带有删除和添加通知的java自定义arraylist

Question

我想在向数组列表添加某些项或从中删除某些项时进行检测。实际上，我有如下代码：

public class myClass {

    MyCustomArrayList<MyObject> al;

    public void method1() {

        al.add(myObject);        
        // Do other works
        al.remove(myObject)
        // Do other works
    }

    private void DoByEachAdd() {
        //I want array list call this method by adding each item to it.
        // It should be in this class because it is doing some works
        // related to this class. for example changing some private variables
    }

    private void DoByEachRemove() {
        // I want array list call this method by adding each item to it.
        // It should be in this class too.
    }

}

我知道数组列表不具备侦听器或某种通知或事件的功能，如果我想检测add，应该有一个自定义数组列表。比如下课：

class MyArrayList<T> {
    private ArrayList<T> list;

    public MyList(){
        list = new ArrayList<>();
    ...
    }
    public void add(T t) {
        list.add(t) {
        //do other things you want to do when items are added 
    }
    public T remove(T t) {
        list.remove(t);
        //do other things you want to do when items are removed
}

（我从here那里得到）

所以问题是：当remove和add方法被激发时，我如何通知调用DoByEachAdd和DoByEachRemove方法的MyArrayList（al）对象。有人有什么想法吗

Answer 1

你需要给myClass到MyArrayList的访问权限

class MyArrayList<T> {
    private ArrayList<T> list;
    private myClass theClass;

public MyList(myClass theClass){
    list = new ArrayList<>();
    this.theClass = theClass;
...
}
public void add(T t) {
    list.add(t) {
    //do other things you want to do when items are added 
    theClass.DoByEachAdd();
}
public T remove(T t) {
    list.remove(t);
    //do other things you want to do when items are removed
    theClass.DoByEachRemove
}

在你的myClass中，把对象放到你的列表中

public class myClass {

   MyCustomArrayList<MyObject> al;


 public myClass(){

  al = new MyCustomArrayList<MyObject>(this);

 }
public void method1() {

    al.add(myObject);        
    // Do other works
    al.remove(myObject)
    // Do other works
}

public void DoByEachAdd() {
    //I want array list call this method by adding each item to it.
    // It should be in this class because it is doing some works
    // related to this class. for example changing some private variables
}

public void DoByEachRemove() {
    // I want array list call this method by adding each item to it.
    // It should be in this class too.
}

}

Answer 2

首先，遵循命名惯例。其次，你为同一个类使用的三个类名MyList、MyArrayList和MyCustomArrayList会让人感到困惑。至于你的问题，你必须在myClass类型的MyArrayList内有一个实例字段（除非你想将DoByEachAdd和DoByEachRemove重构为static）。这可以通过将其添加为构造函数参数来实现，例如

// inside MyArrayList
private ArrayList<T> list;
private final myClass instance;

public MyArrayList(myClass instance) { // <  NOT MyList
    list = new ArrayList();
    this.myClass = myClass;
}

此外，我对你的做法表示怀疑。其他具有MyArrayList实例的类只能使用add和remove方法。如果你想省去很多麻烦，让所有的方法都可见，要么把list声明为public final，要么把MyArrayList变成ArrayList的子类，例如

public class MyArrayList<T> extends ArrayList<T> {
    private final myClass instance;

    public MyArrayList(myClass instance) { // <  NOT MyList
        list = new ArrayList();
        this.myClass = myClass;
    }

    @Override
    public boolean add(T t) {
        boolean returnThis = super.add(t);
        // do some stuff
        instance.DoByEachAdd();
        return returnThis;
    }

    @Override
    public boolean remove(T t) {
        boolean returnThis = super.remove(t);
        // do some stuff
        instance.DoByEachRemove();
        return returnThis;
    }
}

如果坚持要从remove返回T，请声明另一种方法：

public T removeT(T t) {
    remove(t);
    // do some stuff
    return someT;
}

Answer 3

您可能需要使用Callback机制

Java为回调机制提供了接口 使用以下步骤：

1. 接口是用抽象方法定义的 服务器知道其名称

2. 当 服务器事件发生时实现接口，从而 给出抽象方法的代码

3. 当客户机向服务器注册时，服务器 保存引用客户端的实例变量 其类型为接口类型

4. server类通过调用 该客户端的接口方法

参考以下代码，并对其进行修改以满足您的要求： 代码用于信用卡应用程序模拟。该要求与您的要求类似，即，在调用某个方法时自动触发某个方法。在下面的代码中，调用pinChanged()方法时会自动调用changePin()方法

public interface PinChangeListener {
    public void pinChanged();
}

public CreditCard {
    public PinChangeListener pinChangeListener;

    private int pin;

    public changePin(int pin) {
        this.pin = pin;
        if (pinChangeListener != null) {
            pinChangeListener.pinChanged();
        }
    }
}

要将回调/侦听器连接到信用卡，只需实现PinchangeStener方法：

creditCard.pinChangeListener = new PinChangeListener() {
    public void pinChanged() {
        System.out.println("The pin has been changed");
    }
};