Why are the buttons not visible at program start?

Question

Can anybody tell me why the buttons are invisible at Program start? They are only then visible, if the client are is invalidated, in other words, if i resize/minimize the window etc.
public class Calculator extends Frame
private Button[] button;
private Panel p_button;
public Calculator()
super();
SetWindowProperties();
button = new Button[10];
p_button = new Panel();
CreateButtons();
AddPanels();
AddButtons();
SetButtonProperties();
addWindowListener(new WindowHandler());
private void SetWindowProperties()
setTitle("Calculator");
setBackground(Color.white);
setLayout(new BorderLayout());
setSize(400,400);
setLocation(250,200);
setVisible(true);
private void CreateButtons()
for (int i=0; i<=9; i++)
button = new Button(String.valueOf(i));
private void SetButtonProperties()
for (int i=0; i<=9; i++)
button.setSize(50, 50);
button.setBackground(Color.gray);
private void AddButtons()
p_button.setLayout(new GridLayout(5, 3, 5, 5));
for (int i=1; i<=9; i++)
p_button.add(button);
for (int i=1; i<=9; i++)
button.addActionListener(new ButtonHandler());
private void AddPanels()
add(p_button, BorderLayout.CENTER);
}

Answer 1

First, use code tags.
Second, your code doesn't even compile. Please create a Short,Self Contained, Compilable and Executable, Example Program