java 代码细节（Introduce Explaining Variable）

这个观点来自《重构-----改善既有代码的设计》

You have a complicated expression.

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Put the result of the expression, or parts of the expression, in a temporary variable with a name that explains the purpose.

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if ((platform.toUpperCase().indexOf("MAC") > -1) &&   (browser.toUpperCase().indexOf("IE") > -1) &&    wasInitialized() && resize > 0 ) {         // do something}

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final boolean isMacOs     = platform.toUpperCase().indexOf("MAC") > -1;final boolean isIEBrowser = browser.toUpperCase().indexOf("IE")  > -1;final boolean wasResized  = resize > 0; if (isMacOs && isIEBrowser && wasInitialized() && wasResized) {     // do something}

Motivation

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Expressions can become very complex and hard to read. In such situations temporary variables can be helpful to break down the expression into something more manageable.

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Introduce Explaining Variable is particularly valuable with conditional logic in which it is useful to take each clause of a condition and explain what the condition means with a well-named temp. Another case is a long algorithm, in which each step in the computation can be explained with a temp.

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Introduce Explaining Variable is a very common refactoring, but I confess I don’t use it that much. I almost always prefer to use Extract Method if I can. A temp is useful only within the context of one method. A method is useable throughout the object and to other objects. There are times, however, when local variables make it difficult to use Extract Method. That’s when I use Introduce Explaining Variable.

Mechanics

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• Declare a final temporary variable, and set it to the result of part of the complex expression.

• Replace the result part of the expression with the value of the temp.

  If the result part of the expression is repeated, you can replace the repeats one at a time.

• Compile and test.

• Repeat for other parts of the expression.

Example

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I start with a simple calculation:

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double price() {    // price is base price - quantity discount + shipping    return _quantity * _itemPrice -        Math.max(0, _quantity - 500) * _itemPrice * 0.05 +        Math.min(_quantity * _itemPrice * 0.1, 100.0);}

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Simple it may be, but I can make it easier to follow. First I identify the base price as the quantity times the item price. I can turn that part of the calculation into a temp:

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double price() {     // price is base price - quantity discount + shipping     final double basePrice = _quantity * _itemPrice;      return basePrice -         Math.max(0, _quantity - 500) * _itemPrice * 0.05 +         Math.min(_quantity * _itemPrice * 0.1, 100.0);}

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Quantity times item price is also used later, so I can substitute with the temp there as well:

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double price() {    // price is base price - quantity discount + shipping    final double basePrice = _quantity * _itemPrice;     return basePrice -        Math.max(0, _quantity - 500) * _itemPrice * 0.05 +        Math.min(basePrice * 0.1, 100.0);}

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Next I take the quantity discount:

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double price() {    // price is base price - quantity discount + shipping    final double basePrice = _quantity * _itemPrice;    final double quantityDiscount = Math.max(0, _quantity - 500) * _itemPrice * 0.05;     return basePrice - quantityDiscount + Math.min(basePrice * 0.1, 100.0);}

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Finally, I finish with the shipping. As I do that, I can remove the comment, too, because now it doesn’t say anything the code doesn’t say:

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double price() {    final double basePrice = _quantity * _itemPrice;    final double quantityDiscount = Math.max(0, _quantity - 500) * _itemPrice * 0.05;    final double shipping = Math.min(basePrice * 0.1, 100.0);     return basePrice - quantityDiscount + shipping;}

Example with Extract Method

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For this example I usually wouldn’t have done the explaining temps; I would prefer to do that with Extract Method. I start again with

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double price() {    // price is base price - quantity discount + shipping    return _quantity * _itemPrice -        Math.max(0, _quantity - 500) * _itemPrice * 0.05 +        Math.min(_quantity * _itemPrice * 0.1, 100.0);}

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but this time I extract a method for the base price:

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double price() {    // price is base price - quantity discount + shipping    return basePrice() -        Math.max(0, _quantity - 500) * _itemPrice * 0.05 +        Math.min(basePrice() * 0.1, 100.0);}  private double basePrice() {    return _quantity * _itemPrice;}

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I continue one at a time. When I’m finished I get

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double price() {    return basePrice() - quantityDiscount() + shipping();} private double quantityDiscount() {    return Math.max(0, _quantity - 500) * _itemPrice * 0.05;} private double shipping() {    return Math.min(basePrice() * 0.1, 100.0);} private double basePrice() {    return _quantity * _itemPrice;}

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I prefer to use Extract Method, because now these methods are available to any other part of the object that needs them. Initially I make them private, but I can always relax that if another object needs them. I find it’s usually no more effort to use Extract Method than it is to use Introduce Explaining Variable.

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So when do I use Introduce Explaining Variable? The answer is when Extract Method is more effort. If I’m in an algorithm with a lot of local variables, I may not be able to easily use Extract Method. In this case I useIntroduce Explaining Variable to help me understand what is going on. As the logic becomes less tangled, I can always use Replace Temp with Query later. The temp also is valuable if I end up having to use Replace Method with Method Object.