JPanel在按下JButton时更改JFrame中的形状/位置。为什么？

Question

[基本上，我已经将JPanel添加到JFrame，并且每当我按下JPanel中的按钮（这会更改JPanel中组件的可见性）时，JPanel的大小和组件的位置都会改变。我无法终生弄清楚为什么会出现此问题。非常感谢您的帮助。

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import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;

public class Main {

private JFrame window;
private JPanel loginPanel;
private JButton loginButton;
private JTextField usernameField;
private JPasswordField passwordField;
private JLabel usernameLabel, passwordLabel, errorMessage;


public static void main(String[] args) {
    Main program = new Main();
}

public Main() {
    createWindow();
    runProgram();
}

private void createWindow() {

    window = new JFrame("Dentist Program");
    window.setSize(1450, 900);
    window.setVisible(true);
    window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    loginPanel = new JPanel();
    loginPanel.setSize(300, 400);
    loginPanel.setBorder(BorderFactory.createLineBorder(Color.BLACK));

    usernameLabel = new JLabel("Username");
    usernameLabel.setSize(125, 20);
    usernameLabel.setLocation(loginPanel.getWidth()/2 - usernameLabel.getWidth()/2, 100);

    usernameField = new JTextField();
    usernameField.setSize(125, 25);
    usernameField.setLocation(loginPanel.getWidth()/2 - usernameField.getWidth()/2, usernameLabel.getY() + usernameLabel.getHeight());

    passwordLabel = new JLabel("Password");
    passwordLabel.setSize(125, 20);
    passwordLabel.setLocation(loginPanel.getWidth()/2 - passwordLabel.getWidth()/2, usernameField.getY() + usernameField.getHeight() + 10);

    passwordField = new JPasswordField();
    passwordField.setSize(125, 25);
    passwordField.setLocation(loginPanel.getWidth()/2 - passwordField.getWidth()/2, passwordLabel.getY() + passwordLabel.getHeight());

    loginButton = new JButton("Log In");
    loginButton.setSize(80, 25);
    loginButton.setLocation(passwordField.getX(), passwordField.getY() + passwordField.getHeight() + 15);
    loginButton.addActionListener(new ActionListener()
            {

            @Override
            public void actionPerformed(ActionEvent arg0) {
                errorMessage.setText("");
                loginPanel.repaint();
            }
            });

    errorMessage = new JLabel("Username and/or password is incorrect");
    errorMessage.setSize(250, 25);
    errorMessage.setLocation(loginPanel.getWidth()/2 - errorMessage.getWidth()/2, loginButton.getY() + loginButton.getHeight() + 25);
    errorMessage.setForeground(Color.RED);

    loginPanel.add(usernameLabel);
    loginPanel.add(usernameField);
    loginPanel.add(passwordLabel);
    loginPanel.add(passwordField);
    loginPanel.add(loginButton);
    loginPanel.add(errorMessage);

    window.getContentPane().add(loginPanel);
    window.repaint();

}

private void runProgram() {

}
}

Answer 1

您的UI更改的原因是，您认为正确的外观（初始外观）实际上是错误的，并且调整大小的版本是“正确”的版本。