaration, can only be initialized in an initialization block or a constructor. A reference, declared final, doesn't make the object final.

Anonymous said...

Can you please suggest when to use final methods in Java, what are best practices around using final. I mean, I always get confused, should I make a class final or not, is there a rule book which we can follow? I hope my IDE suggest this but not so far.

Jonathan Schneider said...

> "Local final variable must be initializing during declaration."

This doesn't seem to be correct? For example, the following snippet compiles and runs okay under Java 1.7.0:

final int f; // Declare a final local variable
System.out.println(new java.util.Date()); // Do some other things
f = 5; // Initialize the final variable
System.out.println(f); // Use the final variable

If you then attempt to add a second assignment of the f variable, that *does* cause a compile-time error ("variable f might already have been assigned").

Anonymous said...

Can you please provide some examples of final methods from Java API? I am learning final keywords and want to know more about, how to use final with methods.

Imran Husain said...

Can you provide the solution of this Q??
Q:As we know that if at any point some exception occurs in the code then an instance of Exception class (or its subclass) is thrown from that point. So my question is the instantiation comes into picture at runtime while checked exception are checked at compile time , therefore no instantiation is there, so how compiler comes to know that it is a compile time exception.

Sudha said...

One Suggestion
You had cover everything about final here. One thing which as if i know is missing is about final instance variable,final static variable and final local variable (which you cover) missed is about the default values what we get for instance and static variable we will not get for final instance and final static variables , where we can initialize them in constructor and initialization block,final local variable will compile fine if we are not using it in code . Write something about this final keyword will get cover fully .

You can write to me at sudha.enigmatic@gmail.com

Sachin Tiwari said...

"You can not make a class immutable without making it final and hence final keyword is required to make a class immutable in java"

I don't agree with this statement, a very simple example would be:
- a final class can still be mutable if its fields are non-final and are not-private or have setters available for them
- similarly a non-final class can still be immutable if all its fields are final or are either private with no setters provided for them

Anonymous said...

One more thing to note about final variables in Java is that, till Java 7, you can not use a non final local variable inside anonymous class, but from Java 8 you can. JDK 8 introduced a concept called effective final, a variable is considered effective final if it is not modified after initialization in local block. What this means is you can now use local variable without final keyword inside anonymous class or lambda expression, provided they must be effectively final. In short, you can save some keystroke while declaring local final variables indented to be used by anonymous class. Here is an example of using effective final variable in Java 8 :


public class EffectiveFinalJava8 {

public static void main(String[] args) {

String nonFinal = "I am non final local variable";
Runnable r = new Runnable() {
@Override
public void run() {
System.out.println("Using non-final local variable inside anonymous class in Java");
System.out.println("Value of non-final variable is : " + nonFinal);

// compile time error - must be final or effective final
// nonFinal = "Can I change non-final variable inside anonymous class";
}
};
}

}

Bo Wang said...

For the last example, shouldn't it be "Loans.add(“home loan”);
Loans.add("personal loan");" ?

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