如何在 JAVA 中使用字符串变量作为对象名称？

Question

I try to write a program which has a register part. İf any user wants to create a new account then the program creates an object , thats ok . But ı dont have got the number of user , so i can not code to infinite.Everytime the program should create an object with different object name (like an id) for new user. Object names need to be formed in order.I am trying to find how can i use string variable as an object name ? for example

User user001 = new User();

After that the user001 name should be changed by java . The names should be in sequence user002,user003 like

Answer 1

You cannot use it directly. But you can use Map :

Map<String, User> users = new HashMap<>();
users.put("user-001", new User(001));
users.put("user-00N", new User(XXX));
...
User currentProcessed = users.get("user-001");

Using Collection API is the only way in which you can store objects under string identifiers.

EDIT: fixes based on comments below. Thanks guys :)

EDIT2:
To obtain userID's which are always in order you have serveral solutions.

For example:
1. You can use a timestamp number - this will not give you a list started from 0 , but it will always be in order no matter what. And this is the simplest solution.
2. You need to store last saved ID somewhere (file) and retrieve it always when you want to create new user. This is more complicated, but achievable. then, you can create a simple ID generator to pick the next value easly:
(In pseudo code)

class IdGenerator {
    public static int nextId() {
        int lastId = getIdFromFile();
        int current = ++lastId;
        store(current);

        return current;
    }
}

Answer 2

I made some explanations in the comments (The most important part here is to make counter static and to increment ID value with each creation of an instance of User:

class User {
    private static int counter = 0; // static! - this is class member, it's common for all instances of Users
    private int ID = counter++; // new User has its own ID number that is incremented with each creation of new instance of User class
    private String name = "user" + ID; // here is unique name of each User

    public int getID() { // convenience method to show ID number
        return ID;
    }

    public String getName() { // convenience method to show name
        return name;
    }

    public String toString() { // String representation of User
        return name;
    }
}

public class MyClass {
    public static void main(String[] args) {
        Map<Integer, String> map = new LinkedHashMap<>(); // LinkedHashMap, just to make the output clear, Users will be stored in the order of adding them to Map

        for(int i = 0; i < 5; i++) { // Here you create 5 new Users and each has its own new ID
            User user = new User();
            map.put(user.getID(), user.getName()); // just to store new Users in a Map (key is ID number, value is user's name
        }

        for(Map.Entry<Integer, String> entry : map.entrySet()) { // this loop is just to print the results contained in Map to the console
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
    }
}

Output that you get:

0: user0
1: user1
2: user2
3: user3
4: user4

Answer 3

Maps do work here. However for a simpler solution just use a List . Access the elements in order (1, 2, 3, ... ) with get(int) . Note the first toString() method below does this. The auto-increment is provide by List which automatically increases the size of the list for you.

import java.util.ArrayList;
import java.util.Scanner;

public class UserRegistrar {

   private ArrayList<User> userList = new ArrayList<>();

   public static void main( String[] args ) {
      UserRegistrar users = new UserRegistrar();
      Scanner scan = new Scanner( System.in );
      for( ;; ) {
         System.out.println( "Enter a user name to register or a blank line to exit:" );
         System.out.print( "User name: " );
         String name = scan.nextLine().trim();
         if( name.isEmpty() ) break;
         User user = new User();
         user.setName( name );
         System.out.print( "User account no.: " );
         int acc = scan.nextInt();
         scan.nextLine();  // eat the new line
         user.setAccount( acc );
         users.addUser( user );
         System.out.println( users );
      }
   }

   public void addUser( User user ) {
      userList.add( user );
   }

   @Override
   public String toString() {
      StringBuilder stringBuilder = new StringBuilder();
      for( int i = 0; i < userList.size(); i++ ) {
         stringBuilder.append( i );
         stringBuilder.append( ": " );
         stringBuilder.append( userList.get( i ) );
         stringBuilder.append( '\n' );
      }
      return stringBuilder.toString();
   }


   public static class User {
      private String name;
      private int account;

      @Override
      public String toString() {
         return "User{" + "name=" + name + ", account=" + account + '}';
      }

      public String getName() {
         return name;
      }

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

      public int getAccount() {
         return account;
      }

      public void setAccount( int account ) {
         this.account = account;
      }
   }
}