The difference between [C] and String in jmap output

clipboard.png
as shown in the figure, in the heap typed by jmap, [C and String appear at the same time.
it"s a little strange that String itself is saved in final char [], and it is listed here at the same time, which means that [the number of instances and memory footprint of C includes String, or is String counted separately?

Java
Mar.04,2021
The actual memory occupied by

String should be String + [C ] in the figure above.

to verify this, I wrote Mini Program 

import java.io.*;
import java.util.*;

public class MemoryMap {
    static long bytes = 0;

    public static void main(String[] args) throws IOException { // Line 1
        Set<String> opt = new HashSet<String>();
        opt.addAll(Arrays.asList(args));
        List<String> list= new ArrayList< String>();
        if (opt.contains("alloc")) {
            for (int i = 0; i < 100; iPP) {
                String s = foo(1024 * 1024, i); //100Mb
                bytes += s.getBytes().length;
                list.add(s);
            }
        }
        System.out.println("Bytes=" + bytes + ", press Enter to exit...");

        InputStreamReader in = new InputStreamReader(System.in);
        BufferedReader br = new BufferedReader(in);
        if(opt.contains("gc")){
            System.gc();
        }
        String a = br.readLine();
        System.out.println(a);
        System.out.println(list.size());
    }

    private static String foo(long count, int k) { // Line 6
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < count; iPP) {
            sb.append(i % 10);
        }
        sb.append(k);
        return sb.toString();
    } 

}

compile 

javac MemoryMap.java

run

separately 
java MemoryMap alloc gc 

java MemoryMap gc

and open another terminal window to view

respectively.

assignment of 100m + characters 

?  $ jps
12426 MemoryMap
12428 Jps

?  $ jmap -histo 12426

 num     -sharpinstances         -sharpbytes  class name
----------------------------------------------
   1:          1160      209829280  [C
   2:            92        9702688  [I
   3:           485          55304  java.lang.Class
   4:            11          33232  [B
   5:          1147          27528  java.lang.String
   6:           525          26424  [Ljava.lang.Object;
   7:            79           5688  java.lang.reflect.Field

cases where there is no allocation 

?  $ jmap -histo 12912

 num     -sharpinstances         -sharpbytes  class name
----------------------------------------------
   1:            92         685024  [I
   2:          1038         110832  [C
   3:           485          55304  java.lang.Class
   4:            14          33872  [B
   5:           524          25968  [Ljava.lang.Object;
   6:          1025          24600  java.lang.String
   7:            79           5688  java.lang.reflect.Field

the difference in the number of string bytes between the two cases is 27528-24600mm 2928
the difference in the number of bytes in the character array is 209829280-110832209718448

actually 100m + numeric characters account for 200m + bytes in memory

Previous: When idea created the module of java, what was the difference between the two?

Next: Put in the icon, when you press the button, but press the button itself.

css mysql arrays josn react html typescript webpack npm sass R objective-c .net sql-server jquery python-3.x angularjs django angular excel regex iphone ajax linux xml pandas vba spring database wordpress string wpf xcode windows bash postgresql oracle multithreading eclipse list firebase algorithm macos forms image scala visual-studio azure bootstrap spring-boot react-native python-2.7 docker performance function winforms matlab powershell apache dataframe api sqlite numpy rest shell selenium flutter dart maven loops qt swing android-studio csv express file class tensorflow sorting codeigniter perl
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b35068-2b7b2.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b35068-2b7b2.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
Need Help?