Instance analysis of final modifiers in Java

Final modifier:

The final modifier member variable must have the programmer display the specified initial value.
Field of a class: the initial value must be specified in the static initialization block or when the Field is declared.
Instance Field: the initial value must be specified in the declaration Field or constructor in a non-static initial block.

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

What is the difference between a final modified base variable and a reference type variable?

Final modifies the base variable: the base variable cannot be reassigned.
Final modifies the reference variable: only ensures that the address referenced by the reference type does not change, i.e
Always reference the same object, but this object can be changed.

/**

  

*/

import java.util.*;

public class Demo5

{

 public static void main(String[] args)

 {

  final B b = new B(22);

  b.test();

  //Legal   Change the value, but still point to the same reference 

  b.setAge(20);

  System.out.println(b.getAge());

  //Illegal < br / >
  // b = null;

  b.test2();

 }

}

 /**

 fianl Modifier member variable 

*/

class A

{

 //Legal < br / >
 final int a = 10;

 //Specifies an initial value 
 in the constructor or initialization block
 final String str;

 final int c;

 final static double d;

 {

  str = "hello";

  //Illegal < br / >
  // a = 100;

 }

 static 

 {

  d = 100;

 }

 

 //The constructor can specify an initial value 
 for a Field that is not specified in the initialization block
 public A()

 {

  //Illegal < br / >
  // str = "ddd";

  c = 1000;

 }

 public double changFinal()

 {

  //You cannot specify an initial value 
 for final ina normal method
  // return d = 1000.90;

  return 0;

 }

}

 /**

 fianl Modify the array   object 

*/

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));

  // In the face of the Arr To assign a value Illegal < br / >
  // arr = null;

 }

 

 /**

   To deepen the final The understanding of the 

 */

 public void test2()

 {

  String str1 = " The bright future ";

  //Direct reference to "good future" in the constant pool 

  String str2 = " happy "+" In the future ";

  //true

  System.out.println(str1 == str2);

  

  String s1 = " happy ";

  String s2 = " In the future ";

  String s3 = s1+s2;

  //False  S1, s2 is just a variable that cannot be determined at compile time 

  //If you want to confirm at compile time, modify s1, s2
 with final
  System.out.println(str1 == s3);

 }

}

If you are familiar with the final modifier in Java, I believe that it has been explained to you clearly in the comments