JPanel添加到JFrame的問題：java.lang.IllegalArgumentException：無法添加到佈局：約束必須是一個字符串（或NULL）

Question

我遇到了一個編譯器問題，當我運行時，它給了我這個：

Exception in thread "main" java.lang.IllegalArgumentException: cannot add to layout: constraint must be a string (or null) 
    at java.awt.BorderLayout.addLayoutComponent(BorderLayout.java:426) 
    at java.awt.Container.addImpl(Container.java:1120) 
    at java.awt.Container.add(Container.java:998) 
    at javax.swing.JFrame.addImpl(JFrame.java:562) 
    at java.awt.Container.add(Container.java:966) 
    at DealerWindow.<init>(DealerWindow.java:36) 
    at DealerWindow.main(DealerWindow.java:98) 
Java Result: 1 
BUILD SUCCESSFUL (total time: 1 second) 

這似乎是在告訴我，上線35它不能添加到佈局，因爲約束必須是字符串或null

這裏是我的代碼（是它的每一位）：

import java.awt.*; 
import javax.swing.*; 

public class DealerWindow extends JFrame{ 
    private final int WINDOW_WIDTH = 450; 
    private final int WINDOW_HEIGHT = 338; 

    public DealerWindow(){ 

     //Sets the title 
     setTitle("Welcome to X"); 

     //Set the window size 
     setSize(WINDOW_WIDTH, WINDOW_HEIGHT); 

     // Specify an action for the close button 
     setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

     // Set BorderLayout 
     setLayout(new BorderLayout()); 

     // creates a Panel that will contain a Label that will contain the users image 
     JPanel innerPanelCenter = new JPanel(); 
     Icon icon = new ImageIcon(); 
     JLabel Label = new JLabel(icon); 

     //Sets Layout and adds the label and innerPanel 
     innerPanelCenter.setLayout(new FlowLayout(FlowLayout.CENTER)); 
     innerPanelCenter.add(Label); 
     add(innerPanelCenter,new FlowLayout(FlowLayout.CENTER)); 

     // creates innerPanelRight to go on the West side of the form 
     JPanel innerPanelRight = new JPanel(); 
     innerPanelRight.setLayout(new FlowLayout()); 

     //Buttons for innerPanelRight 
     JButton openButton = new JButton("Open"); 

     //Labels for innerPanelRight 
     JLabel makeLabel = new JLabel("Make: "); 
     JLabel modelLabel = new JLabel("Model: "); 
     JLabel yearLabel = new JLabel("Year: "); 
     JLabel commentsLabel = new JLabel("Comments: "); 

     //TextFields for innerPanelRight 
     JTextField makeTextField = new JTextField(); 
     JTextField modelTextField = new JTextField(); 

     //ComboBoxes for innerPanelRight and fills the ComboBox 
     JComboBox yearComboBox = new JComboBox(); 
     for (int i = 1900; i >2013; i++){ 
      yearComboBox.addItem(i); 
     } 

     //TextAreas for innerPanelRight 
     JTextArea commentsTextArea = new JTextArea(); 
     commentsTextArea.setLineWrap(true); 

     //Creates JPanels for each user field and sets Layouts 
     JPanel makePanel = new JPanel(); 
     makePanel.setLayout(new BorderLayout()); 
     JPanel modelPanel = new JPanel(); 
     modelPanel.setLayout(new BorderLayout()); 
     JPanel yearPanel = new JPanel(); 
     yearPanel.setLayout(new BorderLayout()); 
     JPanel commentsPanel = new JPanel(); 
     commentsPanel.setLayout(new BorderLayout()); 

     //adds components to the respective Panels 
     yearPanel.add(yearLabel,BorderLayout.EAST); 
     yearPanel.add(yearComboBox,BorderLayout.CENTER); 

     makePanel.add(makeLabel,BorderLayout.EAST); 
     makePanel.add(makeTextField,BorderLayout.CENTER); 

     modelPanel.add(modelLabel,BorderLayout.EAST); 
     modelPanel.add(modelTextField,BorderLayout.CENTER); 

     commentsPanel.add(commentsLabel,BorderLayout.EAST); 
     commentsPanel.add(commentsTextArea,BorderLayout.CENTER); 

     //add the button and 4 component panels to the innerPanelRight 
     innerPanelRight.add(openButton); 
     innerPanelRight.add(yearPanel); 
     innerPanelRight.add(makePanel); 
     innerPanelRight.add(modelPanel); 

     //add innerPanelRight to the west side of the frame 
     add(innerPanelRight,BorderLayout.WEST); 
    } 
    public static void main(String [] args){ 
     new DealerWindow(); 
    } 
} 

錯誤狀態問題是在線35上：

add(innerPanelCenter,new FlowLayout(FlowLayout.CENTER)); 

Answer 1

使用有效的佈局約束。更換

add(innerPanelCenter,new FlowLayout(FlowLayout.CENTER)); 

與

add(innerPanelCenter, BorderLayout.CENTER);