巢狀條件式通常為為複雜度高的區域。
以下紀錄簡化巢狀條件式的技巧

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1.巢狀 if 轉換成 if then else + and。
先從最簡單的開始，e.g.

    private String nestedCondition1()
    {
        String result = "";
        if (isProcess) {
            if (isDone) {
                return result = result.concat("process is truedone is true");
            }
        }
        return result;
    }

目標是第4和第5行巢狀if，我們使用 if + and，化解巢狀if。

    private String fixByIfThenElse1()
    {
        String result = "";
        if (isProcess && isDone) {
            return result = result.concat("process is truedone is true");
        }
        return result;
    }

---

2.變化：原式 if 和 if 之間有statement。e.g.第5行新增 result = result.concat(“process is true”);

    private String nestedCondition2()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                return result = result.concat("done is true");
            }
        }
        return result;
    }

對於第5行有個轉換的小技巧。在2個 if 之間的 statement 可以轉換為 if(condition1 && ! condition2){statement};
需要注意的是 condition2 必須反轉。e.g.

if(condition1){
    statement1;
    if(condition2){
        statement2;
    }
}
---------trans-----------
if(condition1 && !condition2){
    statement1;
}else if(condition1 && condition2){
    statement1;
    statement2;
}

轉換 if-then-else + and

    private String fixByIfThenElse2()
    {
        String result = "";
        if(isProcess && !isDone){
            result = result.concat("process is true");
        }else if(isProcess && isDone){
            result = result.concat("process is true");
            result = result.concat("done is true");
        }
        return result;
    }

---

 3.變化： 原式最外層 if 產生相對應的 else。新增 9～11行

    private String nestedCondition3()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                return result = result.concat("done is true");
            }
        } else {
            result = result.concat("process is not true");
        }
        return result;
    }

轉換 if-then-else + and，新增8～10行

    private String fixByIfThenElse3()
    {
        String result = "";
        if(isProcess && !isDone){
            result = result.concat("process is true");
        }else if(isProcess && isDone){
            result = result.concat("process is true");
            result = result.concat("done is true");
        }else if(!isProcess){
            result = result.concat("process is not true");
        }
        return result;
    }

---

 4.變化： 原式內層 if 產生相對應 else 。新增 8～10行

    private String nestedCondition4()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                return result = result.concat("done is true");
            } else {
                return result = result.concat("done is not true");
            }
        } else {
            result = result.concat("process is not true");
        }
        return result;
    }

轉換 if-then-else + and

    private String fixAndNestedConditionSimple()
    {
        String result = "";
        if (isProcess && isDone) {
            result = result.concat("process is true");
            return result = result.concat("done is true");
        } else if (isProcess && !isDone) {
            result = result.concat("process is true");
            return result = result.concat("done is not true");
        } else if (!isProcess) {
            result = result.concat("process is not true");
        }
        return result;
    }

---

5.變化：外層 if 的 else 加入其他動作，12~17行

    private String nestedCondition5()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                result = result.concat("done is true");
            } else {
                result = result.concat("done is not true");
            }
        } else {
            result = result.concat("process is not true");
            if (isPrint) {
                result = result.concat("print is true");
            } else {
                result = result.concat("print is not true");
            }
        }
        return result;
    }

轉換對應的 if-then-else + and  為

private String fixByNestedCondition5()
    {
        String result = "";
        if (isProcess && isDone) {
            return result = result.concat("process is truedone is true");
        } else if (isProcess && !isDone) {
            return result = result.concat("process is truedone is not true");
        } else if (!isProcess && isPrint) {
            result = result.concat("process is not true");
            return result = result.concat("print is true");
        } else if (!isProcess && !isPrint) {
            result = result.concat("process is not true");
            return result = result.concat("print is not true");
        }
        return result;
    }

總結：
照這種排列邏輯來看基本上所有的巢狀 if 都可以轉換 if then else + and的結構，差別只在於複雜度與可讀性的不同。

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2.以衛述句取代巢狀條件式。
這個方法是從”重構：改善既有程式的設計”一書來的。
衛述句是指條件式中若有特別的判斷邏輯，必須明確的標示出來並立刻從函式中返回。e.g.

    private String nestedCondition1()
    {
        String result = "";
        if (isProcess) {
            if (isDone) {
                return result = result.concat("process is truedone is true");
            }
        }
        return result;
    }

以衛述句改善為

    private String fixByGuardClause1()
    {
        String result = "";
        if (!isProcess) {
            return result;
        }
        if (isDone) {
            return result = result.concat("process is truedone is true");
        }
        return result;
    }

---

2.變化：原式 if 和 if 之間有statement。e.g.第5行新增 result = result.concat(“process is true”);

    private String fixByGuardClauseOrigin()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
        }else{
            if (isDone) {
                result = result.concat("done is true");
            }else{
                result = result.concat("done is not true");
            }
        }
        return result;
    }

以衛述句改善有2種形式，分別為是否在判斷邏輯加入 and 運算。
加入and運算後可以看到其第1種方式較為簡潔(不需要摻雜result = result.concat(“process is true”))，可讀性較高。
1.在判斷邏輯加入 and 。

    private String fixByGuardClauseWithAnd2()
    {
        String result = "";
        if(isProcess && !isDone){
            return result = result.concat("process is true");
        }
        if(isProcess && isDone){
            result = result.concat("process is true");
            return result= result.concat("done is true");
        }
        return result;
    }

2.不加入 and。

    private String fixByGuardClause2()
    {
        String result = "";
        if(!isProcess){
            return result;
        }
        result = result.concat("process is true");
        if(isDone){
            return result = result.concat("done is true");
        }
        return result;
    }

---

3.變化型： 原式最外層 if 產生相對應的 else。新增 9～11行

    private String nestedCondition3()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                return result = result.concat("done is true");
            }
        } else {
            result = result.concat("process is not true");
        }
        return result;
    }

以衛述句改善為

    private String fixByGuardClause3WithAnd()
    {
        String result = "";
        if(!isProcess){
            return result.concat("process is not true");
        }
        if(isProcess && !isDone){
            return result.concat("process is true");
        }
        if(isProcess && isDone){
            result = result.concat("process is true");
            return result.concat("done is true");
        }
        return result;
    }

---

4.變化： 原式內層 if 產生相對應 else 。新增 8～10行 

    private String nestedCondition4()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                return result = result.concat("done is true");
            } else {
                return result = result.concat("done is not true");
            }
        } else {
            result = result.concat("process is not true");
        }
        return result;
    }

以衛述句改善為

    private String fixByGuardClause4WithAnd()
    {
        String result = "";
        if(isProcess && !isDone){
            result = result.concat("process is true");
            return result = result.concat("done is not true");
        }
        if(isProcess && isDone){
            result = result.concat("process is true");
            return result = result.concat("done is true");
        }
        if(!isProcess){
            return result = result.concat("process is not true");
        }
        return result;
    }

---

5.變化：外層 if 的 else 加入其他動作，新增12~17行

    private String nestedCondition5()
    {
        String result = "";
        if (isProcess) {
            result = result.concat("process is true");
            if (isDone) {
                result = result.concat("done is true");
            } else {
                result = result.concat("done is not true");
            }
        } else {
            result = result.concat("process is not true");
            if (isPrint) {
                result = result.concat("print is true");
            } else {
                result = result.concat("print is not true");
            }
        }
        return result;
    }

以衛述句改善為

    private String fixByGuardClause5WithAnd()
    {
        String result = "";
        if (!isProcess && !isPrint) {
            result = result.concat("process is not true");
            result = result.concat("print is not true");
            return result;
        }
        if (!isProcess && isPrint) {
            result = result.concat("process is not true");
            result = result.concat("print is true");
            return result;
        }
        if (isProcess && isDone) {
            result = result.concat("process is true");
            result = result.concat("done is true");
            return result;
        }
        if (isProcess && !isDone) {
            result = result.concat("process is true");
            result = result.concat("done is not true");
            return result;
        }
        return result;
    }

無論使用衛述句或是if-then-else都可將難以理解的巢狀結構分解成簡單的表達式。