java - 无法用JavaBeans制作程序来监听其他组件

Question

import java.beans.PropertyVetoException;

public class Main {
  public static void main(String[] args) {

    Purchase purch = new Purchase("Computer");

    PurchaseView pView = new PurchaseView();
    purch.addPropertyChangeListener(pView);

    try {
      purch.setData("Notebook");
      System.out.println(purch);

    } catch (PropertyVetoException exc) {
      System.out.println(exc.getMessage());
    }

  }
}

那里有Purchase 和PurchaseView 类:

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.beans.PropertyVetoException;

public class Purchase {

    private String data;

    private PropertyChangeSupport propertyChange = new PropertyChangeSupport(this);

    public Purchase(String data) {
        this.data = data;
    }

    public synchronized void addPropertyChangeListener(PropertyChangeListener listener) {
        propertyChange.addPropertyChangeListener(listener);
    }

    public synchronized void removePropertyChangeListener(PropertyChangeListener l) {
        propertyChange.removePropertyChangeListener(l);
    }

    public String getData() {
        return data;
    }

    public void setData(String data) {
        String oldValue = data;
        this.data = data;
        propertyChange.firePropertyChange("data", oldValue, data);
    }
    }
}  


import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;

public class PurchaseView implements PropertyChangeListener{

    public PurchaseView() {}

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        String propName = evt.getPropertyName();
        if(propName.equals("data")) {
            String oldValue = (String) evt.getOldValue();
            String newValue = (String) evt.getNewValue();
            System.out.println("Change value of: " + evt.getPropertyName() + " from: " + oldValue + " to: " + newValue);
        }
    }
}

我希望程序在数据属性更改时生成如 PurchasingView 类中所示的输出。对我来说似乎实现得很好，但它不起作用。

有什么想法吗？

Answer 1

你的输出是什么？

我认为您在 setData 方法中使用 oldValue 时犯了一个错误。应该是这样的:

public void setData(String data) {
    String oldValue = this.data;
    this.data = data;
    propertyChange.firePropertyChange("data", oldValue, data);
}

请注意，您忘记了 setData 方法第一行的 this 保留字，这意味着您采用方法变量而不是类变量。