J2ME 簡易計算機實作：狀態機設計與按鍵處理邏輯

以狀態機設計實作四則運算計算機，支援整數除法、清除功能，邏輯清晰易擴充。

三狀態設計

1. INPUT_FIRST：輸入第一個數字
2. INPUT_FLAG：輸入運算符號 (+ - * /)
3. INPUT_SECOND：輸入第二個數字

核心邏輯

• 數字累積：value = value * 10 + (keyCode - KEY_NUM0)，利用鍵碼與字元編碼對應關係
• 運算符號切換：按 * 鍵循環切換 + - * /
• 左軟鍵清除：依狀態刪除最後一位數字或運算符
• 右軟鍵計算：執行四則運算，結果存回第一數字，重置狀態

完整原始碼 (CalcCanvas.java)

public class CalcCanvas extends Canvas {
    int status; final int INPUT_FIRST=1, INPUT_FLAG=2, INPUT_SECOND=3;
    int x=150, y=160, lineHeight=20;
    int firstNum=0, secondNum=0, flag=0; // flag: 1=+, 2=-, 3=*, 4=/
    int w, h; String[] sFlag = {"+","-","*","/"};

    public CalcCanvas() {
        status = INPUT_FIRST;
        setFullScreenMode(true);
        h = getHeight(); w = getWidth();
    }

    protected void paint(Graphics g) {
        g.setColor(0xffffff); g.fillRect(0,0,w,h);
        g.setColor(0);
        g.drawString("=", w, h, Graphics.RIGHT|Graphics.BOTTOM);
        g.drawString("C", 0, h, Graphics.LEFT|Graphics.BOTTOM);
        switch(status) {
            case INPUT_FIRST:
                g.drawString(""+firstNum, x, y, Graphics.RIGHT|Graphics.BOTTOM); break;
            case INPUT_FLAG:
                g.drawString(""+firstNum, x, y-lineHeight, Graphics.RIGHT|Graphics.BOTTOM);
                g.drawString(sFlag[flag-1], x, y, Graphics.RIGHT|Graphics.BOTTOM); break;
            case INPUT_SECOND:
                g.drawString(""+firstNum, x, y-2*lineHeight, Graphics.RIGHT|Graphics.BOTTOM);
                g.drawString(sFlag[flag-1], x, y-lineHeight, Graphics.RIGHT|Graphics.BOTTOM);
                g.drawString(""+secondNum, x, y, Graphics.RIGHT|Graphics.BOTTOM); break;
        }
    }

    public void keyPressed(int keyCode) {
        if (keyCode == -6) { // 左軟鍵 C
            switch(status) {
                case INPUT_FIRST: firstNum/=10; break;
                case INPUT_FLAG: status=INPUT_FIRST; flag=0; break;
                case INPUT_SECOND: secondNum/=10; if(secondNum==0) status=INPUT_FLAG; break;
            }
        }
        if (keyCode == -7) { // 右軟鍵 =
            if (status==INPUT_SECOND && secondNum!=0) {
                switch(flag) {
                    case 1: firstNum+=secondNum; break;
                    case 2: firstNum-=secondNum; break;
                    case 3: firstNum*=secondNum; break;
                    case 4: firstNum/=secondNum; break;
                }
                status=INPUT_FIRST; flag=0; secondNum=0;
            }
        }
        if (keyCode>=KEY_NUM0 && keyCode<=KEY_NUM9) {
            switch(status) {
                case INPUT_FIRST: firstNum = firstNum*10 + (keyCode-KEY_NUM0); break;
                case INPUT_SECOND: secondNum = secondNum*10 + (keyCode-KEY_NUM0); break;
                case INPUT_FLAG: status=INPUT_SECOND; secondNum = secondNum*10 + (keyCode-KEY_NUM0); break;
            }
        }
        if (keyCode == KEY_STAR) { // * 鍵切換運算符
            if (status==INPUT_FIRST || status==INPUT_FLAG) {
                flag = (flag < 4) ? flag+1 : 1;
                status = INPUT_FLAG;
            }
        }
        repaint();
    }
}

操作說明：數字鍵輸入數字、* 鍵切換運算子、左軟鍵刪除、右軟鍵計算結果。