英文:
JPanel not adding new components
问题
我正在构建一个GUI，需要动态向JPanel添加一些标签，但出于某种原因代码无法正常工作：
public class ChatClientGUI extends javax.swing.JFrame {
/**
* Creates new form ChatClientGUI
* @param server : server remoto a cui connettersi
*/
public ChatClientGUI(ChatServerIF server) {
initComponents();
...
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
// 这些代码可以正常工作，实际上我看到了两个"Mex:"标签被添加到了JPanel中
}
private void sendButtonActionPerformed(java.awt.event.ActionEvent evt) {
try {
System.out.println("MEX SENT"); // 我在终端中看到这一行
messagesPanel.add(new JLabel("New mex sent", SwingConstants.LEFT), BorderLayout.PAGE_START);
// 这个代码不起作用，没有将任何内容添加到JPanel中
server.sendMessage(client, username.getSelectedItem(), messageText.getText());
} catch (Exception ex) {
ex.printStackTrace();
}
}
希望这可以帮助您解决问题。
英文:
I'm building a GUI where i need to add dynamically to a JPanel some Label, but the code is not working for some reason:
public class ChatClientGUI extends javax.swing.JFrame {
/**
* Creates new form ChatClientGUI
* @param server : server remoto a cui connettersi
*/
public ChatClientGUI(ChatServerIF server) {
initComponents();
...
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
// those works, infact i see 2 "Mex:" label added to the JPanel
}
private void sendButtonActionPerformed(java.awt.event.ActionEvent evt) {
try {
System.out.println("MEX SENT"); // I see this line in the terminal
messagesPanel.add(new JLabel("New mex sent", SwingConstants.LEFT), BorderLayout.PAGE_START);
// this does not work, nothing is added to the JPanel
server.sendMessage(client, username.getSelectedItem(), messageText.getText());
} catch (Exception ex) {
ex.printStackTrace();
}
}
</details>
# 答案1
**得分**: 2
> 我需要动态添加...
当您动态向面板添加组件时，需要调用：
1. `revalidate()` 和
2. `repaint()`（有时需要）
在面板上。
默认情况下，组件的大小为（0，0），因此没有可绘制的内容。
`revalidate()` 将调用布局管理器，`repaint()` 确保整个面板被重新绘制
```java
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
指定 BorderLayout.PAGE_START 似乎没有意义。在 BorderLayout 中，您只能向任何区域添加一个组件。如果看到多个组件，则您的面板必须使用其他布局（而不是 BorderLayout）。因此，该约束是不必要的。
英文:
> i need to add dynamically...
When you add components dynamically to a panel you need to invoke:
revalidate()andrepaint()(sometimes needed)
on the panel.
By default a component has a size of (0, 0) so there is nothing to paint.
The revalidate() will invoke the layout manager and the repaint() makes sure the entire panel is repainted
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
Specifying the BorderLayout.PAGE_START doesn't seem to make sense. You can only add a single component to any area in the BorderLayout. Your panel must be using some other layout (not the BorderLayout) if you see multiple comonents. Therefore that constraint is not needed.
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