英文:
Getting an error message in Java while using Java Swing?
问题
这是我使用GUI和Java Swing的一个非常初级的小项目之一。一切都正常运行，没有错误，直到我调用setText()方法。以下是我的代码：
public class FirstGui implements ActionListener {
private int count = 0;
private JLabel label;
private JFrame frame;
private JButton button;
public FirstGui() {
// 创建窗口
frame = new JFrame();
JPanel panel = new JPanel();
panel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
panel.setLayout(new GridLayout(0, 1));
// 创建面板
frame.add(panel, BorderLayout.CENTER);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setTitle("这是我们的第一个GUI！");
frame.pack();
frame.setVisible(true);
// 创建一个点击按钮
button = new JButton("点击这里！:)");
panel.add(button);
// 当点击按钮时添加动作
button.addActionListener(this);
// 创建一个标签
label = new JLabel("点击次数: ");
panel.add(label);
}
public static void main(String[] args) {
FirstGui user1 = new FirstGui();
}
@Override
public void actionPerformed(ActionEvent e) {
count++;
label.setText("点击次数: " + count); // 这似乎是问题发生的地方。
}
}
我不确定为什么它会给我一个错误消息："无法调用"javax.swing.JLabel.setText(String)"，因为"this.label"为空"。
英文:
This is one of my very first mini-projects using GUI and Java swing. Everything runs fine with no errors until I invoke the setText() method. Here is my code:
public class firstGui implements ActionListener {
private int count = 0;
private JLabel label;
private JFrame frame;
private JButton b;
public firstGui() {
// Creating the window
JFrame frame = new JFrame();
JPanel panel1 = new JPanel();
panel1.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
panel1.setLayout(new GridLayout(0, 1));
// Creating the panel
frame.add(panel1, BorderLayout.CENTER);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setTitle("Here is our first GUI!");
frame.pack();
frame.setVisible(true);
// Creating a button to click on
JButton b = new JButton("Click here! :) ");
panel1.add(b);
// Adding actions to the button when clicked
b.addActionListener(this);
// Creating a label
JLabel label = new JLabel("Number of times clicked: ");
panel1.add(label);
}
public static void main(String[] args) {
firstGui user1 = new firstGui();
}
@Override
public void actionPerformed(ActionEvent e) {
count++;
label.setText("Number of times clicked: " + count); // here is where the problem seems to occur.
}
}
I'm not sure as to why it gives me a:
"Cannot invoke "javax.swing.JLabel.setText(String)" because "this.label" is null" error message.
答案1
得分: 1
发生这种情况的原因是因为您对标签使用了一个新的_object_，而不是使用已声明的_label_字段。
在这里，您的以下字段可以在_firstGui_类中的任何非静态方法中访问。
private int count = 0;
private JLabel label;
private JFrame frame;
private JButton b;
所以，不要使用一个新的_JLabel_对象，在这里
JLabel label = new JLabel("点击次数：");
使用来自类字段的相同对象-在声明中删除对象类型。
label = new JLabel("点击次数：");
英文:
The reason this is happening is because you have used a new object for your label.
Rather than using the declared label field.
Your following fields here, accessible to any non-static method within the firstGui class.
private int count = 0;
private JLabel label;
private JFrame frame;
private JButton b;
So, instead of using a new JLabel object, here
JLabel label = new JLabel("Number of times clicked: ");
Use the same one from the class fields—remove the object type, in the declaration.
label = new JLabel("Number of times clicked: ");
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