http://bj007.blog.51cto.com/1701577/345100/
switch语句是我们日常工作中最常见也是争论最多的(goto被忽视的前提下)。在重构中也把switch语句看成是一种代码的坏味道。但如何改造现有的switch代码并在今后去避免呢?本文从两方面进行探讨。
1 类型转化
在不同的抽象层次上对于同一个对象可能会用不同的定义。举个简单的例子,在计算器中,用户输入的操作符号可能是字符型的,而程序内部实现的时候需要用枚举型。因此可能就会有这样的函数。
在不同的抽象层次上对于同一个对象可能会用不同的定义。举个简单的例子,在计算器中,用户输入的操作符号可能是字符型的,而程序内部实现的时候需要用枚举型。因此可能就会有这样的函数。
public class Calculator
{
public enum OPERATOR {Add, Minus, Multiply, Divide, Unknown};
public static OPERATOR GetOperator(char ch)
{
OPERATOR op = OPERATOR.Unknown;
switch (ch)
{
case '+':
op = OPERATOR.Add;
break;
case '-':
op = OPERATOR.Minus;
break;
case '*':
op = OPERATOR.Multiply;
break;
case '/':
op = OPERATOR.Divide;
break;
default:
break;
}
return op;
}
}
{
public enum OPERATOR {Add, Minus, Multiply, Divide, Unknown};
public static OPERATOR GetOperator(char ch)
{
OPERATOR op = OPERATOR.Unknown;
switch (ch)
{
case '+':
op = OPERATOR.Add;
break;
case '-':
op = OPERATOR.Minus;
break;
case '*':
op = OPERATOR.Multiply;
break;
case '/':
op = OPERATOR.Divide;
break;
default:
break;
}
return op;
}
}
代码不长,不过写的过程中我却拷贝几次(case部分的代码)。原因就是代码的结构性重复还是比较严重的,而且如果以后支持其他新的操作时候,这代码不仅要改变,而且还会不断变冗长。认真想想,避免的办法还是有的,用Dictionary做一个映射表就可以实现。
public class Calculator
{
public enum OPERATOR { Add, Minus, Multiply, Divide, Unknown};
static private var OperatorMap = new Dictionary<char, OPERATOR> {
{ '+', OPERATOR.Add },
{ '-', OPERATOR.Minus },
{ '*', OPERATOR.Divide },
{ '/', OPERATOR.Multiply },
};
public static OPERATOR GetOperator2(char ch)
{
if (OperatorMap.ContainsKey(ch))
return OperatorMap[ch];
else
return OPERATOR.Unknown;
}
}
{
public enum OPERATOR { Add, Minus, Multiply, Divide, Unknown};
static private var OperatorMap = new Dictionary<char, OPERATOR> {
{ '+', OPERATOR.Add },
{ '-', OPERATOR.Minus },
{ '*', OPERATOR.Divide },
{ '/', OPERATOR.Multiply },
};
public static OPERATOR GetOperator2(char ch)
{
if (OperatorMap.ContainsKey(ch))
return OperatorMap[ch];
else
return OPERATOR.Unknown;
}
}
这样,不仅代码简洁了很多,而且从本质上已经从逻辑代码维护变成了映射表的维护。
2 动作调用
接续刚才的例子,这次不仅要实现操作符转化,而且要实现真正的运算。第一个反应又是想到了switch。实现如下:
接续刚才的例子,这次不仅要实现操作符转化,而且要实现真正的运算。第一个反应又是想到了switch。实现如下:
public class Calculator
{
static public int Calculate(char op, int number1, int number2)
{
int result = 0;
switch (GetOperator(op))
{
case OPERATOR.Add:
result = number1 + number2;
break;
case OPERATOR.Minus:
result = number1 - number2;
break;
case OPERATOR.Multiply:
result = number1 * number2;
break;
case OPERATOR.Divide:
result = number1 / number2;
break;
default:
throw new Exception("Unsupported Operation!");
}
return result;
}
}
{
static public int Calculate(char op, int number1, int number2)
{
int result = 0;
switch (GetOperator(op))
{
case OPERATOR.Add:
result = number1 + number2;
break;
case OPERATOR.Minus:
result = number1 - number2;
break;
case OPERATOR.Multiply:
result = number1 * number2;
break;
case OPERATOR.Divide:
result = number1 / number2;
break;
default:
throw new Exception("Unsupported Operation!");
}
return result;
}
}
跟之前遇到的问题有些类似,不过这回需要调用函数。通过.Net环境提供的Dictionary以及Func Delegate的解决如下:
public class Calculator
{
static private Dictionary<OPERATOR, Func<int, int, int>> calculationAction = new Dictionary<OPERATOR, Func<int, int, int>> {
{ OPERATOR.Add, Add },
{ OPERATOR.Minus, Minus },
{ OPERATOR.Multiply, Multiply },
{ OPERATOR.Divide, Divide }
};
public static OPERATOR GetOperator2(char ch)
{
if (OperatorMap.ContainsKey(ch))
return OperatorMap[ch];
else
return OPERATOR.Unknown;
}
static int Add(int number1, int number2)
{
return number1 + number2;
}
// Others are omitted here for brevity
}
{
static private Dictionary<OPERATOR, Func<int, int, int>> calculationAction = new Dictionary<OPERATOR, Func<int, int, int>> {
{ OPERATOR.Add, Add },
{ OPERATOR.Minus, Minus },
{ OPERATOR.Multiply, Multiply },
{ OPERATOR.Divide, Divide }
};
public static OPERATOR GetOperator2(char ch)
{
if (OperatorMap.ContainsKey(ch))
return OperatorMap[ch];
else
return OPERATOR.Unknown;
}
static int Add(int number1, int number2)
{
return number1 + number2;
}
// Others are omitted here for brevity
}
通过这个简单的例子,我们可以看出Switch通常会带来一些结构性的重复,通过利用Dictionary等二维表结构,我们可以尽量去避免switch语句,从而实现更好的灵活性和可维护性。最后需要支持的是,在Switch语句的重构过程中需要注意几点:
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