setPreferredSize不能按預期工作，JPanel不是「打包」

Question

所以我一直在試圖爲牙醫創建這個簡單的用戶界面，當我添加頂部菜單下的標題時，我注意到「setPreferredSize」不工作預期。基本上我想菜單水平擴展，所以我使用GridBagLayout。我還將包含按鈕的JPanel的大小設置爲我的contentPane的全寬度大小。

奇怪的是，如果我調整窗口（JFrame）的大小並使其水平變大，那麼JPanel也會調整爲首選大小。創建時

的JFrame：

這是JFrame中一旦其創建時我手動調整（水平更大通過拖動邊）：

正如你所看到的一旦我使窗口變大，JPanel的大小就會得到糾正。另外，我確定窗口有足夠的空間用於頂層菜單（甚至比頂層菜單大50px），但當設置爲可見時，它仍會像第一個圖像一樣被繪製。

主窗口中的代碼是：

import javax.imageio.ImageIO; 
import javax.swing.*; 
import java.awt.*; 
import java.io.File; 

/** 
* Created by Matheus on 27/10/2016. 
*/ 
public class WindowTest extends JFrame { 

    private JPanel contentPane; 
    private JPanel topMenu; 
    private JPanel header; 
    private JPanel dummy; 
    private JButton home, appointments, healthCare, 
        patients, contact; 


    public WindowTest(final int w,final int h) { 
     super(); 

     // Colours we'll need to paint the UI (RGB format) 
     final Color lightBlue = new Color(200, 200, 255); 
     final Color bgBlue = new Color(112, 205, 255); 
     final Color grey = new Color(128, 128, 128, 40); 
     final Color white = new Color(255,255,255); 
     final Color transWhite = new Color(255,255,255, 100); 

     // Gradient drawing in this area 
     JPanel contentPane = new JPanel(new GridBagLayout()) { 
      public void paintComponent(Graphics g) { 

       super.paintComponent(g); 
       Graphics2D gb = (Graphics2D) g; 
       gb.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY); 
       GradientPaint gp = new GradientPaint(0, 100, bgBlue, 0, h, white); 
       gb.setPaint(gp); 
       gb.fillRect(0, 0, w, h); 

      } 
     }; 
     contentPane.setPreferredSize(new Dimension(w, h)); 

     // Grid bag contraints here - Initial settings 
     GridBagConstraints gbc = new GridBagConstraints(); 
     gbc.anchor = GridBagConstraints.FIRST_LINE_START; 




     // Create the header stuff here. 
     gbc.gridy = 0; 
     gbc.gridx = 0; 
     gbc.weighty = 0; 
     gbc.weightx = 0; 
     try{ 
      File imgFile = new File(getClass().getResource("header.jpg").toURI()); 
      Image headerImg = ImageIO.read(imgFile); 
      header = new ImagePanel(headerImg); 

      contentPane.add(header, gbc); 
     } 
     catch (Exception e){ 
      System.out.println("Failed to load image"); 
     } 




     // Top menu stuff here 
     topMenu = new JPanel(new FlowLayout()); 
     topMenu.setBackground(new Color(0,0,0)); 
     topMenu.setPreferredSize(new Dimension(w, 60)); 
     System.out.println(topMenu.getSize()); 

     // Creating menu items 
     home = new MenuButton(300, 75, "Home", transWhite); 
     appointments = new MenuButton(300, 75, "Appointments", transWhite); 
     patients = new MenuButton(300, 75, "Patients", transWhite); 
     healthCare = new MenuButton(300, 75, "Health Care", transWhite); 
     contact = new MenuButton(300, 75, "Contact", transWhite); 

     home = new JButton("Home"); 
     appointments = new JButton("Appointments"); 
     patients = new JButton("Patients"); 
     healthCare = new JButton("Health Care Plan"); 
     contact = new JButton("Contact"); 

     topMenu.add(home); 
     topMenu.add(appointments); 
     topMenu.add(patients); 
     topMenu.add(healthCare); 
     topMenu.add(contact); 
     topMenu.revalidate(); 

     gbc.gridy = 1; 
     gbc.weightx = 1; 
     gbc.weighty = 0; 
     contentPane.add(topMenu, gbc); 


     // Dummy here for the rest of the screen 
     JPanel dummy = new JPanel(); 
     gbc.gridy = 1; 
     gbc.weighty = 1; 
     gbc.weightx = 1; 
     contentPane.add(dummy, gbc); 


     // Main window settings 
     this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
     this.add(contentPane); 
     this.pack(); 
     this.revalidate(); 
     this.setVisible(true); 
    } 
} 

我還試圖用不同的佈局管理器，但他們給出相同的結果。有人能幫我嗎？

Answer 1

強制組件填充所有空間的最簡單方法是使用BorderLayout。在你的情況下，它應該使用兩次。

// Initialize topMenu as before 

// Create a panel for image and buttons 
JPanel head = new JPanel(new BorderLayout()); 
// Image will be centered and its height will be preserved 
head.add(new JLabel(new ImageIcon(getClass().getResource("header.jpg"))), BorderLayout.PAGE_START); 
// Buttons also will be centered and their height will be preserved 
head.add(topMenu, BorderLayout.PAGE_END); 

// Gradient drawing in this area 
JPanel contentPane = new JPanel(new BorderLayout()) { 
    // paintComponent ... 
}; 

// The height of the head will be preserver, 
// but the width will be equal to the window width 
contentPane.add(head, BorderLayout.PAGE_START); 

// Main component 
// Just for the example 
final JButton main = new JButton("Test"); 
main.setOpaque(false); 
main.setBackground(Color.MAGENTA); 

// Main component will be stretched both vertically and horizontally 
contentPane.add(main, BorderLayout.CENTER);