Clean code and Code Smells

Goals

Readable, maintainable and
extendable code

A simple example
public List<int[]> getThem() {
List<int[]> list1 = new ArrayList<int[]>();
for (int[] x : theList)
if (x[0] == 4)
list1.add(x);
return list1;
}

A simple example
if (x[0] == 4)
list1.add(x);
return list1;
}

This code is quite simple but
what does it do?

A simple example
if (x[0] == 4)
list1.add(x);
return list1;
}

This code is quite simple but
what does it do?
Looking at it we can’t tell
what it is actually doing!

A simple example
public List<int[]> getFlaggedCells() {
List<int[]> flaggedCells = new ArrayList<int[]>();
for (int[] cell : gameBoard)
if (cell[STATUS_VALUE] == FLAGGED)
flaggedCells.add(x);
return flaggedCells;
}

Is this code any better?

A simple example
public List<Cell> getFlaggedCells() {
List<Cell> flaggedCells = new ArrayList<Cell>();
for (Cell cell : gameBoard)
if (cell.isFlagged())
flaggedCells.add(x);
return flaggedCells;
}

What about this?

A simple example
What we have done:

A simple example
What we have done:
used intention flaggedCells
revealing names rather than list1

A simple example
What we have done:
replaced magic numbers cell[STATUS_VALUE]
with constants rather than x[0]

A simple example
What we have done:
replaced magic numbers cell[STATUS_VALUE]
with constants rather than x[0]
created an appropriate Cell cell rather
abstract data type than int[] cell

Another example
int d;
What does it mean?
Days? Diameter? ...

Another example
int d;
What does it mean?
Days? Diameter? ...

int d; //elapsed time in days
Is this any better?

Another example
int d;
What does it mean?
Days? Diameter? ...

int d; //elapsed time in days
Is this any better?

int elapsedTimeInDays;
What about this?

Do one thing
public bool isEdible() {
  if (this.ExpirationDate > Date.Now &&
       this.ApprovedForConsumption == true &&
       this.InspectorId != null) {
    return true;
  } else {
    return false;
  }
}

How many things is the function doing?

Do one thing
  if (this.ExpirationDate > Date.Now && 1.check expiration
       this.ApprovedForConsumption == true &&
       this.InspectorId != null) {
2.check approval
    return true; 3.check inspection
  } else {
    return false; 4.answer the request
  }
}

How many things is the function doing?

Do one thing
    return isFresh()    &&
  isApproved() &&
Now the function is doing one thing!
  isInspected();
}

Do one thing
    return isFresh()    &&
  isApproved() &&
Now the function is doing one thing!
  isInspected();
}

A change in the speciﬁcations turns
into a single change in the code!

Don’t mix levels of abstraction

public void doTheDomesticThings() {
    takeOutTheTrash(); public void doTheDomesticThings() {
    walkTheDog();     takeOutTheTrash();
    for (Dish dish : dirtyDishStack) {     walkTheDog();
       sink.washDish(dish);     doTheDishes();
       teaTowel.dryDish(dish); }
    }
}

Which one is easier to read and understand?

Separate commands and queries

Commands should Queries should
only do something only answer
(One thing) something
public class Car{
    private boolean isOn

    public void turnOn(){
        isOn = true;
    } AVOID SIDE EFFECTS!
    public boolean isOn(){
        return isOn;
    }
}

Use exceptions
public int foo(){
   ...
}

public void bar(){
   if(foo() == OK)
      ...
   else
      // error handling
}

Use exceptions
Errors have to be encoded
   ...
}

   if(foo() == OK)
      ...
   else
}

Use exceptions
   ...
}

Checks (when performed)
   if(foo() == OK) require a lot of code
      ...
   else
}

Use exceptions
   ...
}

Checks (when performed)
   if(foo() == OK) require a lot of code
      ...
   else
} It’s harder to extend such
programs

Use exceptions
public void foo() 
      throws FooException{
   ...
}

   try{
      foo();
      ...
   } catch(FooException){
   }
}

Use exceptions
      throws FooException{ No need to mix return
   ...
}
values and control values
   try{
      foo();
      ...
   }
}

Use exceptions
   ...
}
   try{ Cleaner syntax
      foo();
      ...
   }
}

Use exceptions
   ...
}
   try{ Cleaner syntax
      foo();
      ...
      // error handling Easier to extend
   }
}

Don’t Repeat Yourself
  String [] elements = {“A”, “B”, “C”};
  foo(“A”);
  for(String element : elements){
  foo(“B”);
    foo(element);
  foo(“C”);
  }
}
}

DO NOT EVER COPY AND PASTE CODE

Don’t Repeat Yourself
  String [] elements = {“A”, “B”, “C”};
  foo(“A”);
  for(String element : elements){
  foo(“B”);
    foo(element);
  foo(“C”);
  }
}
}

Logic to handle the elements
it’s written once for all
DO NOT EVER COPY AND PASTE CODE

Explain yourself in the code

Which one is clearer?

// Check to see if the employee is eligible for full benefits
if ((employee.flags & HOURLY_FLAG) && (employee.age > 65))

if (employee.isEligibleForFullBenefits())

Comments
GOOD BAD

API Documentation Redundant
Explanation of intent Obsolete
Clariﬁcation Code commented-out

Other code smells
What we don’t want to see in your code

The bloaters
Something in your code grow too large

Long methods Single responsibility
and large classes principle violated

Primitive obsession
It is a symptom of bad
and too much
design
parameters

Primitive obsession
public Class Car{
private int red, green, blue;

public void paint(int red, int green, int blue){
this.red = red;
this.green = green;
this.blue = blue;
}
}

public Class Car{
private Color color;

public void paint(Color color){
this.color = color;
}
}

The OO abusers
Object orientation is not fully exploited

Switch statements on It is better to use
objects polymorphism

Refused bequest
Poor class hierarchy
Alternative classes with design
different interfaces

Switch vs polymorphism
public Money calculatePay(Employee e)
throws InvalidEmployeeType{
switch(e.type){
case COMMISSIONED:
return calculateCommissionedPay(e);
case HOURLY:
return calculateHourlyPay(e);
case SALARIED:
return calculateSalariedPay(e);
default:
throw new
InvalidEmployeeType(e.type);
}
}

public abstract class Employee{
public abstract Money calculatePay();
}

Refused bequest
Subclass doesn’t use superclass methods and attributes
public abstract class Employee{
private int quota;
public int getQuota();
...
}

public class Salesman extends Employee{ ... }

public class Engineer extends Employee{
...
public int getQuota(){
throw new NotSupportedException();
}
}

Engineer does not use quota. It should be
pushed down to Salesman

The change preventers
Something is making hard to change the code

A class has to be changed
Divergent change
in several parts

A single change requires
Shotgun surgery
changes in several classes

The dispensables
The code contains something unnecessary

A class is not doing
Class not providing logic
enough

Unused or
It isn’t something useful
redundant code

The couplers
Some classes are too tightly coupled

Feature Envy Misplaced responsibility

Classes should know as little
Inappropriate Intimacy
as possible about each other

Message Chains Too complex data access

Feature Envy
public class Customer{
private PhoneNumber mobilePhone;

...

public String getMobilePhoneNumber(){
return “(” +
mobilePhone.getAreaCode() + “)” +
mobilePhone.getPrefix() + “-” +
mobilePhone.getNumber();
}
}

Feature Envy
public class Customer{
private PhoneNumber mobilePhone;

...

return “(” +
mobilePhone.getAreaCode() + “)” +
mobilePhone.getPrefix() + “-” +
mobilePhone.getNumber();
}
}

return mobilePhone.toFormattedString();
}

Message chains
a.getB().getC().getD().getTheNeededData()

a.getTheNeededData()

Law of Demeter: Each unit should
only talk with friends

Clean code and Code Smells

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Editor's Notes