Storage principle of string

Yes String String to make a brief introduction

1. String Represents string type , Belong to Reference data type , Not a basic data type
2. stay Java Use casually in In double quotation marks The are String object , for example :“abc” “hello”
3. Java Specified in the , A string in double quotation marks , yes immutable Of , in other words "abc" From birth to death , immutable , Can't be "abcd", Nor can it become "ab"
4. stay JDK The string enclosed in double quotation marks , for example :“abc” "def" All are Directly stored in the string constant pool of the method area Of
5. Why are strings stored in the string constant pool ? Strings are used too often in actual development , in order to Improve the efficiency of execution

example1:

        String s1 = "abcdef";
        String s2 = "abcdef"+"xy";

These two lines of code create 3 String objects , All in String constant pool in

String s3 = new String("xyz");

Call the constructor to create a string , Use new A string object created in a way , In this code xyz Is from the string constant pool ; Everything enclosed in double quotation marks has a copy in the string constant pool .new Object is to open up space in heap memory
Code corresponding memory map :

note： Local variables are all created in stack memory , Local variables in code s3 What is kept is String The address of the object in heap memory

example2:

class User：

public class User {
    
    private int id;
    private String name;
    public User(int id, String name) {
    
        this.id = id;
        this.name = name;
    }
}

class UserTest:

public class UserTest {
    
    public static void main(String[] args) {
    
        User user = new User(110," Zhang San ");
    }
}

Strings enclosed in double quotation marks open up memory space in the string constant pool in the method area , so " Zhang San " In the string constant pool ; the new Of User Object opens up space in heap memory , in other words id = 110 Space opened up in heap memory , and name What is saved is its... In the string constant pool Memory address ;user As a local variable, the address is opened in the stack memory , What is saved is the memory address of the corresponding object in the heap memory ; The corresponding memory diagram is as follows :

Add :
Interview questions : The following code creates several objects ?

        String s1 = new String("hello");
        String s2 = new String("hello");

answer ： altogether 3 Objects ！ String constant pool 1 Objects (“hello”), In heap memory 2 Objects (2 individual String object )！( Memory diagram as above )

example3:

note: Garbage collector GC Don't release ( Recycling ) Constant

1. In the following code ,s2 Of ”hello“ Whether it will be created ? What is the result of the output ?

        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1==s2);

Can't ！ Double quotes "hello" Is stored in the string pool of the method area ,s1 It has been created , so s2 No more creation , Instead, it refers directly to its address in the string constant pool . The output is zero true, because **== The double equal sign compares memory addresses **, Variables s1 and s2 It's all preserved "hello" Memory address in string constant pool , Therefore, the output true!
2. In the following code , What is the result of the output ?

        String x = new String("xyz");
        String y = new String("xyz");
        System.out.println(x==y);

false! local variable x,y Separately saved String The address of the object in heap memory , and x、y The saved String The object is different , Are created in the heap memory respectively ！ Therefore, the output false!
The memory diagram corresponding to the above code :

Through this case, we can also know , Comparison of string objects cannot be done with "“, and You should use String Class equals Method .Objec The bottom layer of the class is also ”",String Class overridden equals!