Java：檢索具有唯一ID的ArrayList的信息只會打印出添加到其中的最后一個元素的信息

Question

首先，我會告訴您我的問題是什么，下面是代碼：

當我運行Class1我可以注冊我的第一個對象，該對象具有唯一的ID。 之后，我注冊第二個對象，該對象也具有唯一的ID。 然后，我嘗試通過詢問第一個對象的ID來檢索我的第一個對象信息 。 現在的問題是，當我請求添加到ArrayList的第一個對象時，以下內容將始終被打印出來：“ ID錯誤。您無法檢索對象信息。” 但是 ，當我嘗試檢索第二個對象（該對象最后添加到ArrayList ）時，它將按我的需要打印出信息。

我的問題是，為什么我只能訪問添加到ArrayList的最后一個對象，並且該如何打印具有唯一ID的第一個對象信息？

這是Class1 ：

import java.util.Scanner;
import java.util.*;

public class Class1 {

   public static void main(String[] args){
       Class1 class1 = new Class1();
       class1.presentoptions();
       class1.createObject();
   }

   Scanner sc = new Scanner(System.in);
   String name;
   String ID;
   double salary;
   Class2 class2;
   final String END_LINE = System.lineSeparator();

   public void presentoptions(){
       class2 = new Class2();
       while (true){
           System.out.println("=== Welcome === ");
           System.out.println("Choose an option below: ");
           System.out.println(" ");
           System.out.println("1. Register an object. ");
           System.out.println("2. Retrieve an objects information. ");
           System.out.println("3. Quit this program. ");
           int option = sc.nextInt();

           switch (option) {
               case 1:
                   System.out.println("What type of object? " + END_LINE
                        + " - Worker. " + END_LINE);
                        // other objects
                   String type = sc.nextLine();
                   createObject(); // creating the specified employee
                   break;

               case 2:
                   class2.retrieveObject();
                   break;
               case 3:
                   System.out.println("You've quitted the program.");
                   System.exit(0);

               default:
                   System.out.println("Error. Please try again.");
                   break;
           }
       }
}


   public void createObject(){
       class2 = new Class2();
       System.out.println("Write the name of the object (worker): ");
       String typeofobject = sc.nextLine();

       UUID uniqueID = UUID.randomUUID();
       String x = "" + uniqueID;
       System.out.println("The new ID of the " + typeofobject + " is: " + uniqueID + ".");
       System.out.println();

       System.out.println("What's the name of the new " + typeofobject + "?");
       name = sc.nextLine();
       System.out.println("What's the salary of the new " + typeofobject + "?");
       salary = sc.nextDouble();
       Employee employee = new Employee(x, name, salary);

       switch (typeofobject) {
           case "Worker":
               class2.registerObject(employee);
               break;

           default:
               System.out.println("You missspelled the type of object. Run the program again.");
               break;
       }
   }
}

這是Class2 ：

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

class Class2 extends Class1{

   Scanner input = new Scanner(System.in);
   ArrayList<Employee> employees = new ArrayList<Employee>();
   final String END_OF_LINE = System.lineSeparator();

   public void registerObject(Employee employee){
       employees.add(employee);
   }

   public void retrieveObject() {

       System.out.println("Which objects information do you want to retrieve? Type in his/her ID.");
       String inputNewID = input.nextLine();
       for(Employee employee: employees){
           if(inputNewID.equals(employee.getID())){
               System.out.println("ID: " + employee.ID + END_OF_LINE + "Name: " + employee.name + END_OF_LINE + "Salary: " + employee.grossSalary);
           } else {
               System.out.println("The ID is wrong. You can't retrieve the objects information.");
           }
       }
   }
}

最后， Employee類：

import java.util.*;

public class Employee {

   protected String ID;
   protected String name;
   protected double grossSalary;
   final String END_OF_LINE = System.lineSeparator();


   public String getID() {
       return ID;
   }

   public Employee (String ID, String name, double grossSalary){
       this.ID = ID;
       this.name = name;
       this.grossSalary = grossSalary;
   }
}

謝謝。

Answer 1

你有線

class2 = new Class2();

在您的createObject()方法中。 但是class2是您擁有員工ArrayList的地方。 因此，每次調用createObject()時，您就只是吹散了舊的ArrayList。

我認為您可以擺脫這一限制。

Answer 2

注意for循環：

for(Employee employee: employees){
       if(inputNewID.equals(employee.getID())){
           System.out.println("ID: " + employee.ID + END_OF_LINE + "Name: " + employee.name + END_OF_LINE + "Salary: " + employee.grossSalary);
       } else {
           System.out.println("The ID is wrong. You can't retrieve the objects information.");
       }
   }

它會在員工列表中打印FOR EACH對象，無論它是否等於inputNewID，我認為目的是一次打印。

您可以使用布爾值標志獲得所需的行為：

boolean isEmployeeFound = false;
for(Employee employee: employees){
       if(inputNewID.equals(employee.getID())){
           isEmployeeFound = true;
           System.out.println("ID: " + employee.ID + END_OF_LINE + "Name:"+employee.name + END_OF_LINE + "Salary: " + employee.grossSalary);
       } 
   }
 if(!isEmployeeFound){System.out.println("The ID is wrong. You can't retrieve the objects information.");}

就像其他提到的一樣，擺脫createObject函數中的class2 = new Class2（）行​​。

Answer 3

問題出在Class2的for循環中的if / else，或更具體地說是else。 在循環的第一個元素上，將觸發else並給出消息。 處理此問題的一種方法是添加一個布爾值以檢查是否找到了ID，並在循環后將else塊更改為if檢查，如下所示：

   boolean idFound = false;
   for(Employee employee: employees){
       if(inputNewID.equals(employee.getID())){
           idFound = true;
           System.out.println("ID: " + employee.ID + END_OF_LINE + "Name: " + employee.name + END_OF_LINE + "Salary: " + employee.grossSalary);
       } 
   }
   if(! idFound) {
       System.out.println("The ID: " + inputNewID + " is not found. You can't retrieve the objects information.");
   }