5.Introduce Explaining Variable If you have a complicated expression,put the result of the expression, or parts of the expression , in a temporary variable with a name that explains the purpose. Introduce Explaining Variable is particulaly valuable with conditional logic in which it is useful to take each clause of a condition and explain what the condition means a well-named temp. Another case is a long algorithm,in which each step in the computation can be explained with a temp.Mechanics (1)Declare a final temporary variable,and set it to the result of part of the complex expression. (2)Replace the result part of the expression with the value of the temp.Before:double price(){ return quantity*itemPrice-Math.max(0,quantity-500)*itemPrice*0.05+Math.min(quantity*itemPrice*0.1,100.0);}After:double price(){ final double basePrice= quantity*itemPrice; final double quantityDiscount=Math.max(0,quantity-500)*itemPrice*0.05 ; final double shipping=Math.min(quantity*itemPrice*0.1,100.0); return basePrice-quantityDiscount+shipping;}
Appendix: using Extract Method to complete refactoring. The benefit of using Extract Method is that theses methodologies are available to any other part of the object that needs them.Although they are private at first,but you can relax that if another object needs them.
double price(){ return basePrice()-quantityDiscount()+shipping();}private Double basePrice(){ return quantityDiscount*itemPrice;}private Double quantityDiscount(){ return Math.min(quantity*itemPrice*0.1,100.0);}private Double shipping(){ return Math.min(quantity*itemPrice*0.1,100.0);}
6.Split Temporary Variable If a temporary variable assigned to more than once,but is not a loop variable nor a collecting temporary variable(If the later assignments are of the form i=i+some expression, that indicates that it is a collceting temporary variable),you should use Split Temporary Method.Any variable with more than one responsibility should be replaced with a temp for each responsibility.Michanics: (1)Change the name of a temp at its declaration and its first assignment. (2)Declare the new temp as final. (3)Change all references of the temp up to its second assignment.(4) Declare the temp at its second assignment.Before:double getDistanceTravelled(int time){ double result; double acc=primaryForce/mass; int primaryTime=Math.min(time,delay); result = 0.5*acc*primaryTime*primaryTime; int secondaryTime=time-delay; if(secondaryTime>0){ double primaryVel=acc*delay; acc=(primaryForce+secondaryForce)/mass; result+=primaryVel*secondaryTime+0.5*acc*secondaryTime*secondaryTime; } return result;}After:double getDistanceTravelled(int time){ double result; final double primaryAcc=primaryForce/mass; int primaryTime=Math.min(time,delay); result = 0.5*primaryAcc*primaryTime*primaryTime; int secondaryTime=time-delay; if(secondaryTime>0){ double primaryVel=primaryAcc *delay; final double secondaryAcc=(primaryForce+secondaryForce)/mass; result+=primaryVel*secondaryTime+ 0.5*secondaryAcc*secondaryTime*secondaryTime; } return result;}
Appendix: Complete refactoringdouble getDistanceTravelled(int time){ double result; result = 0.5*getPrimaryAcc()*Math.pow(getPrimaryTime(time),2); if(getSecondaryTime(time)>0){ double primaryVel=getPrimaryAcc() *delay; final double secondaryAcc=(primaryForce+secondaryForce)/mass; result+=primaryVel*getSecondaryTime(time)+ 0.5*secondaryAcc*Math.pow(getSecondaryTime(time)，2); } return result;}private int getPrimaryTime(int time){ return Math.min(time,delay);}private double getPrimaryAcc(){ return primaryForce/mass;}private int getSecondaryTime(int time){ return time-delay;}
7.Remove Assignments to ParametersBefore:int discount(int inputVal, int quantity, int yearToDate){ if(inputVal>50) inputVal-=2; if(quantity>100) inputVal-=1; if(yearToDate>10000) inputVal-=4; return inputVal;}
After:int discount(int inputVal, int quantity, int yearToDate){ int result=inputVal; if(inputVal>50) result-=2; if(quantity>100) result-=1; if(yearToDate>10000) result-=4; return result;}
8.Replace Method with Method Object If there is a long method that uses local variables in such a way that you cannot apply Extract Method.Turn the mothod into its own object so that all the local variables became fields on that object.You can then decompose the method into other methods on the same object.Michanics: (1)Create a new class ,name it after the method. (2)Give the new class a final field for the object that hosted the original method and a field for each temporary variable and each parameter int the method. (3)Give the new class a constructor that takes the source object and each parameter. (4)Give the new class a method named “compute” and copy the body of the original method into it.
Before:class Account{int gamma(int inputVal,int quantity,int yearToDate){int importantVal1=(inputVal*quantity)+delta();int importantVal2=(inputVal*yearToDate)+100;if((yearToDate-importantVal1>100)) importantVal2-=20;int importantVal3=importantVal2*7;return importantVal3-2*importantVal1;}}
After:class Account{int gamma(int inputVal,int quantity,int yearToDate){return new Gamma(this, inputVal, quantity, yearToDate);}}
class Gamma{ public Gamma(Account account,int inputVal,int quantity,int yearToDate){ account=account; inputVal=inputVal; quantity=quantity; yearToDate=yearToDate; } final Account account; int importantVal1; int importantVal2; int importantVal3; int inputVal; int quantity; int yearToDate;
int compute(){ int importantVal1=(inputVal*quantity)+account.delta();int importantVal2=(inputVal*yearToDate)+100;if((yearToDate-importantVal1>100)) importantVal2-=20;int importantVal3=importantVal2*7; return importantVal3-2*importantVal1; }}
9.Substitue AlgorithmReplace an algorithm with one that is clearer.