In programming, we sometimes hope that certain data cannot be changed. At this time, final comes into play. Final is a keyword in Java, which means "this part cannot be modified." There are two reasons for not wanting to be changed: efficiency and design. There are three situations where final is used: data, methods, and classes.

##           1. Final data

##                                                     The constancy of data is very useful when it can reduce the burden on the system during runtime. For these constant data, I can call them "constants". "Constant" is mainly used in the following two places:

1. Compilation-time constants can never be changed.

2. When initializing during runtime, we hope that it will not be changed. ##​​

​​​

public class Person {
    private String name;

    Person(String name){
        this.name = name;
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

public class FinalTest {
    private final String final_01 = "chenssy";    //编译期常量，必须要进行初始化，且不可更改
    private final String final_02;                //构造器常量，在实例化一个对象时被初始化
    
    private static Random random = new Random();
    private final int final_03 = random.nextInt(50);    //使用随机数来进行初始化
    
    //引用
    public final Person final_04 = new Person("chen_ssy");    //final指向引用数据类型
    
    FinalTest(String final_02){
        this.final_02 = final_02;
    }
    
    public String toString(){
        return "final_01 = " + final_01 +"   final_02 = " + final_02 + "   final_03 = " + final_03 +
               "   final_04 = " + final_04.getName();
    }
    
    public static void main(String[] args) {
        System.out.println("------------第一次创建对象------------");
        FinalTest final1 = new FinalTest("cm");
        System.out.println(final1);
        System.out.println("------------第二次创建对象------------");
        FinalTest final2 = new FinalTest("zj");
        System.out.println(final2);
        System.out.println("------------修改引用对象--------------");
        final2.final_04.setName("chenssy");
        System.out.println(final2);
    }
}

------------------
Output:
------------第一次创建对象------------
final_01 = chenssy   final_02 = cm   final_03 = 34   final_04 = chen_ssy
------------第二次创建对象------------
final_01 = chenssy   final_02 = zj   final_03 = 46   final_04 = chen_ssy
------------修改引用对象--------------
final_01 = chenssy   final_02 = zj   final_03 = 46   final_04 = chenssy

​ ​
The only point explained here is: don’t think that if some data is final, you can know its value at compile time. We will know it through final_03. Here, it is initialized with a random number, and its value can only be known at runtime. .

##, Final Method

All methods that are marked by Final cannot be inherited and changed, so The first reason for using a final method is method locking to prevent any subclasses from modifying it. As for the second reason, it is the efficiency issue. I don’t understand this efficiency issue very clearly. I excerpted this passage from the Internet:

# I don’t understand this passage very well, so I copied it. Can that Java expert explain it to me? !

​

public class Custom extends Person{
    public void method1(){
        System.out.println("Person's  method1....");
    }
    
//    Cannot override the final method from person：子类不能覆盖父类的final方法
//    public void method2(){
//        System.out.println("Person's method2...");
//    }
}

3. Final class

If a class is modified with final, it indicates that the class is the final class and it does not want or allow others to inherit it. In programming, for safety or other reasons, we do not allow any changes to this class, and we do not want it to have subclasses. At this time, we can use final to modify the class.

对于final修饰的类来说，它的成员变量可以为final，也可以为非final。如果定义为final，那么final数据的规则同样适合它。而它的方法则会自动的加上final，因为final类是无法被继承，所以这个是默认的。

四、 final参数

在实际应用中，我们除了可以用final修饰成员变量、成员方法、类，还可以修饰参数、若某个参数被final修饰了，则代表了该参数是不可改变的。

如果在方法中我们修改了该参数，则编译器会提示你：The final local variable i cannot be assigned. It must be blank and not using a compound assignment。

public class Custom {
    public void test(final int i){
      //i++;     ---final参数不可改变
        System.out.println(i);
    }
    
    public void test(final Person p){
     //p = new Person();    --final参数不可变
     p.setName("chenssy");
    }
}

       同final修饰参数在内部类中是非常有用的，在匿名内部类中，为了保持参数的一致性，若所在的方法的形参需要被内部类里面使用时，该形参必须为final。详情参看：//m.sbmmt.com/。

       五、final与static

       final和static在一起使用就会发生神奇的化学反应，他们同时使用时即可修饰成员变量，也可修饰成员方法。

       对于成员变量，该变量一旦赋值就不能改变，我们称它为“全局常量”。可以通过类名直接访问。

       对于成员方法，则是不可继承和改变。可以通过类名直接访问。

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