爲什麼JApplet顯示空白的灰色屏幕？

所以我一直在試圖弄清楚爲什麼我的JApplet開始，並沒有提供任何錯誤，但沒有任何顯示。我有一個調用setup_layout（）的init（）方法，但我認爲我可能做了錯誤的佈局。任何人幫助？爲什麼JApplet顯示空白的灰色屏幕？

import java.awt.BorderLayout; 
import java.awt.Color; 
import java.awt.FlowLayout; 
import java.awt.GridLayout; 
import java.awt.event.ActionEvent; 
import java.awt.event.ActionListener; 

import javax.swing.JApplet; 
import javax.swing.JButton; 
import javax.swing.JPanel; 
import javax.swing.JTextArea; 


public class Assignment8 extends JApplet implements ActionListener 
{ 
    public static final int WIDTH = 500; 
    public static final int HEIGHT = 500; 
    public static final int NUMBER_OF_DIGITS = 30; 

    private JTextArea operand, result; 
    private double answer = 0.0; 
    private JButton sevenB, eightB, nineB, divideB, fourB, fiveB, sixB, multiplyB, oneB, twoB, threeB, subtractB, 
    zeroB, dotB, addB, resetB, clearB, sqrtB, absB, expB; 

    /** public static void main(String[] args) 
    { 
     Assignment8 aCalculator = new Assignment8(); 
     aCalculator.setVisible(true); 
    }**/ 

    public void init() 
    { 
     setup_layout(); 
    } 

    private void setup_layout() { 
     setSize(WIDTH, HEIGHT); 

     JPanel MainPanel = new JPanel(); 
     JPanel buttonPanel = new JPanel(); 
     JPanel calcPanel = new JPanel(); 
     JPanel textPanel = new JPanel(); 

     buttonPanel.setLayout(new GridLayout(3, 4)); 
     calcPanel.setLayout(new GridLayout(3, 3)); 
     textPanel.setLayout(new FlowLayout()); 
     MainPanel.setLayout(new BorderLayout()); 
     MainPanel.setBackground(Color.red); 

     operand = new JTextArea(); 
     result = new JTextArea(); 
     operand.setEditable(false); 
     result.setEditable(false); 
     textPanel.add(operand); 
     textPanel.add(result); 

     sevenB = new JButton("7"); 
     eightB = new JButton("8"); 
     nineB = new JButton("9"); 
     divideB = new JButton("/"); 
     fourB = new JButton("4"); 
     fiveB = new JButton("5"); 
     sixB = new JButton("6"); 
     multiplyB = new JButton("*"); 
     oneB = new JButton("1"); 
     twoB = new JButton("2"); 
     threeB = new JButton("3"); 
     subtractB = new JButton("-"); 
     zeroB = new JButton("0"); 
     dotB = new JButton("."); 
     addB = new JButton("+"); 
     resetB = new JButton("Reset"); 
     clearB = new JButton("Clear"); 
     sqrtB = new JButton("Sqrt"); 
     absB = new JButton("Abs"); 
     expB = new JButton("Exp"); 

     sevenB.addActionListener(this); 
     eightB.addActionListener(this); 
     nineB.addActionListener(this); 
     divideB.addActionListener(this); 
     fourB.addActionListener(this); 
     fiveB.addActionListener(this); 
     sixB.addActionListener(this); 
     multiplyB.addActionListener(this); 
     oneB.addActionListener(this); 
     twoB.addActionListener(this); 
     threeB.addActionListener(this); 
     subtractB.addActionListener(this); 
     zeroB.addActionListener(this); 
     dotB.addActionListener(this); 
     addB.addActionListener(this); 
     resetB.addActionListener(this); 
     clearB.addActionListener(this); 
     absB.addActionListener(this); 
     expB.addActionListener(this); 
     sqrtB.addActionListener(this); 

     buttonPanel.add(sevenB); 
     buttonPanel.add(eightB); 
     buttonPanel.add(nineB);   
     buttonPanel.add(fourB); 
     buttonPanel.add(fiveB); 
     buttonPanel.add(sixB);   
     buttonPanel.add(oneB); 
     buttonPanel.add(twoB); 
     buttonPanel.add(threeB); 
     buttonPanel.add(zeroB); 
     buttonPanel.add(dotB); 

     calcPanel.add(subtractB); 
     calcPanel.add(multiplyB); 
     calcPanel.add(divideB); 
     calcPanel.add(sqrtB); 
     calcPanel.add(absB); 
     calcPanel.add(expB); 
     calcPanel.add(addB); 
     calcPanel.add(resetB); 
     calcPanel.add(clearB); 

     MainPanel.add(buttonPanel); 
     MainPanel.add(calcPanel); 
     MainPanel.add(textPanel); 
     setVisible(true); 
    } 

    public void actionPerformed(ActionEvent e) 
    { 
     try 
     { 
      assumingCorrectNumberFormats(e); 
     } 
     catch (NumberFormatException e2) 
     { 
      result.setText("Error: Reenter Number."); 
     } 
     catch (DivideByZeroException e1) { 
      result.setText("Error: CANNOT Divide By Zero. Reenter Number."); 
     } 
    } 


    //Throws NumberFormatException. 
    public void assumingCorrectNumberFormats(ActionEvent e) throws DivideByZeroException 
    { 
     String actionCommand = e.getActionCommand(); 

     if (actionCommand.equals("1")) 
     { 
      int num = 1; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("2")) 
     { 
      int num = 2; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("3")) 
     { 
      int num = 3; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("4")) 
     { 
      int num = 4; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("5")) 
     { 
      int num = 5; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("6")) 
     { 
      int num = 6; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("7")) 
     { 
      int num = 7; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("8")) 
     { 
      int num = 8; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("9")) 
     { 
      int num = 9; 
      operand.append(Integer.toString(num)); 
     } 
     else if (actionCommand.equals("0")) 
      { 
       int num = 0; 
       String text = operand.getText(); 
       //double check = stringToDouble(text); 
       if(text.isEmpty()||text.contains(".")) { 
        operand.append(Integer.toString(num)); 
       } 
      } 
     else if (actionCommand.equals(".")) 
     { 
      String text = operand.getText(); 
      if(!text.contains(".")) { 
       operand.append("."); 
      } 
     } 

     else if (actionCommand.equals("+")) 
     { 
      answer = answer + stringToDouble(operand.getText()); 
      result.setText(Double.toString(answer)); 
      operand.setText(""); 
     } 
     else if (actionCommand.equals("-")) 
     { 
      answer = answer - stringToDouble(operand.getText()); 
      result.setText(Double.toString(answer)); 
      operand.setText(""); 

     } 
     else if (actionCommand.equals("*")) 
     { 
      answer = answer * stringToDouble(operand.getText()); 
      result.setText(Double.toString(answer)); 
      operand.setText(""); 

     } 
     else if (actionCommand.equals("/")) 
     { 
      double check = stringToDouble(operand.getText()); 
      if(check >= -1.0e-10 && check <= 1.0e-10) { 
       throw new DivideByZeroException(); 
      } 
      else { 
       answer = answer/stringToDouble(operand.getText()); 
       result.setText(Double.toString(answer)); 
       operand.setText(""); 
      } 

     } 
     else if (actionCommand.equals("Sqrt")) 
     { 
      double check = stringToDouble(result.getText()); 
      if(check < 0) { 
       throw new NumberFormatException(); 
      } 
      else { 
       answer = Math.sqrt(stringToDouble(result.getText())); 
       result.setText(Double.toString(answer)); 
       operand.setText(""); 
      } 

     } 
     else if (actionCommand.equals("Abs")) 
     { 
      answer = Math.abs(stringToDouble(result.getText())); 
      result.setText(Double.toString(answer)); 
      operand.setText(""); 

     } 
     else if (actionCommand.equals("Exp")) 
     { 
      answer = Math.pow(stringToDouble(result.getText()),stringToDouble(operand.getText())); 
      result.setText(Double.toString(answer)); 
      operand.setText(""); 
     } 
     else if (actionCommand.equals("Reset")) 
     { 
      answer = 0.0; 
      result.setText(""); 
      operand.setText(""); 
     } 
     else if (actionCommand.equals("Clear")) 
     { 
      operand.setText(""); 
     } 
     else 
      result.setText("Unexpected error."); 
    } 


    //Throws NumberFormatException. 
    private static double stringToDouble(String stringObject) 
    { 
     return Double.parseDouble(stringObject.trim()); 
    } 

    //Divide by Zero Exception Class 
    public class DivideByZeroException extends Exception { 
     DivideByZeroException() { 
     } 
    }

}

來源

2014-11-20 user3002486

你從不添加的mainPanel的小應用程序本身。

即

add(MainPanel);

而且，由於使用的mainPanel BorderLayout的，你需要用一個恆定的BorderLayout.XXX添加子面板。即，

變化這樣的：

MainPanel.add(buttonPanel); 
    MainPanel.add(calcPanel); 
    MainPanel.add(textPanel);

到：

MainPanel.add(buttonPanel, BorderLayout.CENTER); 
    MainPanel.add(calcPanel, BorderLayout.PAGE_END); // wherever you want to add this 
    MainPanel.add(textPanel, BorderLayout.PAGE_START);

注意您的代碼可以通過使用陣列或列表大大簡化。

或者可以簡化它在其他方面，比如：

public void assumingCorrectNumberFormats(ActionEvent e) 
    throws DivideByZeroException { 
    String actionCommand = e.getActionCommand(); 
    String numbers = "1234567890"; 

    if (numbers.contains(actionCommand)) { 
    operand.append(actionCommand); 

    // if you need to work with it as a number 
    int num = Integer.parseInt(actionCommand); 
    }

我自己，我會使用不同的功能，不同的ActionListeners，例如用於數字和.輸入按鈕的ActionListener一個和另一個用於操作按鈕。

來源

2014-11-20 20:33:36

謝謝！我覺得我的代碼效率很低。我將如何使用數組？我可以看到它只用於創建編號按鈕。 – user3002486 2014-11-20 20:39:31

爲什麼JApplet顯示空白的灰色屏幕？

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