Java Swing 計算機完整實作：四則運算、科學鍵、記憶體功能

以 JApplet/JFrame 雙模式實作標準計算機，支援四則運算、平方根、倒數、百分比、正負號切換、退位鍵、清除/全部清除、記憶體（MC/MR/MS/M+）完整功能。介面採 GridLayout 矩陣排列，顏色區分數字鍵（藍）與功能鍵（紅），可直接編譯執行。

按鍵陣列定義

private final String[] KEYS = {
    "7","8","9","/","sqrt",
    "4","5","6","*","%",
    "1","2","3","-","1/x",
    "0","+/-",".","+","="
};
private final String[] COMMAND = {"Backspace","CE","C"};
private final String[] M = {" ","MC","MR","MS","M+"};

核心運算邏輯

數字鍵處理

private boolean firstDigit = true;
private void handleNumber(String key) {
    if (firstDigit) display.setText(key);
    else if (key.equals(".") && display.getText().indexOf(".") < 0)
        display.setText(display.getText() + ".");
    else if (!key.equals("."))
        display.setText(display.getText() + key);
    firstDigit = false;
}

運算子處理（含除零、倒數零保護）

private double number = 0.0;
private String operator = "=";
private void handleOperator(String key) {
    double displayVal = getNumberFromDisplay();
    switch (operator) {
        case "/":  if (displayVal == 0) { showError("除數不能為零"); return; }
                   number /= displayVal; break;
        case "1/x": if (number == 0) { showError("零沒有倒數"); return; }
                    number = 1 / number; break;
        case "+": number += displayVal; break;
        case "-": number -= displayVal; break;
        case "*": number *= displayVal; break;
        case "sqrt": number = Math.sqrt(number); break;
        case "%": number /= 100; break;
        case "+/-": number *= -1; break;
        case "=": number = displayVal; break;
    }
    // 整數顯示優化
    long l = (long) number;
    display.setText((number - l == 0) ? String.valueOf(l) : String.valueOf(number));
    operator = key;
    firstDigit = true;
}

介面佈局

// 頂列：指令鍵 (Backspace, CE, C)
JPanel command = new JPanel(new GridLayout(1,3,3,3));
// 中列：數字/運算鍵 4x5
JPanel calckeys = new JPanel(new GridLayout(4,5,3,3));
// 左列：記憶體鍵 5x1
JPanel calms = new JPanel(new GridLayout(5,1,3,3));
// 組合
JPanel panel1 = new JPanel(new BorderLayout(3,3));
panel1.add(command, BorderLayout.NORTH);
panel1.add(calckeys, BorderLayout.CENTER);
JPanel top = new JPanel(new BorderLayout());
top.add(display, BorderLayout.CENTER);
getContentPane().setLayout(new BorderLayout(3,5));
getContentPane().add(top, BorderLayout.NORTH);
getContentPane().add(panel1, BorderLayout.CENTER);
getContentPane().add(calms, BorderLayout.WEST);

雙模式啟動

• Applet：init() 自動建構介面、註冊監聽器。
• Application：main() 建立 JFrame、加入 Calculator 實例、設定視窗屬性。

編譯執行

javac Calculator.java
# Applet 模式
appletviewer Calculator.html
# 應用程式模式
java Calculator

擴充建議

1. 引入 ScriptEngineManager (JavaScript) 實現運算式解析，取代手寫邏輯。
2. 加入鍵盤綁定 (KeyBinding) 支援實體鍵盤輸入。
3. 支援歷史紀錄面板、科學函數 (sin/cos/log/^)。
4. 使用 BigDecimal 解決浮點數精度問題。