Explain final, finally, and finalize

One 、final keyword

1.1 decorator

When it modifies a class, it means that the class cannot be inherited , Because abstract classes are used to be inherited , therefore abstract Key words and final Keywords cannot coexist .final All member methods in a class are implicitly defined as final Method .

1.2 Modification methods

When a method is declared as final when , This method does not allow any subclass to override this method , But subclasses can still use this method .
PS： The difference between rewriting and overloading ： Method overloading and rewriting are ways to implement polymorphism , The difference is that the former implements compile time polymorphism , The latter implements runtime polymorphism .

heavy load ：

 Overload occurs in a class , If a method with the same name has a different parameter list （ Different parameter types 、 The number of parameters is different or both are different ） be 
 Considered overloaded , Overload has no special requirements for return type , Cannot differentiate by return type .

rewrite ：

1. Occurs between parent and child classes 
2. Method name , parameter list , Return type （ Except that the return type of the method in the subclass is the subclass of the return type in the parent class ） It has to be the same 
3. The access modifier limit must be greater than the access modifier of the overridden method （public>protected>default>private)
4. The overriding method must not throw a new check exception or a more general check exception than the overridden method declaration 

1.3 Modifier constant

When final When modifying a basic data type , Indicates that the value of the basic data type cannot change once it is initialized ; If final When modifying a reference type , After it is initialized, it can no longer point to other objects , But the content of the object that the reference refers to can change . It's one thing in essence , Because the value of the reference is an address ,final Required value , That is, the value of the address does not change .—— Reference immutable
final Member variables represent constants , Can only be assigned once , The value does not change after assignment .final Modify a member variable （ attribute ）, Initialization must be displayed . There are two initialization methods , One is to initialize when a variable is declared ; The second method is to declare a variable without assigning an initial value , However, the initial value should be assigned to the variable in all constructors of the class in which the variable is located .
As shown below ：

When a This attribute is decorated as final, Instead of static When , It belongs to the resource of the instance object of the class . When static final When decorating at the same time , It belongs to class resources .
When a class is loaded into memory , Because it is the resource of the instance object , This property is not allocated memory space , It just defines a variable a, Only when the class is instantiated （ Create objects ） This attribute is used to allocate memory space , When instantiating, the constructor is executed at the same time , So the property is initialized , This is consistent with the need to initialize when it is allocated memory space .

Two 、finally keyword

finally As part of exception handling , It can only be found in try/catch In the sentence , And a statement block is attached to indicate that the statement must be executed in the end , Often used in situations where resources need to be released .
finally Not being carried out ：
（1） Only with finally Corresponding try When the statement block gets executed ,finally Statement block will execute .
（2）try Terminated in statement block Java The running or thread of the virtual machine is interrupted interrupted Or terminate .

public static int a() {
         int a = 0;
        try{
            a++;
            return a;
        }catch (Exception e){
            a++;
            return a;
        }finally {
            a++;
            return a;
        }
    }

The output of the above code is ：2.
finally Statement must be executed in the modified code , Throw or not throw the exception , Will have a first time a++, And then finally Once in a++.
Sum up , In both return Under the circumstances ,finally Medium return For the end result .

3、 ... and 、finalize() Method

Refer to another article of mine ：JVM series （2）—— Garbage collection
finalize() The method is Object class , Because all classes inherit from Object class , So all classes have finalize Method .
Before an object is actually declared dead , It will be marked twice ： The first is after reachability analysis , Found recyclable , Will be marked ; The second time is whether it is necessary to call finalize() Method filtering , Screening rules ： Is it useful to rewrite finalize() Method . Second execution finalize() Method , It is determined that there is no need to call .
If you need to call , Then mark it for the second time , Then the object is recycled .
This method is strongly discouraged , The uncertainty is too great , The order of object calls cannot be guaranteed .