如何在Java中更改面板的大小？

Question

I'm trying to create a simple login screen consisting of different panels. I want help in resizing panels. The Panel I want resized is coloured in Green. I want to make it a bit smaller. The Panel in Green is the North Panel and is set to Border Layout. I want to make the green panel smaller since i feel its too big

I tried northPanel.setSize(150,150); but I got no result

This is my code:

JLabel lblWelcome = new JLabel("Welcome To The Login Screen", SwingConstants.CENTER);
JPanel northPanel = new JPanel(new BorderLayout());
northPanel.setBackground(Color.green);
northPanel.add(lblWelcome, BorderLayout.CENTER);

Answer 1

You are using a BorderLayout which has five positions to put components in the layout. The five positions are PAGE_START, PAGE_END, LINE_START, LINE_END, and CENTER. Here is a diagram of these positions:

For this layout, you want to put the component that should take the remaining space of the frame in the CENTER position. For this reason, the northPanel is probably better suited in the PAGE_START position while the JPanel that houses the login labels and submit button should be in the CENTER position. Using this positioning will allow you to resize the northPanel and allow the panel housing the login labels and submit button to take up the remaining frame space.

Answer 2

I want to make the green panel smaller

Looks to me like the two panels are the same size which tells me you are using a GridLayout for your frame.

Don't use a GridLayout, instead keep the default BorderLayout of the frame.

Then your code would be something like:

JLabel lblWelcome = new JLabel("Welcome To The Login Screen", SwingConstants.CENTER);
JPanel northPanel = new JPanel(new BorderLayout());
northPanel.setBackground(Color.green);
northPanel.add(lblWelcome, BorderLayout.CENTER);

frame.add(northPanel, BorderLayout.PAGE_START);
frame.add(centerPanel, BorderLayout.CENTER);

Now the green panel will only be as big as the JLabel. If you want the panel to be bigger, then add an EmptyBorder to the northPanel. Read the section from the Swing tutorial on How to Use Borders for more information.