Java switch 陷阱：忘記 break 導致的 case 穿透與 IDE 預防配置

現場還原：一個經典 Bug

public class SwitchDemo {
    public static void main(String[] args) {
        System.out.println("2 = " + numCase(2));
    }

    public static String numCase(int n) {
        String chineseNumber = "";
        switch (n) {
            case 0: chineseNumber = "零";
            case 1: chineseNumber = "壹";
            case 2: chineseNumber = "貳";
            case 3: chineseNumber = "三";
            case 4: chineseNumber = "肆";
            case 5: chineseNumber = "伍";
            case 6: chineseNumber = "陸";
            case 7: chineseNumber = "柒";
            case 8: chineseNumber = "捌";
            case 9: chineseNumber = "玖";
        }
        return chineseNumber;
    }
}

輸出結果

2 = 玖

根因分析：Case 穿透

Java switch 默認順序執行，匹配到 case 2 後，依續執行 case 3~9 所有賦值語句，最終 chineseNumber 為最後一條 "玖"。這稱為 Fall-through（穿透）。

正確寫法

switch (n) {
    case 0: chineseNumber = "零"; break;
    case 1: chineseNumber = "壹"; break;
    case 2: chineseNumber = "貳"; break;
    case 3: chineseNumber = "三"; break;
    case 4: chineseNumber = "肆"; break;
    case 5: chineseNumber = "伍"; break;
    case 6: chineseNumber = "陸"; break;
    case 7: chineseNumber = "柒"; break;
    case 8: chineseNumber = "捌"; break;
    case 9: chineseNumber = "玖"; break;
    default: chineseNumber = "未知"; break;
}

何時故意使用穿透？

// 合法場景：多值共用邏輯
switch (month) {
    case 3: case 4: case 5: season = "春季"; break;
    case 6: case 7: case 8: season = "夏季"; break;
    case 9: case 10: case 11: season = "秋季"; break;
    default: season = "冬季";
}

最佳實踐：故意穿透處必須加註釋 // fall through，並通過 Checkstyle/SpotBugs 強制檢查。

IDE 智能預防配置

Eclipse

1. Window → Preferences → Java → Compiler → Errors/Warnings
2. 展開 Potential programming problems
3. ‘switch’ case fall-through 設為 Error（紅叉）或 Warning（黃線）
4. Apply → 以後漏寫 break 直接報錯

IntelliJ IDEA

1. Settings → Editor → Inspections → Java → Control flow issues
2. 勾選 Fallthrough in 'switch' statement → Severity 選 Error
3. 代碼編輯器實時紅線提示，Alt+Enter 可一鍵補全 break

VS Code (Java Extension Pack)

預設即啟用 switch.fallthrough 警告，settings.json 可調整：

"java.compile.nullAnalysis.mode": "automatic",
"java.lint.switchFallthrough": "error"

現代替代方案：Java 12+ Switch Expression

// 箭頭語法：無穿透、賦值表達式、編譯期保證窮盡
String chineseNumber = switch (n) {
    case 0 -> "零";
    case 1 -> "壹";
    case 2 -> "貳";
    case 3 -> "三";
    case 4 -> "肆";
    case 5 -> "伍";
    case 6 -> "陸";
    case 7 -> "柒";
    case 8 -> "捌";
    case 9 -> "玖";
    default -> "未知";
};

優勢：語法層面消除穿透風險、函數式風格、類型安全、編譯器強制覆蓋所有情況。

總結

• switch 默認穿透是歷史設計，每個 case 必須以 break/return/throw 結束
• 善用 IDE 靜態檢查將錯誤攔截在編譯期
• 新項目優先使用 Java 12+ Switch Expression，從語言層面根治
• Code Review 重點檢查 switch 結構，見無 break 必問「是否故意穿透？」