在arraylist中存储对象并返回这些对象

Question

import java.util.*;
import java.util.ArrayList;

class Employee{

    int eid;
    String eName;
    String eAddr;
    int eSalary;

    public Employee(int eid,String eName,String eAddr,int eSalary){
        this.eid   = eid;
        this.eName = eName;
        this.eAddr = eAddr;
        this.eSalary=eSalary;
    }
}

class EmployeeDB{

    ArrayList<Employee> al = new ArrayList<Employee>();

    public boolean addEmployee(Employee e){
        return al.add(e);
    }

    public Employee[] listAll() {
        Employee[] array = new Employee[al.size()];
        System.out.println("SIZE is : "+al.size());
        return al.toArray(array);
    }
}



public class Main
{
    public static void main(String[] args) {
        System.out.println("  ");

        Employee e1 = new Employee(111,"Employee-1","loc-1",100);
        EmployeeDB d1 = new EmployeeDB();
        d1.addEmployee(e1);


        Employee e2 = new Employee(222,"Employee-2","loc-2",200);
        EmployeeDB d2 = new EmployeeDB();
        d2.addEmployee(e2);

        EmployeeDB x = new EmployeeDB();
        Employee[] arr = x.listAll();

        for(Employee e : arr){
            System.out.println( e.eid );
            System.out.println( e.eName );
            System.out.println( e.eAddr );
            System.out.println("**************");
        }
    }
}

这里我将两个对象添加到数组列表中。最后，我尝试在main类函数中显示这两个元素。

但是没有添加元素。在添加两个对象之后，它告诉我数组列表的大小是0。并且不显示任何元素。

所以请告诉我怎样才能得到想要的结果。

Answer 1

在列出EmployeeDB中的所有员工之前，您尚未向其中添加任何employee实例。相反，您需要将它们添加到不同的EmployeeDB实例中。尝试下面的代码片段。

public static void main(String[] args) {
        System.out.println("  ");

        Employee e1 = new Employee(111,"Employee-1","loc-1",100);
        Employee e2 = new Employee(222,"Employee-2","loc-2",200);

        EmployeeDB x = new EmployeeDB();
        x.addEmployee(e1);
        x.addEmployee(e2);
        Employee[] arr = x.listAll();

        for(Employee e : arr){
            System.out.println( e.eid );
            System.out.println( e.eName );
            System.out.println( e.eAddr );
            System.out.println("**************");
        }
    }

Answer 2

您每次都会向新的EmployeeDB对象添加Employee对象。而不是向现有EmployeeDB对象添加Employee对象。

必须使用EmployeeDB object x来添加employee对象以及检索添加的employee对象x.listAll()

public class Main
    {
        public static void main(String[] args) {
            System.out.println("  ");

            EmployeeDB x = new EmployeeDB();

            Employee e1 = new Employee(111,"Employee-1","loc-1",100);
            //EmployeeDB d1 = new EmployeeDB();
            x.addEmployee(e1);


            Employee e2 = new Employee(222,"Employee-2","loc-2",200);
          //  EmployeeDB d2 = new EmployeeDB();
            x.addEmployee(e2);

           // EmployeeDB x = new EmployeeDB();
            Employee[] arr = x.listAll();

            for(Employee e : arr){
                System.out.println( e.eid );
                System.out.println( e.eName );
                System.out.println( e.eAddr );
                System.out.println("**************");
            }
        }
    }

Answer 3

看起来你使用的是公共对象而不是数组。只创建一个对象数组会简单得多。这样你就可以正确地检查属性和列表大小。您将不需要所有这些复杂、混乱的代码、指针和.thises。如果您需要方便地存储数据库，这也可以让您使用JSON。