what is final in java

Basic usage of the final keyword

In Java, the final keyword can be used to modify classes, methods and variables (including member variables and local variables). Let’s learn about the basic usage of the final keyword from these three aspects. (Recommended learning: java course )

1, decorative class

When a class is modified with final, it indicates that this class cannot be inherited. In other words, if you will never let a class be inherited, you can modify it with final. Member variables in a final class can be made final as needed, but be aware that all member methods in a final class will be implicitly designated as final methods.

When using final to modify a class, you should choose carefully. Unless this class really will not be used for inheritance in the future or for security reasons, try not to design the class as a final class.

2. Modification method

The following passage is excerpted from page 143 of the fourth edition of "Java Programming Thoughts":

"Use There are two reasons for the final method. The first reason is to lock the method to prevent any inherited class from modifying its meaning; the second reason is efficiency. In early Java implementation versions, the final method was converted to inline Call.

But if the method is too large, you may not see any performance gains from inline calls. In recent Java versions, there is no need to use final methods for these optimizations."

Therefore, set a method to final only if you want to explicitly prohibit the method from being overridden in subclasses. That is, the final method of the parent class cannot be overridden by the subclass, which means that the subclass cannot have the same method as the parent class.

The final modified method means that this method is already the "final and final" meaning, that is, this method cannot be overridden (multiple final modified methods can be overloaded).

One thing to note here is: because the premise of rewriting is that the subclass can inherit this method from the parent class, if the final modified method in the parent class also has access control permissions of private, it will cause the child to This method cannot be directly inherited from the class.

Therefore, the same method name and parameters can be defined in the subclass at this time. At this time, there is no longer a conflict between overwriting and final, but a new method is redefined in the subclass. (Note: The private method of the class will be implicitly designated as the final method.)

3. Modified variables Modified variables are the most commonly used place for final, and it is also what this article will focus on next. .

Final member variables represent constants and can only be assigned once, and the value will not change after assignment.

When final modifies a basic data type, it means that the value of the basic data type cannot change once it is initialized; if final modifies a reference type, it cannot change after it is initialized. Let it point to other objects, but the content of the object pointed to by the reference can change. Essentially it's the same thing, because the referenced value is an address, and final requires the value, that is, the value of the address not to change.

final modifies a member variable (attribute) and must be initialized explicitly. There are two initialization methods. One is to initialize the variable when it is declared. The second method is to not assign an initial value when declaring the variable, but to assign it to the variable in all constructors of the class where the variable is located. initial value.

When the parameter type of a function is declared as final, it means that the parameter is read-only. That is, you can read and use the parameter, but you cannot change the value of the parameter.

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