Adding JPanel to JFrame

问题:

I have a program in which a JPanel is added to a JFrame:

public class Test{

    Test2 test = new Test2();
    JFrame frame = new JFrame();

    Test(){

    ...
    frame.setLayout(new BorderLayout());
    frame.add(test, BorderLayout.CENTER);
    ...

    }

    //main

    ...

    }

    public class Test2{

    JPanel test2 = new JPanel();

    Test2(){

    ...

    }

}

I get an error asking me to change type of 'panel' to 'component'. I do I fix this error? It wants me to do: Component panel = new Component();

回答1:

public class Test{

Test2 test = new Test2();
JFrame frame = new JFrame();

Test(){
...
frame.setLayout(new BorderLayout());
frame.add(test, BorderLayout.CENTER);
...
}

//main
...
}

//public class Test2{
public class Test2 extends JPanel {

//JPanel test2 = new JPanel();

Test2(){
...
}

回答2:

do it simply

public class Test{
    public Test(){
        design();
    }//end Test()

public void design(){
    JFame f = new JFrame();
    f.setSize(int w, int h);
    f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    f.setVisible(true);
    JPanel p = new JPanel(); 
    f.getContentPane().add(p);
}

public static void main(String[] args){
     EventQueue.invokeLater(new Runnable(){
     public void run(){
         try{
             new Test();
         }catch(Exception e){
             e.printStackTrace();
         }

 }
         );
}

}

回答3:

Instead of having your Test2 class contain a JPanel, you should have it subclass JPanel:

public class Test2 extends JPanel {

Test2(){

...

}

More details:

JPanel is a subclass of Component, so any method that takes a Component as an argument can also take a JPanel as an argument.

Older versions didn't let you add directly to a JFrame; you had to use JFrame.getContentPane().add(Component). If you're using an older version, this might also be an issue. Newer versions of Java do let you call JFrame.add(Component) directly.

回答4:

Test2 test = new Test2();
...
frame.add(test, BorderLayout.CENTER);

Are you sure of this? test is NOT a component! To do what you're trying to do you should let Test2 extend JPanel !

回答5:

Your Test2 class is not a Component, it has a Component which is a difference.

Either you do something like

frame.add(test.getPanel() );

after you introduced a getter for the panel in your class, or you make sure your Test2 class becomes a Component (e.g. by extending a JPanel)