setLayout(null)时,JPanel上的组件未显示 [英] Component on JPanel not showing when setLayout(null)
问题描述
有人可以说出组合框为什么不显示?我有一个控制器:
Someone can tell why the combobox is not showing ? I have a Controller:
public class TestController extends JPanel {
TestView cgView;
public TestController()
{
setLayout(null);
cgView=new TestView();
add(cgView);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame fr = new JFrame("testt");
fr.setSize(1200,1000);
fr.setResizable(false);
TestController cgc=new TestController();
fr.setBackground(Color.white);
fr.setVisible(true);
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
fr.add(cgc);
}
});
}
}
还有一个视图
public class TestView extends JPanel{
private static final long serialVersionUID = 1L;
public JComboBox<String> comboBox;
public TestView() {
comboBox= new JComboBox<>(new String[] {"option1", "option2" });
comboBox.setBounds(100,500, 100, 20);
add(comboBox);
}
}
由于TestController中的 setLayout(null),我看不到comboBox.如果我在我的TestContoller()中添加 add(cgView.comboBox),那么它看起来像这样:
Because of setLayout(null) in TestController, I can't see the comboBox. If I add add(cgView.comboBox) to my TestContoller(), so that it looks like this:
public TestController()
{
setLayout(null);
cgView=new TestView();
add(cgView);
add(cgView.comboBox);
}
比我看得到的多.有人可以告诉为什么吗?
Than I can see it. Can someone tell why?
所以我的解决方案是总是将组件添加到TestController中,或者将TestController作为TestView的一个属性来传递(所以在TestView()中,我会这样添加它们:this.parentPanel.add(comboBox).是否还有其他解决方案?
So my solution is to always add the components in TestController, or to pass TestController as an atribute to TestView (so in TestView() I would add them like this this.parentPanel.add(comboBox). Is there any other solution?
推荐答案
- 几乎不使用空布局
- 相反,请使用嵌套在JPanels中的布局的最佳组合来为您的GUI实现令人愉悦的布局.
- 如果您确实使用空布局,那么您将完全负责设置添加到该容器中的所有组件的大小和位置.
- 您当前的问题是,您永远不会给TestView一个尺寸或位置,而是将其添加到使用布局的空容器中.
- 您不应将一个组件(以上是JComboBox)添加到多个容器中.
- 在 之后,您要在JFrame上调用
setVisible(true)
.在您添加所有组件并在其上调用pack()
之前. - Don't use null layout, almost ever
- Instead use the best combination of layouts nested in JPanels to achieve a pleasing layout for your GUI.
- If you do use null layout then you are fully responsible for setting the size and location of all components added to that container.
- Your current problem is that you never give TestView a size or location and have then added it to a null layout-using container.
- You shouldn't add a component (above, your JComboBox) to more than one container.
- Don't call
setVisible(true)
on the JFrame until after you've added all components and calledpack()
on it.
例如
import java.awt.*;
import javax.swing.*;
public class TestController extends JPanel {
private static final int PREF_W = 1000;
private static final int PREF_H = 800;
TestView cgView;
public TestController() {
setLayout(null);
cgView = new TestView();
cgView.setSize(getPreferredSize());
add(cgView);
}
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame fr = new JFrame("testt");
// fr.setSize(1200, 1000);
fr.setResizable(false);
TestController cgc = new TestController();
fr.setBackground(Color.white);
// fr.setVisible(true);
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
fr.add(cgc);
fr.pack(); //!! added
fr.setVisible(true); // !! moved
}
});
}
}
但是最好使用布局:
import java.awt.*;
import javax.swing.*;
public class TestController extends JPanel {
private static final int PREF_W = 1000;
private static final int PREF_H = 800;
TestView cgView;
public TestController() {
//!! setLayout(null);
cgView = new TestView();
//!! cgView.setSize(getPreferredSize());
add(cgView);
}
@Override
public Dimension getPreferredSize() {
return new Dimension(PREF_W, PREF_H);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
JFrame fr = new JFrame("testt");
// fr.setSize(1200, 1000);
fr.setResizable(false);
TestController cgc = new TestController();
fr.setBackground(Color.white);
// fr.setVisible(true);
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
fr.add(cgc);
fr.pack(); //!! added
fr.setVisible(true); // !! moved
}
});
}
}
class TestView extends JPanel {
private static final long serialVersionUID = 1L;
public JComboBox<String> comboBox;
public TestView() {
comboBox = new JComboBox<String>(new String[] { "option1", "option2" });
// comboBox.setBounds(100, 500, 100, 20);
add(comboBox);
}
}
修改
OP在评论中问:
Edit
The OP asked in a comment:
几乎不会"?在哪种情况下,您会使用[null布局]?
'Almost never'? In which cases you would use it [the null layout]?
我很少使用它,例如当我想通过动画或使用MouseListener移动组件时,但是即使如此,许多人还是建议您创建自己的布局来处理该布局,例如Rob Camick的
I use it rarely, such as when I want to move components around via animation or with a MouseListener, but even then, many suggest that you create your own layout to handle that such as Rob Camick's Drag Layout
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