Final methods override java

The “final” Keyword in Java

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1. Overview

While inheritance enables us to reuse existing code, sometimes we do need to set limitations on extensibility for various reasons; the final keyword allows us to do exactly that.

In this tutorial, we’ll take a look at what the final keyword means for classes, methods, and variables.

2. Final Classes

Classes marked as final can’t be extended. If we look at the code of Java core libraries, we’ll find many final classes there. One example is the String class.

Consider the situation if we can extend the String class, override any of its methods, and substitute all the String instances with the instances of our specific String subclass.

The result of the operations over String objects will then become unpredictable. And given that the String class is used everywhere, it’s unacceptable. That’s why the String class is marked as final.

Any attempt to inherit from a final class will cause a compiler error. To demonstrate this, let’s create the final class Cat:

And let’s try to extend it:

public class BlackCat extends Cat

We’ll see the compiler error:

The type BlackCat cannot subclass the final class Cat

Note that the final keyword in a class declaration doesn’t mean that the objects of this class are immutable. We can change the fields of Cat object freely:

Cat cat = new Cat(); cat.setWeight(1); assertEquals(1, cat.getWeight()); 

If we follow the rules of good design strictly, we should create and document a class carefully or declare it final for safety reasons. However, we should use caution when creating final classes.

Notice that making a class final means that no other programmer can improve it. Imagine that we’re using a class and don’t have the source code for it, and there’s a problem with one method.

If the class is final, we can’t extend it to override the method and fix the problem. In other words, we lose extensibility, one of the benefits of object-oriented programming.

3. Final Methods

Methods marked as final cannot be overridden. When we design a class and feel that a method shouldn’t be overridden, we can make this method final. We can also find many final methods in Java core libraries.

Sometimes we don’t need to prohibit a class extension entirely, but only prevent overriding of some methods. A good example of this is the Thread class. It’s legal to extend it and thus create a custom thread class. But its isAlive() methods is final.

This method checks if a thread is alive. It’s impossible to override the isAlive() method correctly for many reasons. One of them is that this method is native. Native code is implemented in another programming language and is often specific to the operating system and hardware it’s running on.

Let’s create a Dog class and make its sound() method final:

Now let’s extend the Dog class and try to override its sound() method:

public class BlackDog extends Dog < public void sound() < >>

We’ll see the compiler error:

- overrides com.baeldung.finalkeyword.Dog.sound - Cannot override the final method from Dog sound() method is final and can’t be overridden

If some methods of our class are called by other methods, we should consider making the called methods final. Otherwise, overriding them can affect the work of callers and cause surprising results.

If our constructor calls other methods, we should generally declare these methods final for the above reason.

What’s the difference between making all methods of the class final and marking the class itself final? In the first case, we can extend the class and add new methods to it.

In the second case, we can’t do this.

4. Final Variables

Variables marked as final can’t be reassigned. Once a final variable is initialized, it can’t be altered.

4.1. Final Primitive Variables

Let’s declare a primitive final variable i, then assign 1 to it.

And let’s try to assign a value of 2 to it:

public void whenFinalVariableAssign_thenOnlyOnce() < final int i = 1; //. i=2; >

The final local variable i may already have been assigned

4.2. Final Reference Variables

If we have a final reference variable, we can’t reassign it either. But this doesn’t mean that the object it refers to is immutable. We can change the properties of this object freely.

To demonstrate this, let’s declare the final reference variable cat and initialize it:

If we try to reassign it we’ll see a compiler error:

The final local variable cat cannot be assigned. It must be blank and not using a compound assignment

But we can change the properties of Cat instance:

cat.setWeight(5); assertEquals(5, cat.getWeight());

4.3. Final Fields

Final fields can be either constants or write-once fields. To distinguish them, we should ask a question — would we include this field if we were to serialize the object? If no, then it’s not part of the object, but a constant.

Note that according to naming conventions, class constants should be uppercase, with components separated by underscore (“_”) characters:

static final int MAX_WIDTH = 999;

Note that any final field must be initialized before the constructor completes.

For static final fields, this means that we can initialize them:

For instance final fields, this means that we can initialize them:

Otherwise, the compiler will give us an error.

4.4. Final Arguments

The final keyword is also legal to put before method arguments. A final argument can’t be changed inside a method:

public void methodWithFinalArguments(final int x)

The above assignment causes the compiler error:

The final local variable x cannot be assigned. It must be blank and not using a compound assignment

5. Conclusion

In this article, we learned what the final keyword means for classes, methods, and variables. Although we may not use the final keyword often in our internal code, it may be a good design solution.

As always, the complete code for this article can be found in the GitHub project.

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

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Can We Override Final Method in Java?

A method in a subclass is said to Override a method in its superclass if that method has a signature identical to a method in its superclass. When this method is called by an object of the subclass, then always the Overridden version will be called. In other words, the method which was Overridden in the superclass will become hidden.

The JVM has 2 choices, it can either call the original method present in the superclass or the Overridden method present in the subclass. Due to late binding, this decision is taken at run-time and not at compile-time.

Can We Override a Final Method?

No, the Methods that are declared as final cannot be Overridden or hidden. For this very reason, a method must be declared as final only when we’re sure that it is complete.

• It is noteworthy that abstract methods cannot be declared as final because they aren’t complete and Overriding them is necessary.
• Methods are declared final in java to prevent subclasses from Overriding them and changing their behavior, the reason this works is discussed at the end of this article.
• The advantage of the final keyword is that it stops developers from intentionally or unintentionally changing the behavior of methods that should not be changed for security or other reasons.

Result of Overriding a Final Method

• The class IntegralOperations has two methods, add and subtract that perform the expected operation.
• Both add and subtract are declared final.
• The class child extends IntegralOperations and tries to Override add and subtract.

Java

prog.java:13: error: add(int,int) in child cannot override add(int,int) in IntegralOperations @Override int add(int a, int b) < return a - b; >^ overridden method is final prog.java:16: error: subtract(int,int) in child cannot override subtract(int,int) in IntegralOperations @Override int subtract(int a, int b) < return a * b; >^ overridden method is final 2 errors

As evident from the above Output, attempting to Override a final method causes a compile-time error.
This proves that final methods cannot be overridden in Java.

Why Does The final Keyword Prevent Overriding?

In the beginning, it was discussed that methods to be Overridden follow Dynamic Method Dispatch (late binding)But the methods which are declared as final follow static binding (early binding), which means that the method definition will be grouped with a body at compile-time itself.

In other words, JVM must know exactly which method to call at compile-time.To accomplish this, for every final method definition there must be a unique body. But if Overriding is allowed then there can be multiple bodies for a method definition and then the compiler won’t be able to choose one of them.

This issue is seen in the above example where we have two bodies each for add and subtract methods. This prompts the compiler to throw an error at compile-time. In this way, the final keyword prevents Overriding and guards the semantics of a method.

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