扩展 ArrayList 并创建新方法 [英] Extending ArrayList and Creating new methods

问题描述

我在抓住某些东西时遇到了一些问题 - 我可能完全错误.

I'm having a bit of a problem grasping something - I might be going about this completely wrong.

我正在尝试创建一个扩展 ArrayList 的类，但有几个方法可以增加功能(至少对于我正在开发的程序而言).

I am trying to create a class which extends ArrayList but has several methods which increase the functionality (at least for the program I am developing.)

其中一种方法是 findById(int id)，它搜索每个 ArrayList 对象以查找特定的 id 匹配项.到目前为止它正在工作，但它不会让我做 for (Item i : this) { i.getId();}

One of the methods is a findById(int id), which searches each ArrayList object for a particular id match. So far it's working, but it won't let me do for (Item i : this) { i.getId(); }

我不明白为什么?

完整代码:

public class CustomArrayList<Item> extends ArrayList<Item> {

    // declare singleton instance
    protected static CustomArrayList instance;

    // private constructor
    private CustomArrayList(){
        // do nothing
    }

    // get instance of class - singleton
    public static CustomArrayList getInstance(){
        if (instance == null){
            instance = new CustomArrayList();
        }
        return instance;
    }

    public Item findById(int id){
        Item item = null;
        for (Item i : this) {
            if (i.getId() == id) {
                      // something
         }
        }
        return item;
    }
    public void printList(){
        String print = "";
        for (Item i : this) {
            print += i.toString() + "\n";
        }
        System.out.println(print);
    }
}

推荐答案

更改

public class CustomArrayList<Item> extends ArrayList<Item> {

到

public class CustomArrayList extends ArrayList<Item> {

<小时>

我怀疑 Item 是您要存储在列表中的类的名称.通过在 CustomArrayList 之后添加 ，您引入了一个 类型参数，它隐藏了这个类.

---

I suspect Item is the name of the class that you want to store in the list. By adding <Item> after CustomArrayList you're introducing a type parameter which shadows this class.

使用参数，你的代码等于

public class CustomArrayList<T> extends ArrayList<T> {
    // ...
        for (T i : this) { i.getId(); }
    // ...
}

这显然不会总是有效，因为 T 可能指任何类型.

which obviously won't always work, as T may refer to any type.

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