Estou correndo, então idc sobre a duplicata. Ainda estou tentando aprender Java e os termos, até o final deste semestre. Eu usei um modelo. Estou usando uma imagem de fundo ("painel"), o que complica tudo.

Basicamente, os botões só aparecem quando eu passo sobre eles. Tem algo a ver com os JPanels, claramente.

Excluí o código que você provavelmente pediria para que, com sorte, alguém me ajude dessa vez, porque meus botões não são como os que eu vi ao procurar em outras postagens recomendadas.

Além disso, posso fazer do JFrame um tamanho fixo (o tamanho no código da classe Test)?

O código pode ser redundante, mas estou apenas tentando fazer tudo funcionar. Tenha em mente que eu sou novo em Java.

CLASSE DE TESTE:

public class TestCalculator {    
    public static void main(String[] args) {
        JFrame frame = new JFrame(TestCalculator.class.getSimpleName());
        ImagePanel panel = new ImagePanel(new ImageIcon("01_Crane_AGweb.jpg").getImage());
        SimpleArithmeticCalculator calc = new SimpleArithmeticCalculator();        
        calc.SetColors(null , Color.white , new Color(72,61,139));
        calc.setVisible(true);
        calc.setOpaque(false);
        panel.setVisible(true);
        panel.setOpaque(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        calc.add(panel);
        frame.add(panel);
        frame.add(calc);       
        frame.getContentPane().add(calc);   
        frame.setPreferredSize(new Dimension(358,379));
        frame.setMinimumSize(new Dimension(358,379));
        frame.setVisible(true);
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
    }
}
class ImagePanel extends JPanel {
    private Image img;
    public ImagePanel(String img) {
    this(new ImageIcon(img).getImage());
}
public ImagePanel(Image img) {
    this.img = img;
    Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
    this.setPreferredSize(new Dimension(size));
    this.setMinimumSize(new Dimension(size));
    this.setMaximumSize(new Dimension(size));
    this.setSize(new Dimension(size));
    this.setLayout(null);
}
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    g.drawImage(img, 0, 0, this.getWidth(),this.getHeight(),this);
}
}

A PRINCIPAL:

    public class SimpleArithmeticCalculator extends JPanel implements ActionListener {

         //BUTTONSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS !
         ...
         // jpanels, buttons, font, values, etc

        public SimpleArithmeticCalculator() {

          super();

      //I THINK this is the problem:  

      buttonPanel.setForeground(null);
      textPanel.setForeground(null);
      calcPanel.setForeground(null);

      textPanel.setLayout(new GridLayout(0,1,0,0));
      buttonPanel.setLayout(new GridLayout(0 , 5 , 5 , 5));

      displayText = new JTextField("" , 20);
      displayText.setHorizontalAlignment(JTextField.RIGHT);
      displayText.setFont(font);
      displayText.setEditable(false);

      textPanel.add(displayText);
      buttons = new JButton[NUM_BUTTONS];

      for (int i = 0 ; i < NUM_BUTTONS ; ++i) {

          buttons[i] = new JButton("" + buttonTexts[i]);
          buttons[i].setMnemonic(buttonKeys[i]);
          buttons[i].setFont(font);
          buttons[i].setMinimumSize(new Dimension(50,50));
          buttons[i].setActionCommand("" + buttonTexts[i]);
          buttons[i].addActionListener(this);
          buttonPanel.add(buttons[i]);
      }

      buttons[BTN_POWER].setText("^");
      buttons[BTN_PERCENT].setText("%");

      buttonPanel.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
      textPanel.setBorder(BorderFactory.createEmptyBorder(5,5,5,5));
      calcPanel.setLayout(new BorderLayout());
      calcPanel.add(textPanel , BorderLayout.NORTH);
      calcPanel.add(buttonPanel , BorderLayout.CENTER);
      add(calcPanel);

      setMinimumSize(new Dimension(358,379));

      setPreferredSize(new Dimension(359,381));

       for (int i = 0 ; i < NUM_BUTTONS ; ++i) {
           buttons[i].setMaximumSize(buttons[i].getSize());
       }

   }

     public void SetColors(Color bg , Color textbg , Color textcolor) {

       calcPanel.setBackground(bg);
       calcPanel.setVisible(true);
       calcPanel.setOpaque(false);
       buttonPanel.setBackground(bg);
       buttonPanel.setVisible(true);
       buttonPanel.setOpaque(false);
       textPanel.setBackground(bg);
       textPanel.setOpaque(true);
       textPanel.setVisible(true);
       displayText.setBackground(textbg);
       displayText.setForeground(textcolor);

       for (int i = 0 ; i < NUM_BUTTONS ; ++i) { 
            buttons[i].setForeground(textcolor);
       }

     }

     //ACTION PERFORMED STUFF & OPERATIONS, BLAH

     public boolean isOpCharacter(char c) {

          return ((c == buttonTexts[BTN_MULT]) ||
                  (c == buttonTexts[BTN_DIV]) ||
                  (c == buttonTexts[BTN_PLUS]) ||
                  (c == buttonTexts[BTN_MINUS]) ||
                  (c == buttonTexts[BTN_POWER]) ||
                  (c == buttonTexts[BTN_PERCENT]));
     }

     public boolean isNumericCharacter(char c) {

      return ((c == buttonTexts[BTN_ZERO]) ||
        (c == buttonTexts[BTN_ONE]) ||
        (c == buttonTexts[BTN_TWO]) ||
        (c == buttonTexts[BTN_THREE]) ||
        (c == buttonTexts[BTN_FOUR]) ||
        (c == buttonTexts[BTN_FIVE]) ||
        (c == buttonTexts[BTN_SIX]) ||
        (c == buttonTexts[BTN_SEVEN]) ||
        (c == buttonTexts[BTN_EIGHT]) ||
        (c == buttonTexts[BTN_NINE]) ||
        (c == buttonTexts[BTN_DECIMAL]));

     }

     public boolean isNonZeroNumber(char c) {

      return (c == buttonTexts[BTN_ONE] ||
        c == buttonTexts[BTN_TWO] ||
        c == buttonTexts[BTN_THREE] ||
        c == buttonTexts[BTN_FOUR] ||
        c == buttonTexts[BTN_FIVE] ||
        c == buttonTexts[BTN_SIX] ||
        c == buttonTexts[BTN_SEVEN] ||
        c == buttonTexts[BTN_EIGHT] ||
        c == buttonTexts[BTN_NINE]);

     }

        public static void main(String[] args) { }
 }