如何显示 arraylist 中的元素？

Question

I am writing a program for cashier queueing system. The program should add, edit, and delete customers info: id, lastname, firstname, and age. The program should prioritize or display first the customers that age 60 above and then the other customers will follow.

Here is my CustomerInfo class.

public class CustomerInfo {
  //instance variables
  public int custID;
  public String lastname, firstname;
  public int age;

  //overloading constructors
  public CustomerInfo(int custID, String lastname, String firstname, int age) {
     this.custID = custID;
     this.lastname = lastname;
     this.firstname = firstname;
     this.age = age;
  }
  public CustomerInfo() {
     custID = 0;
     lastname = "";
     firstname = "";
     age = 0;
  }
  //setters or mutator methods
  public void setCustID(int custID) {
     this.custID = custID;
  }
  public void setLastName(String lastname) {
     this.lastname = lastname;
  }
  public void setFirstName(String firstname) {
     this.firstname = firstname;
  }
  public void setAge(int age) {
     this.age = age;
  }
  //getters or accessor methods
  public int getCustID() {
     return custID;
  }
  public String getLastName() {
     return lastname;
  }
  public String getFirstName() {
     return firstname;
  }
  public int getAge() {
     return age;
  }
}

And this is my add and view code:

import java.util.*;
import javax.swing.*;

public class FinalProject {
    public static void main(String[] args) throws IOException, InterruptedException {
        Scanner input = new Scanner(System.in);
        CustomerInfo[] cusInfo = new CustomerInfo[5];
        ArrayList<Integer> cusId = new ArrayList<>();
        ArrayList<String> lastNames = new ArrayList<>();
        ArrayList<String> firstNames = new ArrayList<>();
        ArrayList<Integer> ages = new ArrayList<>();
        ArrayList<Integer> cusIdSenior = new ArrayList<>();
        ArrayList<String> lastNamesSenior = new ArrayList<>();
        ArrayList<String> firstNamesSenior = new ArrayList<>();
        ArrayList<Integer> agesSenior = new ArrayList<>();
        int count = 0, countSenior = 0;
        CustomerInfo ci = new CustomerInfo();
        int customernum = 0;

        System.out.println("ADD CUSTOMER DETAILS");
        
        for(int i = 0; i < 5; i++) {
           System.out.print("Customer's ID           : ");
           int custID = input.nextInt();
           System.out.print("Customer's Last Name    : ");
           String lastname = input.next();
           System.out.print("Customer's First Name   : ");
           String firstname = input.next();
           System.out.print("Customer's Age          : ");
           int age = input.nextInt();
           CustomerInfo c = new CustomerInfo(custID, lastname, firstname, age);
                    
           cusInfo[customernum] = c;
           customernum++;
           System.out.println();
        }

        //VIEW CUSTOMER DETAILS
        if(customernum == 0) {
          JOptionPane.showMessageDialog(null, "QUEUE IS EMPTY. NOTHING TO VIEW.");
        }
        else{
           System.out.println("VIEW CUSTOMER DETAILS");
           for(int i = 0; i < customernum; i++) {
              cusId.add(ci.custID);
              lastNames.add(ci.lastname);
              firstNames.add(ci.firstname);
              ages.add(ci.age);
              count++;
           }
           for(int i = 0; i < count; i++) {
              if(ages.get(i) >= 60) {
                cusIdSenior.add(cusId.get(i));
                lastNamesSenior.add(lastNames.get(i));
                firstNamesSenior.add(firstNames.get(i));
                ages.add(ages.get(i));
                countSenior++;
              }
           }
           for(int i = 0; i < countSenior; i++) {
             System.out.printf("Customer ID         : %d\n", cusIdSenior.get(i));
             System.out.printf("Customer Last Name  : %s\n", lastNamesSenior.get(i));
             System.out.printf("Customer First Name : %s\n", firstNamesSenior.get(i));
             System.out.printf("Customer Age        : %s\n", agesSenior.get(i));
           }
           for(int i = 0; i < count; i ++) {
              if(ages.get(i) < 60 && ages.get(i) > 0) {
                 System.out.printf("Customer ID         : %d\n", cusId.get(i));
                 System.out.printf("Customer Last Name  : %s\n", lastNames.get(i));
                 System.out.printf("Customer First Name : %s\n", firstNames.get(i));
                 System.out.printf("Customer Age        : %s\n", ages.get(i));
              }
           }
        }
    }
}

The problem is after I run and input all the elements, the it won't display the statements and the program will exit immediately.

Answer 1

I am glad you improved your question and it shows that you've tried. So here is what I was saying earlier. Using a comparator, add the following to your code:

Add this class

public class SortByAge implements Comparator<CustomerInfo> {

    @Override
    public Comparator<CustomerInfo> reversed() {
        return Comparator.super.reversed();
    }

    @Override
    public int compare(CustomerInfo o1, CustomerInfo o2) {
        return o1.getAge() - o2.getAge();
    }
  
}

Then in the main

Scanner input = new Scanner(System.in);
        ArrayList<CustomerInfo> customerInfo = new ArrayList<>();

        System.out.println("ADD CUSTOMER DETAILS");
        for(int i = 0; i < 2; i++) {
           System.out.print("Customer's ID           : ");
           int custID = input.nextInt();
           System.out.print("Customer's Last Name    : ");
           String lastname = input.next();
           System.out.print("Customer's First Name   : ");
           String firstname = input.next();
           System.out.print("Customer's Age          : ");
           int age = input.nextInt();
           CustomerInfo c = new CustomerInfo(custID, lastname, firstname, age);
           customerInfo.add(c);
           System.out.println();
        }
        
        SortByAge sba = new SortByAge();
        Collections.sort(customerInfo,sba.reversed());
        
        for(CustomerInfo ci : customerInfo){
            System.out.printf("Customer ID         : %d\n", ci.getCustID());
            System.out.printf("Customer Last Name  : %s\n", ci.getFirstName());
            System.out.printf("Customer First Name : %s\n", ci.getLastName());
            System.out.printf("Customer Age        : %s\n", ci.getAge());
        }

Answer 2

The logic of the program seems to be redudantly complicated, it is not quite clear why you duplicate the array of customer data cusInfo with the several lists, store the input data in the array and do not use it at all in the loop after "VIEW CUSTOMER DETAILS" message.

When you use ci variable, the lists are populated with empty strings and 0 age.

Simple fixes for the existing code just to make it work could be:

System.out.println("VIEW CUSTOMER DETAILS");
for (int i = 0; i < customernum; i++) {
    ci = cusInfo[i];              //  use the data stored in the array!
    cusId.add(ci.custID);
    lastNames.add(ci.lastname);
    firstNames.add(ci.firstname);
    ages.add(ci.age);
    count++;
}

for (int i = 0; i < count; i++) {
    if (ages.get(i) >= 60) {
        cusIdSenior.add(cusId.get(i));
        lastNamesSenior.add(lastNames.get(i));
        firstNamesSenior.add(firstNames.get(i));
        agesSenior.add(ages.get(i));              // fixed typo, should be agesSenior
        countSenior++;
    }
}

---

However, if the actual task is to sort the customers by their age and move senior people to the top, a custom Comparator should be implemented and used when calling Arrays.sort (eg as a lambda).

The logic of sorting could be like this: if only one of the customers is senior, place it first; else compare the customers by age as is: 60 -> 65 -> 70 -> 35 -> 45 -> 59

Also, the redundant code of populating unnecessary lists may be removed completely, thus resulting in the following:

Scanner input = new Scanner(System.in);
CustomerInfo[] cusInfo = new CustomerInfo[5];

System.out.println("ADD CUSTOMER DETAILS");

for (int i = 0; i < cusInfo.length; i++) {
   System.out.print("Customer's ID           : ");
   int custID = input.nextInt();
   System.out.print("Customer's Last Name    : ");
   String lastname = input.next();
   System.out.print("Customer's First Name   : ");
   String firstname = input.next();
   System.out.print("Customer's Age          : ");
   int age = input.nextInt();
   CustomerInfo c = new CustomerInfo(custID, lastname, firstname, age);
            
   cusInfo[i] = c;
   System.out.println();
}

System.out.println("VIEW CUSTOMER DETAILS");
Arrays.sort(cusInfo, (c1, c2) -> c1.age >= 60 ^ c2.age >= 60 // check if _only one_ is senior
        ? (c1.age >= 60 ? -1 : 1) // put senior first
        : Integer.compare(c1.age, c2.age) // compare by age
);
   
for (CustomerInfo ci : cusInfo) {
    System.out.printf("Customer ID         : %d\n", ci.custID);
    System.out.printf("Customer Last Name  : %s\n", ci.lastname);
    System.out.printf("Customer First Name : %s\n", ci.firstname);
    System.out.printf("Customer Age        : %s\n", ci.age);
}

Sample output:

VIEW CUSTOMER DETAILS
Customer ID         : 1
Customer Last Name  : aa
Customer First Name : bb
Customer Age        : 60
Customer ID         : 4
Customer Last Name  : eg
Customer First Name : jg
Customer Age        : 72
Customer ID         : 5
Customer Last Name  : ww
Customer First Name : nh
Customer Age        : 39
Customer ID         : 3
Customer Last Name  : gg
Customer First Name : qq
Customer Age        : 48
Customer ID         : 2
Customer Last Name  : rr
Customer First Name : ww
Customer Age        : 58