Refactoring. Refactoring. Refactoring. Refactoring. Refactoring. Refactoring. Lesson Five: Conditionals

Transcription

1 Lesson Five: should not be too complex. If they are complex, refactor them to make simple methods making readability and understandability better. Learning objective have simple conditionals which are self documenting with meaningful names In our program with explaining variable names, there are still some conditionals which are difficult to understand. We find them in mainly in bestmove(). FOR EXAMPLE int bestmove(int computerstatus, int userstatus) { if (((computerstatus & (1 << potentialcomputermove)) == 0) && ((userstatus & (1 << potentialcomputermove)) == 0)) if (((potentialcomputerstatus & (1 << j)) == 0) && ((userstatus & (1 << j)) == 0)) if (((computerstatus & (1 <<firstavailablecomputermove)) == 0) && ((userstatus & (1 <firstavailablecomputermove)) == 0)) { REFACTORINGS Decomposing Conditional Consolidating Conditional Statements Consolidate Duplicate Conditional Fragments Replace Nested Conditional with Guard Clauses Remove Control Flag 1

2 Decompose Conditional You have a complicated conditional (if-then-else) statement. Extract methods from the condition, then part, and else parts. Complexity exist in a heavy if then else structure and you want to make the code more readable and understandable. You extract methods and name them where the code is readable and make small boolean methods to make the logic more understandable. if (date.before (SUMMER_START) date.after(summer_end)) charge qualitity * _winterrate + _winterservicecharge; Consider the following initial if statement. else charge quantity * _summerrate; GOES TO if (notsummer_date)) if (date.before (SUMMER)START) date.after(summer_end)) charge qualitity * _winterrate + _winterservicecharge; else charge quantity * _summerrate; charge = wintercharge(quantity); else charge = summercharge (quantity); Extract the conditional and each leg as follows. If(notSummer(date) charge = wintercharge(quantity); else charge = summercharge (quantity); private boolean notsummer (Date date) { return date.before (SUMMER_START) date.after (SUMMER_END); } // end not summer private double summercharge (int quantity) { Extract the condition into its own method Extract the then part then the else part Compile and test return quantity * _summerrate; } // end summercharge private double wintercharge (int quantity) { return quantity * _winterrate + )winterservicecharge; } 2

3 Consolidate Conditional Expressions You have a sequence of conditional tests with the same result. Sometimes you have a series of conditional checks in which the conditional statements are different but the effects are the same. Combine them into a single conditional expression and extract it. double disabilityamount() { if (_seniority < 2) return 0; if (_monthsdisabled > 12) return 0; if (_isparttime) return 0; // compute the disability amount GOES TO. Double diabilityamount () { if (isnoteligablefordisability ()) return 0; // computer the disability amount Assume the following. double disabilityamount() { if (_seniority < 2) return 0; if (_monthsdisabled > 12) return 0; if (_isparttime) return 0; // compute the disability amount These statements may be equivalent of a complex or statement double diabilityamount () { if (_seniority < 2) (_monthsdisabled > 12) (_isparttime)) return 0; // computer the disability amount Look at the condition and use extract method to communicate what the condition is looking for. double disabilityamount() { if (iseligiblefordiability()) return 0; // computer the disability amount This can be done with ands. if (onvacation()) if (lengthofservice*( > 10 ) return 1; return 0.5; Boolean iseligiblefordisability() { return ((_seniority < 2) (_monthsdisabled > 12) (_isparttime)); } // end is ElibibleFor Disability if (onvacation () && lengthofservice () > 10) return 1; else return 0.5; 3

4 Given the following if (onvacation() && lengthofservice*( > 10 ) return 1; return 0.5; Test only the condition and returns a value turn the routine into a single return statements using the ternary operator. if (onvacation () && lengthofservice () > 10)? 1; 0.5; Check that none of the conditional has side effects. Replace the string of conditionals with a single conditional statement using logical operators. Compile and test. Consider using Extract Method on the condition. Consolidate Duplicate Conditional Fragments The same fragment of code is in all branches of a conditional expression. Consolidate Duplicate Conditional Fragments: Sometime you find the same code executed in all legs of a conditional. This requires you move the code outside the conditional to make it more clear what remains constant. Move it outside of the expression. Consolidate Duplicate Conditional Fragments: You begin with the following if (isspecialdeal() ) { total = price * 0.95; send (); } // end if else { total price * 0.98; send(); Move the send method out since it is used by both if (isspecialdeal() ) total = price * 0.95; else total =price * 0.98; send(); Consolidate Duplicate Conditional Fragments: Identify code that is executed. If common code is at the beginning, do it before conditional. If the code is at the end, move it after the conditional If the common code is in the middle, see if you can move it forward or backward. If there is more than one line of code extract method. 4

5 Remove Control Flag You have a variable that is acting as a control flag for a series of boolean expressions. You have a control flag that allows exit from the conditional statements. These control flags add complexity. Get rid of the control flag. Use a break or return instead. Function checks a people list for suspicious characters void checksecurity (String [ ] people) { boolean found = false; for (int i = 0; i < people.length; i++) { if (!found) { // this is the flag if (people[i].equals ( Don )) { sendalert(); found = true; } if (people[i].equals ( John )) { sendalert(); found = true; } } // end if } // end for } // end checksecurity The control flag is the piece that sets the found variable. Step 1: Set breaks in one at a time. Leave in flag. void checksecurity (String [ ] people) { boolean found = false; for (int i = 0; i < people.length; i++) { if (!found) { if (people[i].equals ( Don )) { sendalert(); found = true; break; } if (people[i].equals ( John )) { sendalert(); found = true; break; } } // end if } // end for } // end checksecurity Step 2: Remove control flag references. You can use the return statement with function returning a String. void checksecurity (String [ ] people) { for (int i = 0; i < people.length; i++) { if (people[i].equals ( Don )) { sendalert(); break; } if (people[i].equals ( John )) { sendalert(); break; } } // end for } // end checksecurity String checksecurity (String [ ] people) { for (int i = 0; i < people.length; i++) { if (people[i].equals ( Don )) { sendalert(); return Don ; } if (people[i].equals ( John )) { sendalert(); return John ; } } // end for return ; // returns null if neither } // end checksecurity 5

6 Find the value of the control flag that gets you out of the logic statement. Replace assignments of the break-out value with a break or continue statement. Compile and test after each replacement. Replace Nested Conditional with Guard Clauses A method has conditional behavior that does not make clear the normal path of execution. Use guard clauses for all the special cases. Replace Nested Conditional with Guard Clauses: Expressional statement may be of two forms, One form is whether either course is part of the normal behavior And the second form is a situation in which one answer from the conditional indicates normal behavior and the other indicates an unusual condition. Replace Nested Conditional with Guard Clauses: (con t) Each type of conditions have different intensions. If both are part of normal behavior, use a condition with an if and an else leg. If the condition is a unusual condition, check the condition and return if the condition is true. This is called a guard clause. Replace Nested Conditional with Guard Clauses: Given double getpayamount() { double result; if (_isdead) result = deathamount(); // this is NOT a normal course else { if (_isseparated) result = separatedamount(); // this is a normal course else { if (_isretired) result = retiredamount(); // this is a normal course else result = normalpayamount(); // this is the normal course return result } // end getpayamount Replace Nested Conditional with Guard Clauses: GOES TO separating out the behaviors. double getpayamount() { if (_isdead) return deadamount(); if (_isseparated) return separatedamount(); if (_isretired) return retiredamount(); return normalpayamount(); } // end getpayamount 6

7 Replace Nested Conditional with Guard Clauses: Given this initial code as shown before -- this is not unlikely code double getpayamount() { double result; if (_isdead) result = deadamount(); else { if (_isseparated) result = separatedamount(); else { if (_isretired) result = retiredamount(); else result = normalpayamount(); return result } // end getpayamount Replace Nested Conditional with Guard Clauses: Checking masking the normal course of action behind the checking. So we use a guard clause one at a time. double getpayamount() { double result; if (_isdead) return deadamount(); // this is masking of normals else { if (_isseparated) result = separatedamount(); else { if (_isretired) result = retiredamount(); else result = normalpayamount(); return result } // end getpayamount Replace Nested Conditional with Guard Clauses: For each check put in the guard clause. Compile and test after each check is replaced with a guard clause. Replace Conditional with Polymorphism You have a conditional that chooses different behavior depending on the type of an object. Move each leg of the conditional to an overriding method in a subclass. Make the original method abstract. Avoid writing explicit conditional statements when you have objects whose behavior varies depending on the type of object you are dealing with. Polymorphism gives you the advantage. Given the following speeds for different types of birds double getspeed() { switch ()type) { case EUROPEAN: return getbasespeed(); case AFRICAN: return getbasespeed() getloadfactor() * _numberofcoconuts; case NORWEGIAN_BLUE: return (_isnailed)? 0 ; getbasespeed (_voltage); } // end switch throw new RuntimeException ( Should be unreachable ); } // end getspeed 7

8 GOES TO Bird getspeed Method: 1. Use Extract Method to take apart the conditional. 2. Use either Replace Type Code with SubClasses OR Bird getspeed Bird getspeed Bird getspeed Use Replace Type Code with State/Strategy 3. Use Move Method to place condition in an isa structure. 4. Create sub-classes with overriding methods. Suppose you have the following inheritance structure Employee Employee Type ENGINEER SALESMAN MANAGER With the following code class Employee.. int payamount() { switch (gettype)() { case EmployeeType.ENGINEER: return _monthlysalary; case EmployeeType.SALESMAN: return _monthlysalary + commission; case EmployeeType.MANAGER: return _monthlysalary + bonus; default; throw new RuntimeException ( Incorrect Employee ); } // end switch } // end Employee With the following code int gettype () { return _type.gettypecode(); } // returns type of object private EmployeeType _type; // an instance of employee type in the program abstract class EmployeeType. // is the abstract class of employee type abstract int gettypecode(); class Engineer extends EmployeeType // defines the subclass int gettypecode() { return Employee.ENGINEER; } // this returns the type The case statement is extracted so I move it into employee type. class EmployeeType // this code used to be in Employee int payamount (Employee emp) { switch (gettypecode ()) { case ENGINEER: return emp.getmonthlysalary (); case SALESMAN: return emp.getmonthlysalary () + emp.getcommission(); case MANAGER: return emp.getmonthlysalary() + emp.getbonus(); default: throw new RuntimeException ( Incorrect Employee ); 8

9 Change the payamount method to delegate to the new class. class Employee int payamount () { return _type.payamount (this)) Work on Case statement copy engineer leg to engineer class class Engineer int payamount (Employee emp) { return emp.getmonthlysalary(); } The new method overrides the case statement for engineers I can place a trap here to make sure I made no mistakes class EmployeeType int payamount (Employee emp) { switch (gettypecode ()) { case ENGINEER: throw new RuntimeException ( Should be being overridden ); case SALESMAN: return emp.getmonthlysalary () + emp.getcommission(); case MANAGER: return emp.getmonthlysalary() + emp.getbonus(); default: throw new RuntimeException ( Incorrect Employee ); I can then replace this switch statement entirely With a polymorphic call. class Employee int payamount () { return _type.payamount (this)) class Engineer int payamount (Employee emp) { return emp.getmonthlysalary(); } If the conditional statement is one part of a larger method, take apart the conditional statement and use Extract Method. If necessary use Move method to place the conditional at the top of the inheritance structure. Pick one of the subclasses. Remove the copied leg of the conditional statement. Compile and test. if (((computerstatus & (1 << firstavailablecomputermove)) == 0) && ((userstatus & (1 << firstavailablecomputermove)) == 0)) BECOMES public boolean cellempty (int computerstatus,int potentialcomputermove,int userstatus) { return ((computerstatus & (1 << potentialcomputermove)) == 0) && ((userstatus & (1 << potentialcomputermove)) == 0); } // end cellempty if ((getcomputerstatus() & (1 << i))!= 0) BECOMES public boolean squareoccupied (int i, int playerstatus) { return ((playerstatus & (1 << i))!= 0); } // end squareoccupied REUSED WITH 3 times if (squareoccupied (i, getcomputerstatus())) 9