Question

我想检测何时将某些项添加到数组列表或从中删除某些项。其实我有一些代码如下：

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监听器或某种通知或事件的能力，如果我想检测添加应该有自定义数组列表。像下面这样的东西：

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
}

（我是从here获得的）

所以问题是：当MyArrayList和al时，如何通知DoByEachAdd DoByEachRemove）调用remove和add方法的对象已触发{{1}}方法。有些机构有什么想法吗？

Answer 1

您需要授予myClass至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
}

并在myClass中将对象提供给您的列表

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

您可能需要使用Callback机制。

Java提供带接口的回调机制使用以下步骤：

使用抽象方法定义接口其名称为服务器所知。
每个客户端在执行操作时都要执行操作服务器事件发生实现接口，从而为抽象方法提供代码。
当客户端注册服务器时，服务器拥有一个引用客户端的实例变量其类型是接口类型。
服务器类通过调用来调用客户端操作该客户端的接口方法。

请参阅以下代码并根据您的要求进行修改：该代码用于信用卡应用程序模拟。该要求与您的要求类似，即在调用某种方法时自动触发某些方法。在下面的代码中，pinChanged()方法会在changePin()时自动调用方法被称为。

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/监听器连接到信用卡，您只需要实现PinChangeListener方法：

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

Answer 3

首先，遵循命名约定。其次，您用于同一个班级的三个班级名称MyList，MyArrayList和MyCustomArrayList会让人感到困惑。至于您的问题，您必须在MyArrayList类型的myClass内有一个实例字段（除非您想重构DoByEachAdd和DoByEachRemove为static ）。这可以通过将其添加为构造函数参数来完成，例如

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

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

另外，我质疑你的做法。其他具有MyArrayList实例的类只能使用add的{{1}}和remove方法。如果您想节省很多麻烦并且所有方法都可见，请将ArrayList声明为list或将public final设为MyArrayList的子类，例如

ArrayList

如果您坚持能够从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; } }返回T，请声明另一种方法：

remove

自定义arraylist，包含删除和添加通知

3 个答案: