Java ArrayList 的对象的 add() 方法不起作用

Java ArrayList of objects' add() method not working

提问by InamTaj

Im new to JAVA, and Im facing a beginer's problem, I know :-P

我是 JAVA 新手，我面临初学者的问题，我知道：-P

IN PLAIN ENGLISH => I'm trying to create an ArrayList of objects, and to add new objects in the array when required.

IN PLAIN ENGLISH => 我正在尝试创建一个对象的 ArrayList，并在需要时在数组中添加新对象。

OK, so here is a compact version of my code

好的，这是我的代码的紧凑版本

package ACP.Employee;   //created package
import java.util.ArrayList;     //imported arraylist class
import ACP.Employee.EmployeeClass;  //imported employee class of same package

public class ClientClass
{
    ArrayList<EmployeeClass> empArray = new ArrayList<EmployeeClass>(); //created a new Array List

    public static void main (String[] args)
    {
        int objcount = 0;   //variable to store objct count

        empArray.add(objcount, EmployeeClass obj);
    }
}

The IDE (Eclipse) gives me following errors on empArray.addline

IDE (Eclipse) 在empArray.add行上给我以下错误

• Cannot make a static reference to the non-static empArray
• EmployeeClass cannot be resolved to a variable
• Syntax error on token obj, delete this token.

• 无法对非静态 empArray 进行静态引用
• EmployeeClass 无法解析为变量
• 标记 obj 上的语法错误，删除此标记。

I have also tried to replace that line with the following syntax,

我还尝试用以下语法替换该行，

empArray.add(new EmployeeClass());

BUTthe following error remains.

但以下错误仍然存​​在。

• Cannot make a static reference to the non-static empArray

• 无法对非静态 empArray 进行静态引用

Kindly help out here, I have seen API Spec of ArrayList's add() method, which is following:::

请在这里提供帮助，我已经看到了 ArrayList 的 add() 方法的 API Spec，如下所示：::

void add(int index, Object element) ==>> Inserts the specified element at the specified position index in the list boolean add(Object o) ==>> Appends the specified element to the end of this list. SOURCE (http://www.tutorialspoint.com/java/java_arraylist_class.htm)

void add(int index, Object element) ==>> 在列表中的指定位置 index 插入指定元素 boolean add(Object o) ==>> 将指定元素附加到此列表的末尾。来源（http://www.tutorialspoint.com/java/java_arraylist_class.htm）

采纳答案by arynaq

There are two problems with your code.

您的代码有两个问题。

1. You are trying to access a member of the class ClientClassfrom a static context (main). This is only possible if that member is static. Since empArrayhas no modifiersit will default to package-private which is not static. You will either have to make it accessible in a static context by declaring it static:

   static ArrayList<EmployeeClass> empArray = new ArrayList<>();
   
   or create an instance of ClientClass and access its member

   ArrayList<EmployeeClass> empArray = new ArrayList<EmployeeClass>();
   
   public static void main(String[] args) {
       ClientClass t = new ClientClass();
       t.empArray.add(new Employee());
   
   }
   

2. The way you are passing employees to the list to be added will not compile in java. Here is a good tutorialon how to create objects in java. In fact it seems you are new to Java, so I recommend you start on page one of the tutorials, they are very good for getting familiar with the language fast and you will be productive in no time.

1. 您正在尝试ClientClass从静态上下文 (main)访问类的成员。这只有在该成员是静态的情况下才有可能。由于empArray没有修饰符，它将默认为非静态的 package-private。您必须通过将其声明为静态来使其在静态上下文中可访问：

   static ArrayList<EmployeeClass> empArray = new ArrayList<>();
   
   或者创建一个 ClientClass 的实例并访问它的成员

   ArrayList<EmployeeClass> empArray = new ArrayList<EmployeeClass>();
   
   public static void main(String[] args) {
       ClientClass t = new ClientClass();
       t.empArray.add(new Employee());
   
   }
   

2. 您将员工传递给要添加的列表的方式不会在 java 中编译。这是一个关于如何在java中创建对象的好教程。事实上，您似乎是 Java 的新手，所以我建议您从教程的第一页开始，它们非常有助于快速熟悉该语言，而且您很快就会提高工作效率。

回答by Ilya

From staticmethod you have access only to staticfields.
From not-staticmethod you have access to all fields.
E.g

从static方法您只能访问static字段。
从not-static方法中，您可以访问所有字段。
例如

private int i = 0;
private static int j = 0;

public void increment()
{
   i++; // correct
   j++; // correct
}
public static void staticIncrement()
{
   i++; // compilation error
   j++; // correct
}  

So, in you example empArrayshould be static.

所以，在你的例子中empArray应该是static.

public class ClientClass
{
    static ArrayList<EmployeeClass> empArray = new ArrayList<EmployeeClass>(); 

    public static void main (String[] args)
    {
       int objcount = 0;   //variable to store objct count

       empArray.add(objcount, new EmployeeClass());
       empArray.add(new EmployeeClass());
    }
}