Analysis of final modifier examples in java

Final modifier:

Final modified member variables must have an initial value explicitly specified by the programmer.
Field of class: The initial value must be specified in a static initialization block or when declaring the Field.
Instance Field: The Field must be declared in a non-static initialization block or the initial value must be specified in the constructor.

Final local variables: must be explicitly initialized by the programmer.

What is the difference between final modified basic variables and reference type variables?

Final modified basic variables: Basic variables cannot be reassigned.
Final modified reference variable: Only the address referenced by this reference type is guaranteed not to change, that is,
always refers to the same object, but this object can completely change.

/**
  
*/
import java.util.*;
public class Demo5
{
 public static void main(String[] args)
 {
  final B b = new B(22);
  b.test();
  //合法  改变值，但仍是指向同一个引用
  b.setAge(20);
  System.out.println(b.getAge());
  // 非法
  // b = null;
  b.test2();
 }
}
 /**
 fianl修饰成员变量
*/
class A
{
 //合法
 final int a = 10;
 //构造器或初始化化块中指定初始值
 final String str;
 final int c;
 final static double d;
 {
  str = "hello";
  //非法
  // a = 100;
 }
 static 
 {
  d = 100;
 }
 
 // 构造器可对没在初始化块中指定的Field指定初始值
 public A()
 {
  // 非法
  // str = "ddd";
  c = 1000;
 }
 public double changFinal()
 {
  // 不能再普通方法中为final指定初始值
  // return d = 1000.90;
  return 0;
 }
}
 /**
 fianl修饰数组 对象
*/
class B
{
 private int age;
 public B()
 {
 
 }
 
 public B(int age)
 {
  this.age = age;
 }
 
 public void setAge(int age)
 {
  this.age = age;
 }
 
 public int getAge()
 {
  return this.age;
 }
 
 public void test()
 {
  final int[] arr={23,434,56,898};
  System.out.println(Arrays.toString(arr));
  Arrays.sort(arr);
  System.out.println(Arrays.toString(arr));
  arr[2] = 200;
  System.out.println(Arrays.toString(arr));
  //下面对Arr重新赋值 非法
  // arr = null;
 }
 
 /**
  加深对final的理解
 */
 public void test2()
 {
  String str1 = "美好未来";
  //直接引用常量池中的"美好未来"
  String str2 = "美好"+"未来";
  //true
  System.out.println(str1 == str2);
  
  String s1 = "美好";
  String s2 = "未来";
  String s3 = s1+s2;
  //false  s1 s2只是变量 在编译时期不能确定 
  //若想在编译时期确定 则用 final修饰 s1 s2
  System.out.println(str1 == s3);
 }
}

Do you guys know something about the final modifier in Java? I believe it has been explained clearly in the comments, so I won’t go into details here

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