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24 - Refactoring: Simplifying Conditional
Expressions and Method Calls
CMPSCI 220 (291A)
Programming Methodology
Spring 2009
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 2
Goal for the week
Build your catalog of familiar refactorings
Increase sense of incremental nature of refactoring
Today: Conditional Expressions (REF 9) and Method
Calls (REF 10)
Next time: Composing methods (REF 6), Moving
features around (REF 7), Data (REF 8)
Later: Generalization (REF 12) and Big Refactorings
(REF 13)
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 3
Simplifying Conditional Expressions
General motivation: Conditional logic ( if and
switch statements) tends to get tricky
Decompose Conditional is a key: it separates the
switching logic from what happens
Some other refactorings address important special
cases
Usually replace switches with polymorphism
Introduce Null Object is a non-obvious case of that
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Decompose Conditional
Before:
if (date.before(SUMMER_START) || date.after(SUMMER_END))
charge = quantity * _winterRate + _winterServiceCharge;
else charge = quantity * _summerRate;
After:
if (notSummer(date))
charge = winterCharge(quantity);
else charge = summerCharge(quantity);
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 5
Decompose Conditional
Motivation: Reasons why things happen get
obscured in details of condition(al)s
Mechanics
Extract condition to its own method
Extract then and else parts to their own methods
While the methods are often short, just the use of
names like notSummer and winterCharge make the
code much clearer
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 6
Consolidate Conditional Expression
Combine sequence of tests with same result:
Before:
double disabilityAmount () {
if (_seniority < 2) return 0;
if (_monthsDisabled > 12) return 0;
if (_isPartTime) return 0;
// compute disability amount …
After:
double disabilityAmount() {
if (isNotEligibleForDisability()) return 0;
// compute disability amount …
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 7
Consolidate Conditional Expression
Motivations:
Clarifies that the sequence is really one check of an “or” of conditions Often sets the stage for Extract Method
Mechanics:
The conditions must not have side-effects. Replace the string of ifs with one if using logical operators Compile and test Consider applying Extract Method to the result
Note: Can do the same thing with and-ed conditionals, etc.
Can sometimes use a ternary (? :) expression in a return
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Consolidate Duplicate Conditional
Fragments
Before:
if (isSpecialDeal()) { total = price * 0.95; send(); } else { total = price * 0.98; send(); }
After:
if (isSpecialDeal()) { total = price * 0.95; } else { total = price * 0.98; } send();
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 9
Consolidate Duplicate Conditional
Fragments
Also works for common code at the beginning
Motivation: clarifies what varies and what stays the
same between the different cases
If there is more than one statement in common,
consider extracting it into a method
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Remove Control Flag
There is a variable acting as a control flag for a
series of boolean expressions; use break or return
instead.
Motivation:
Code arises from one-entry/one-exit style of coding
As usual, refactoring improves clarity
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Remove Control Flag
Generic structure being replaced:
set done to false while not done if (condition) do something set done to true next step of loop
Mechanics
Determine the value that gets you out Replace assignments of that value with break/continue
Compile and test after each replacement
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 12
Remove Control Flag
Generic structure being replaced:
set done to false while not done if (condition) do something set done to true next step of loop
Alternate refactoring
Extract the logic into a method Determine the value that gets you out Replace assignments of that value with a return Compile and test after each replacement
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Remove Control Flag:
Example 2 – After Extract Method
void checkSecurity (String[] people) { String found = foundMiscreant(people); someLaterCode(found); } String foundMiscreant (String[] people) { String found = “”; for (String person : people) { if (found.equals(“”)) { if (person.equals(“Don”)) { sendAlert(); found = “Don”; } if (person.equals(“John”)) { sendAlert(); found = “John”; } } } return found; }
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 20
Remove Control Flag:
Example 2 – After first return
String foundMiscreant (String[] people) { String found = “”; for (String person : people) { if (found.equals(“”)) { if (person.equals(“Don”)) { sendAlert(); return “Don”; } if (person.equals(“John”)) { sendAlert(); found = “John”; } } } return found; }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 21
Remove Control Flag:
Example 2 – After second return
String foundMiscreant (String[] people) { String found = “”; for (String person : people) { if (found.equals(“”)) { if (person.equals(“Don”)) { sendAlert(); return “Don”; } if (person.equals(“John”)) { sendAlert(); return “John”; } } } return found; }
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 22
Remove Control Flag:
Example 2 – Remove flag variable
String foundMiscreant (String[] people) { String found = “”; for (String person : people) { if (found.equals(“”)) { if (person.equals(“Don”)) { sendAlert(); return “Don”; } if (person.equals(“John”)) { sendAlert(); return “John”; } } } return “”; }
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Remove Control Flag:
Example 2 – End
String foundMiscreant (String[] people) { for (String person : people) { if (person.equals(“Don”)) { sendAlert(); return “Don”; } if (person.equals(“John”)) { sendAlert(); return “John”; } } return “”; }
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Replace Nested Conditional with
Guard Clauses
Normal path not clear:
double getPayAmount () { double result; if (_isDead) result = deadAmount(); else { if (_isSeparated) result = separatedAmount(); else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); }; } return result; }
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Replace Nested Conditional with
Guard Clauses
Use guard clauses for special cases:
double getPayAmount () { if (_isDead) return deadAmount(); if (_isSeparated) return separatedAmount(); if (_isRetired) return retiredAmount(); return normalPayAmount(); }
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 26
Replace Nested Conditional with
Guard Clauses
Motivation: clarify when ifs are testing for special
cases, versus situation where then/else are both
normal
Mechanics:
For each check, put in a guard clause
Should either return or throw an exception
Compile and test after each replacement
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 27
Replace Nested Conditional with
Guard Clauses: Example 1 - Start
double getPayAmount () { double result; if (_isDead) result = deadAmount(); else { if (_isSeparated) result = separatedAmount(); else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); }; } return result; }
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Replace Nested Conditional with Guard
Clauses: Example 1 – First guard
double getPayAmount () { double result; if (_isDead) return deadAmount(); else { if (_isSeparated) result = separatedAmount(); else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); }; } return result; }
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Replace Nested Conditional with Guard
Clauses: Example 1 – Second guard
double getPayAmount () { double result; if (_isDead) return deadAmount(); if (_isSeparated) return separatedAmount(); else { if (_isRetired) result = retiredAmount(); else result = normalPayAmount(); }; return result; }
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Replace Nested Conditional with Guard
Clauses: Example 1 – Third guard
double getPayAmount () { double result; if (_isDead) return deadAmount(); if (_isSeparated) return separatedAmount(); if (_isRetired) return retiredAmount(); else result = normalPayAmount(); return result; }
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Replace Nested Conditional with Guard
Clauses: Example 2 – Explicit results
public double getAdjustedCapital () { // example requires reversing sense of conditionals double result = 0.0; if (_capital <= 0.0) return 0.0; if (intRate <= 0.0 || _duration <= 0)) return 0.0; result = (_income / _duration) * ADJ_FACTOR; return result; }
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Replace Nested Conditional with Guard
Clauses: Example 2 – Remove temp
public double getAdjustedCapital () { // example requires reversing sense of conditionals double result = 0.0; if (_capital <= 0.0) return 0.0; if (intRate <= 0.0 || _duration <= 0)) return 0.0; return (_income / _duration) * ADJ_FACTOR; return result; }
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Replace Nested Conditional with Guard
Clauses: Example 2 – End
public double getAdjustedCapital () { // example requires reversing sense of conditionals // Note: May want to combine tests into one conditional, // or separate into three conditionals if (_capital <= 0.0) return 0.0; if (intRate <= 0.0 || _duration <= 0)) return 0.0; return (_income / _duration) * ADJ_FACTOR; }
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Replace Conditional with Polymorphism
Skipping, since it’s already familiar …
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Introduce Null Object
You have repeated checks for a null value. Replace the
null value with a null object (and use polymorphism)
if (customer == null) plan = BillingPlan.basic(); else plan = customer.getPlan();
Add a new NullCustomer class and use it:
Plan = customer.getPlan();
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Introduce Null Object
Motivation: Eliminate many tests and conditional
code
Also makes NullPointerException less likely
But can hide the same logical condition
This sometimes making debugging slightly harder
Benefits outweight disadvantages
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Introduce Null Object: Mechanics
Create a Null version subclass. Implement an isNull() method for the original and new class. The null object should be a singleton of the null class
Compile
Replace setting to Java null with the null object
Replace comparisons with null by isNull() calls
Compile and test
For each conditional, add a method to the Null class that overrides; compile
Remove the condition check; compile and test
Repeat until all conditionals removed
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Introduce Null Object:
Example - Classes
class Site …
Customer _customer;
Customer getCustomer () { return _customer; }
class Customer …
public String getName () { … }
public BillingPlan getPlan () { … }
public PaymentHistory getHistory () { … }
class PaymentHistory …
int getWeeksDelinquentInLastYear () { … }
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Introduce Null Object:
Example – Code fragment - Start
Customer customer = site.getCustomer();
BillingPlan plan;
if (customer == null) plan = BillingPlan.basic();
else plan = customer.getPlan();
String customerName;
if (customer == null) customerName = “occupant”;
else customerName = customer.getName();
int weeksDelinquent;
if (customer == null) weeksDelinquent = 0;
else weeksDelinquent =
customer.getHistory().getWeeksDelinquentInLastYear();
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Introduce Null Object:
Example – Adding NullCustomer
class Customer … public boolean isNull () { return false; } protected Customer () {} // needed by subclass … class NullCustomer extends Customer { @Override public boolean isNull () { return true; } } // optional: use Nullable interface interface Nullable { boolean isNull(); } // have Customer implement this interface
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Introduce Null Object:
Example – A factory method is good …
class Customer …
static Customer newNull() { return new NullCustomer(); // or … set up to use singleton pattern } …
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Introduce Null Object:
Example – Update a source of nulls …
class Site … Customer getCustomer () { return (customer == null)? Customer.newNull() : _customer; }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 55
Introduce Null Object:
Example – … and simplify once more
Customer customer = site.getCustomer();
BillingPlan plan = customer.getPlan();
String customerName = customer.getName();
int weeksDelinquent =
customer.getHistory().getWeeksDelinquentInLastYear();
if (customer.isNull()) weeksDelinquent = 0;
else weeksDelinquent =
customer.getHistory().getWeeksDelinquentInLastYear();
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 56
Introduce Null Object:
Example – End
Customer customer = site.getCustomer();
BillingPlan plan = customer.getPlan();
String customerName = customer.getName();
int weeksDelinquent =
customer.getHistory().getWeeksDelinquentInLastYear();
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 57
Introduce Null Object: Comments
You may have several kinds of null object
NoCustomer vs. UnknownCustomer, for example
The null class gets the most common behavior
But specific cases can still use isNull if they are
different
More generally, may have several special case objects
This pattern tends to remove exceptions, replacing
with special case behavior in the null objects
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 58
Introduce Assertion
Code assumes something implicitly: make it explicit!
double getExpenseLimit () { // should have a limit or a primary project return (_expenseLimit != NULL_EXPENSE)? _expenseLimit : _primaryProject.getMemeberExpenseLimit(); }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 59
Introduce Assertion
With the explicit assertion:
double getExpenseLimit () { assert (_expenseLimit != NULL_EXPENSE) || (_primaryProject != null); return (_expenseLimit != NULL_EXPENSE)? _expenseLimit : _primaryProject.getMemeberExpenseLimit(); }
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 60
Introduce Assertion
Use only for conditions that should never be false
according to the logic of the program
Use other exceptions for bad input, I/O failure, etc.
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 61
Recap of Simplifying Conditionals
Decompose Conditional
Consolidate Conditional Expressions
Consolidate Duplicate Conditional Fragments
Remove Control Flag
Replace Nested Conditional with Guard Clauses
Replace Conditional with Polymorphism
Introduce Null Object
Introduce Assertion
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Making Method Calls Simpler
Many, perhaps most, of these are simple to apply
So we won’t belabor the easy ones
May seem trivial, so why are they important?
“Naming is a key tool in communication.”
Interfaces are key to good [OO] software
Being clear about mutating vs. non-mutating methods
Show what you have to, hide the rest
Use exceptions appropriately
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Rename Method
Simple to do
Humans need good names
Compare getinvcdtlmt vs. getInvoiceableCreditLimit Choosing good names is a skill that you develop
The sort of thing that Eclipse automates
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Add Parameter
Simple to do
The sort of thing that Eclipse automates
But beware; ask these questions:
Can an existing parameter get the info you need?
How about adding a method to an existing parameter?
Should the behavior be on a different object?
Perhaps you are creating a data clump, or should
consider Introduce Parameter Object?
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Remove Parameter
Simple to do
The sort of thing that Eclipse automates
Generally good to do if parameter no longer used in
the method
But may be necessary in the case of an inherited
method
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Separate Query from Modifier
Query = a method that reads but does not modify
Modifier = a method that modifies object state
Generally: if a method returns a value, it should not
modify state
If it does, try to separate the query from the modifier
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Separate Query from Modifier
Example – … and change its name
void sendAlert (String[] people) {
for (String person : people) {
if (person.equals(“Don”)) {
sendAlert();
return;
if (person.equals(“John”)) {
sendAlert();
return;
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 74
Separate Query from Modifier
Example – … and apply Substitute Algorithm
void sendAlert (String[] people) { if (! foundPerson(people).equals(“”)) sendAlert(); }
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Parameterize Method
Before:
fivePercentRaise() {…}
tenPercentRaise() {…}
After:
raise(float percentage) {…}
This case is pretty obvious. Here is one that is more
subtle …
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Parameterize Method – Subtle Example
Dollars baseCharge () { double result = Math.min(lastUsage(),100) * 0.03; if (lastUsage() > 100) { result += (Math.min(lastUsage(),200)-100)0.05; }; if (lastUsage() > 200) { result += (lastUsage() – 200) * 0.07; }; return new Dollars(result); }*
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Parameterize Method – Subtle Example;
combines with Extract Method (implicitly)
Dollars baseCharge () {
double result = usageInRange(0, 100) * 0.03;
result += usageInRange(100, 200) * 0.05;
result += usageInRange(200, Integer.MAX_VALUE) * 0.07;
return new Dollars(result);
protected int usageInRange (int start, int end) {
if (lastUsage() > start)
return Math.Min(lastUsage(), end) – start;
else
return 0;
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 78
Replace Parameter with
Explicit Methods
void setValue (String name, int value) { if (name.equals(“height”)) { _height = value; return; } if (name.equals(“width”)) { _width = value; return; } assert false; } // change to new methods: void setHeight (int arg) { _height = arg; } void setWidth (int arg) { _width = arg; }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 79
Replace Parameter with
Explicit Methods - Motivation
Use when you do largely different things depending on the
parameter value, and the parameter value is usually a
constant
Caller has to supply the parameter anyway, so could just as
well call an explicit method (in most cases)
Clearer
Better compile-time checking
Possible parameter values explicit
Might consider using a setter, or Replace Conditional with
Polymorphism, in some cases
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 80
Replace Parameter with
Explicit Methods - Mechanics
Create a method for a leg; compile
Change callers for that case; compile; test
Repeat until all cases handled
Delete original method; compile; test
We skip showing it in detail since it is straightfoward
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Preserve Whole Object
Before:
int low = daysTempRange().getLow ();
int high = daysTempRange().getHigh();
withinPlan = plan.withinRange(low, high);
After:
withinPlan = plan.withinRange(daysTempRange());
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 82
Preserve Whole Object - Motivation
Reduces changes to callers if called routine’s needs
change in the future
Downside: creates dependency on an object as
opposed to separate values
Consider also Move Method
May need to do Introduce Parameter Object first
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Preserve Whole Object - Mechanics
Add a parameter for the whole object, to the parameter list
and to each call; compile; test
Replace uses of one parameter in called method to get value
from whole object; compile; test
Delete that parameter (from list and each call); compile; test
Repeat until no more parameters can be deleted
Delete code in called that obtains parameters (if possible);
compile; test
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Preserve Whole Object
Example - Start
class Room … boolean withinPlan (HeatingPlan plan) { int low = daysTempRange().getLow(); int high = daysTempRange().getHigh(); return plan.withinRange(low, high); }
class HeatingPlan … private TempRange _range; boolean withinRange (int low, int high) { return (low >= _range.getLow() && high <= _range.getHigh()); }
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Preserve Whole Object
Example – End
class Room …
boolean withinPlan (HeatingPlan plan) {
return plan.withinRange(daysTempRange());
class HeatingPlan …
private TempRange _range;
boolean withinRange (TempRange roomRange) {
return (roomRange.getLow () >= _range.getLow () &&
roomRange.getHigh() <= _range.getHigh());
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Preserve Whole Object
Example – Can apply Move Method
class HeatingPlan …
private TempRange _range;
boolean withinRange (TempRange roomRange) {
return (_range.includes(roomRange));
class TempRange …
boolean includes (TempRange arg) {
return (arg.getLow () >= this.getLow () &&
arg.getHigh() <= this.getHigh());
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Replace Parameter with Method
Before:
int basePrice = _quantity * _itemPrice;
discountLevel = getDiscountLevel();
double finalPrice =
discountedPrice(basePrice, discountLevel);
After:
int basePrice = _quantity * _itemPrice;
double finalPrice = discountedPrice (basePrice);
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Replace Parameter with Method –
When?
Invoke a method
And: pass its result back to another method
And: target of second call can call the first method
for itself
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 95
Replace Parameter with Method –
Motivation
If a method can get a value some other way, avoid
passing it as a parameter
If parameter is there for future flexibility, still
consider removing it and dealing with that later
Skip mechanics since it’s straightforward
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 96
Introduce Parameter Object
A group of parameters that naturally go together:
replace with an object.
Before:
amountInvoicedIn(Date start, Date end) { … }
amountReceivedIn(Date start, Date end) { … }
amountOverdueIn(Date start, Date end) { … }
After:
amountInvoicedIn(DateRange range) { … }
amountReceivedIn(DateRange range) { … }
amountOverdueIn(DateRange range) { … }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 97
Introduce Parameter Object -
Motivation
A data clump should be an object for clarity
Reduces size of parameter lists: also clearer
The accessors make the code more consistent
Often a prelude to moving behavior to the clump
Thus, often reduces duplication
Details similar to things previously shown, so we
skip them
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 98
Remove Setting Method
You have a field that should be set at creation time and never
altered: Remove any setting method for it
Motivation:
Makes intent clear Removes possibility of change
Mechanics
Mostly obvious, so we skip In a subclass, may need a setter in superclass for setting some private fields, but give it a name that makes clear it is not an ordinary setter
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 99
Hide Method
You have a method that is not used by any other class:
make it private
Motivation:
Keeps interface as narrow as possible
Tends to happen as classes evolve richer behavior
Mechanics
Obvious
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 100
Replace Constructor with Factory
Method
We studied factories early in the course.
The mechanics are straightforward.
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 101
Encapsulate Downcast
A method returns an object that callers will need to
downcast: Move the downcast inside the method
Before:
Object lastReading () {
return readings.lastElement();
After:
Reading lastReading () {
return (Reading)readings.lastElement();
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 102
Encapsulate Downcast
Motivation
Not as necessary when you use generics
But clone() is a good example that remains Recall that we put the downcast inside the memento() method
(We also put an exception handler there: same principle)
Make the object’s interface better typed
Mechanics: Straightforward
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 109
Replace Exception with Test
Example – Start
class ResourcePool …
private ArrayList _available = …;
Resource getResource () {
try {
return _available.remove(_available.size()-1);
} catch (IndexOutOfBoundsException e) {
return new Resource();
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 110
Replace Exception with Test
Example – With test for exceptional case
class ResourcePool …
private ArrayList _available = …;
Resource getResource () {
if (_available.isEmpty()) {
return new Resource();
try {
return _available.remove(_available.size()-1);
} catch (IndexOutOfBoundsException e) {
return new Resource();
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 111
Replace Exception with Test
Example – Add assertion (then test)
class ResourcePool … private ArrayList _available = …; Resource getResource () { if (_available.isEmpty()) { return new Resource(); }; try { return _available.remove(_available.size()-1); } catch (IndexOutOfBoundsException e) { assert false; return new Resource(); } }
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 112
Replace Exception with Test
Example – Remove try block
class ResourcePool … private ArrayList _available = …; Resource getResource () { if (_available.isEmpty()) { return new Resource(); }; try { return _available.remove(_available.size()-1); } catch (IndexOutOfBoundsException e) { assert false; return new Resource(); } }
UU NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERSTMHERST •• Department of Computer ScienceDepartment of Computer Science 113
Replace Exception with Test
Example – End
class ResourcePool …
private ArrayList _available = …;
Resource getResource () {
if (_available.isEmpty()) {
return new Resource();
return _available.remove(_available.size()-1);
U U NIVERSITY OFNIVERSITY OF MM ASSACHUSETTSASSACHUSETTS , A, A MHERST •MHERST• Department of Computer ScienceDepartment of Computer Science 114
Next time …
Continue expanding your catalog of refactorings …
