Update JLabel when custom object property changes

Question

I have a Java class and I want one of its properties to be displayed by a JLabel in a Swing desktop application:

class Item {
    private String name;
    private Integer quantity;

    // getters, setters...
}

class Frame {
    Item item = new Item();
    ...

    JLabel label = new JLabel();
    label.setText(item.getQuantity().toString());
    ...
}

How do I get the label to update its text whenever the quantity property changes on the item?

Answer 1

Something will have to update the text of your label (with the setText method you already know). Probably the easiest thing is to let the Item class fire PropertyChangeEvent s when its properties are changed, and attach a listener to the item which updates the label.

final JLabel label = new JLabel();
label.setText(item.getQuantity().toString());
item.addPropertyChangeListener( new PropertyChangeListener(){
   @Override
   public void propertyChange( PropertyChangeEvent event ){
     if ( "quantity".equals( event.getPropertyName ) ){
        //I assume this happens on the EDT, otherwise use SwingUtilities.invoke*
        label.setText( (String)event.getNewValue() );
     }
   }
});

The PropertyChangeSupport class makes it easy to manage the listeners and fire the events in your Item class

Answer 2

通过调用Component附带的repaint（）。

Answer 3

我可能会向您的Item对象添加一个ObjectChangeListener，然后重写它的objectChanged方法以更新JLabel并调用repaint（）。