java if 语句在 if(item instanceof nomclass) 中不起作用

Question

我刚刚为我的保存按钮编写了一个actionPerformed，它将数据保存到数组列表中，但在此之前我必须确保所有字段都不为空，因此如果文本字段为空，我想显示一个对话框并将所有字段都为空红色背景颜色的文本字段

这是我的代码

//Field outside constructor
private List<Component> comp;





//inside constructor
comp = getAllComponents(this);
//method
public static List<Component> getAllComponents(final Container c) {
    Component[] comps = c.getComponents();
    List<Component> compList = new ArrayList<Component>();
    for (Component comp : comps) {
        compList.add(comp);
        if (comp instanceof Container)
            compList.addAll(getAllComponents((Container) comp));
    }
    return compList;
}

``

//actionperformed
if(e.getSource() == savebtn){
        for(Component item:comp){
            if(item.isVisible()){
                if(item instanceof JTextField){
                    JTextField txtField = (JTextField)item;
//here is my problem: with no if statement my program works fine and puts all textfields in red but I want to highlight just empty textfields;
                    if(txtField.getText() == null)
                    txtField.setBackground(Color.RED);
                }
            }
        }


    }

那么我该如何解决这个问题呢？非常感谢

Answer 1

if(txtField.getText() == null)

这里不检查是否为空，参见 public String getText()¹ 的文档:

Returns the text contained in this TextComponent. If the underlying document is null, will give a NullPointerException. Note that text is not a bound property, so no PropertyChangeEvent is fired when it changes. To listen for changes to the text, use DocumentListener.

我会使用DocumentListener相反，类似的东西:

myTextField.getDocument().addDocumentListener(new DocumentListener() {

    public void changedUpdate(DocumentEvent e) { checkIfEmpty(); }
    public void removeUpdate(DocumentEvent e) { checkIfEmpty(); }
    public void insertUpdate(DocumentEvent e) { checkIfEmpty();}

    public void checkIfEmpty() {
       if (myTextField.getText().equals("")){
           //set color to red
       } else {
           //set color back
       }   
    }
});

¹ 另请注意该方法的签名，它返回一个 String。