Question

I have a problem with JFrame and the key word “this”. When I use frame.getContentPane, components don’t show up unless I replace frame with “this”. It seems getContentPane doesn’t get the JFrame content pane, it gets something else I don’t know what it is. I’m afraid I have two different JFrame components even though I declared only one. Can anybody explain this problem?
This is my code:

public class Form1 extends JFrame
{
    private static final long serialVersionUID = 1L;
    JFrame frame = new JFrame("TableLayout");
    Container content = frame.getContentPane();//this doesn't work unless I replace "frame" with the key word "this"///
public Form1 ()//constructor
{
    String label[] = {"Top", "Bottom", "Add", "Delete", "Center", "Overlap"};
     double border = 5;
     double size[][] =
      {{border, 0.20, border, TableLayout.FILL, border, 0.80, border},  // Columns
       {border, 0.15, border, TableLayout.FILL, border, 0.10, border}}; // Rows  
     JButton button[] = new JButton[label.length];
     for (int i = 0; i < label.length; i++)
     {
         button[i] = new JButton(label[i]);
     }
     content.add (button[0], "1, 1, 5, 1"); // Top (row,column)
     content.add (button[1], " 1, 5, 5, 5"); // Bottom
     content.add ((button[2], "1, 3      "); // Left
     content.add (button[3],"5, 3,      "); // Right
     this.pack();
    }
}

Answer 1

public class Form1 extends JFrame
{
    private static final long serialVersionUID = 1L;
    JFrame frame = new JFrame("TableLayout");

Your class is a frame and also has a Frame attribute named frame. Of course there are two frames!

Change this..

public class Form1 extends JFrame
{
    private static final long serialVersionUID = 1L;
    JFrame frame = new JFrame("TableLayout");
    Container content = frame.getContentPane();//this doesn't work unless I replace "frame" with the key word "this"///
public Form1 ()//constructor
{

To something more like (definite answers come with an SSCCE in the question)..

public class Form1 extends JFrame
{
    private static final long serialVersionUID = 1L;
    Container content;

public Form1 ()//constructor
{
    super("TableLayout");
    content = getContentPane();