java 扩展 ArrayList 并创建新方法
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Extending ArrayList and Creating new methods
提问by Cody
I'm having a bit of a problem grasping something - I might be going about this completely wrong.
我在抓住某些东西时遇到了一些问题 - 我可能完全错误地处理了这个问题。
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.)
我正在尝试创建一个扩展 ArrayList 的类，但有几种方法可以增加功能（至少对于我正在开发的程序而言）。
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(); }
其中一种方法是 findById(int id)，它在每个 ArrayList 对象中搜索特定的 id 匹配项。到目前为止它的工作，但它不会让我做for (Item i : this) { i.getId(); }
I don't understand why?
我不明白为什么？
Full code:
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);
}
}
回答by aioobe
Change
改变
public class CustomArrayList<Item> extends ArrayList<Item> {
to
到
public class CustomArrayList extends ArrayList<Item> {
I suspect Itemis the name of the class that you want to store in the list. By adding <Item>after CustomArrayListyou're introducing a type parameterwhich shadows this class.
我怀疑Item是您要存储在列表中的类的名称。通过在引入隐藏此类的类型参数<Item>之后添加。CustomArrayList
Withthe <Item>parameter, your code is equal to
随着该<Item>参数，你的代码是等于
public class CustomArrayList<T> extends ArrayList<T> {
// ...
for (T i : this) { i.getId(); }
// ...
}
which obviously won't always work, as Tmay refer to any type.
这显然并不总是有效，因为T可能指的是任何类型。
回答by Jon Skeet
What is getId()? Presumably it's a method in someclass, but we don't know whichclass.
什么是getId()？大概是某个类中的一个方法，但是我们不知道是哪个类。
If you've actually got a class called Itemwith a getId()method, which this is meant to be a list of, you simply need to stop your class from being generic. So instead of this:
如果你真的有一个Item用getId()方法调用的类，这意味着它是一个列表，你只需要阻止你的类成为泛型。所以而不是这个：
public class CustomArrayList<Item> extends ArrayList<Item> {
you want:
你要：
public class CustomArrayList extends ArrayList<Item> {
Currently within your class, Itemdoesn't refer to a class called Item, it refers to a type parametercalled Item.
目前你的类中，Item并非指一类叫做项目，它是指一个类型的参数，称为Item。
Now personally:
现在个人：
- I wouldn't avoid creating singletons unless you really have to
- If you have to, I'd avoid creating them in the way you have (which isn't thread-safe)
- I wouldn't extend
ArrayList<>unless I really had to, preferring composition over inheritance
- 我不会避免创建单身人士，除非你真的必须这样做
- 如果必须，我会避免以您拥有的方式创建它们（这不是线程安全的）
ArrayList<>除非我真的必须，否则我不会扩展，更喜欢组合而不是继承
