Patterns

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Intention revealing selector

Naming based on side effects

Double-dispatching method

Accessing methods

Query methods

Boolean property setting

Converter methods

Constructors

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Composed Method

How big should a method be?

Write methods that perform one identifiable task.

Few lines per method.

Consistent level of abstraction.

Minimizes number of methods you have to change in subclass.

Minimizes code copying in subclass.

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Composed Method UsageTop down

- self input; process; output

Bottom up

- common expressions

- long loop bodies

- comments

From client - two or more messages to another object is suspicious

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Methods from Comments

Comments indicate "identifiable task"

If you need to comment a block of code, it probably should be a separate method.

Turn method comment into method name.

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Reversing MethodGood code has a rhythm that makes it easy to

understand. Code that breaks the rhythm is harder to read and understand.

Point

printOn: aStream

x printOn: aStream.

aStream nextPutAll: ‘ @’.

y printOn: aStream6

Reversing method

Point

printOn: aStream

aStream

print: x;

nextPutAll: ‘ @’;

print: y

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Reversing method

Make a method on the parameter.

Derive its name from the original message.

Take the original receiver as a parameter.

Implement the method by sending the original message to the original receiver.

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Method object

What do you do about a method that is too big but that can’t be decomposed because of all the temporary variables that are used?

Turn method into a class. Temporary variables and arguments become instance variables. Then you can decompose the method.

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Example of Method Object

In class Class

readNewSubclassOn: aCodeReader in: aNameSpace

| fullName numInstVars numMetaInstVars

metaclassFormat allMetaclassInstVars metaclassInstVars classFormat allClassInstVars classInstVars newMeta class bookkeepingInfoStart bookkeepingInfoSize |

fullName := aCodeReader readByteSymbol.10

aCodeReader fileFormat >= 10

ifTrue:

[metaclassFormat := aCodeReader readLong.

numMetaInstVars := aCodeReader readLong.

allMetaclassInstVars := aCodeReader readByteStringCollection.

metaclassInstVars := allMetaclassInstVars

copyFrom: allMetaclassInstVars size - numMetaInstVars + 1

to: allMetaclassInstVars size.

classFormat := aCodeReader readLong.

numInstVars := aCodeReader readLong.

allClassInstVars := aCodeReader readByteStringCollection.

classInstVars := allClassInstVars copyFrom: allClassInstVars size - numInstVars + 1 to: allClassInstVars size]

ifFalse:

[metaclassFormat := aCodeReader readLong.

metaclassInstVars := aCodeReader readByteStringCollection.

numMetaInstVars := metaclassInstVars size.

classFormat := aCodeReader readLong.

classInstVars := aCodeReader readByteStringCollection.

numInstVars := classInstVars size].

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Make class NewSubclassReader, a subclass of Object.

Make instance variables for arguments, receiver, and temporaries: aCodeReader aNameSpace superclass fullName numInstVars numMetaInstVars metaclassFormat allMetaclassInstVars metaclassInstVars classFormat allClassInstVars classInstVars newMeta class

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readNewSubclassOn: aCodeReader in: aNameSpace

(NewSubclassReader on: aCodeReader in: aNameSpace) readNewSubclass

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readNewSubclass

self initializeClassInformation.

self makeNewMetaclass.

self makeNewClass.

self isNewClassIncompatibleWithSuperclass ifTrue:

[CodeReader invalidClassFormatSignal

raiseWith: fullName

errorString: ' : ', fullName].

self checkShapeChange.

self readSubclassInfoFromCodeReader.

^class

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Extract method

Turn an expression into a method of its own References to arguments or temporaries

turn into arguments of new method. References to instance variables don’t

change ^ in the expression can be a problem

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Execute Around Method

Pairs of actions that must be taken together Open file / close file Start tag / end tag

aCursor showWhile: [ … ]

aFile openDuring: [ aThing printOn: aFile]

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Simple Superclass NameWhat should we call the root of a hierarchy?

Complex name conveys full meaning.

Simple name is easy to say, type, extend.

But need to show that subclasses are related.

Simple Superclass Name

Give superclasses simple names: two or (preferably) one word

- Number

- Collection

- VisualComponent

Qualified Subclass Name

What should you call a subclass that plays a role similar to its superclass?

Unique name conveys most information

Derived name communicates relationship to superclass

Qualified Subclass NameUse names with obvious meaning. Otherwise,

prepend an adjective to most important superclass.

- OrderedCollection

- UndefinedObject

- CloneFigureCommand, CompositeCommand, ConnectionCommand

Variables: Roles vs. TypesTypes are specified by classes

aRectangle

aCollection

aView

Roles - how an object is used

location

employees

topView

Role Suggesting Instance Variable

What should you name an instance variable?

Type is important for understanding implementation. But class comment can describe type.

Role communicates intent, and this is harder to understand than type.

Role Suggesting Instance Variable

Name instance variables for the role they play. Make the name plural if the variable is a collection.

Point: x, y

Interval: start, stop, step

Polyline: vertices

Type Suggesting Parameter Name

Name of variable can either communicate type or role.

Keywords communicate their parameter's role, so name of variable should give new information.

Type Suggesting Parameter Name

Name parameters according to their most general expected class, preceded by "a" or "an". If there is more than one parameter with the same expected class, precede the class with a descriptive word.

Temporaries

Name temporaries after role they play.

Use temporaries to:

collect intermediate results

reuse result of an expression

name result of an expression

Methods are simpler when they don't use temporaries!

Class Comments

Every class should have a comment that describes the purpose of the class and the types of its instance variables

Method comments

Name of method should describe what it does.

Name of arguments should describe what they are.

Name of method should describe return type.

Method comment should describe anything else that isn’t obvious.

Common State

Different instances of a class have different values for some state. How do you represent it?

Declare an instance variable in the class.

Name the variable using Role Suggesting Instance Variable Name. Initialize it with either Lazy Initialization, Explicit Initialization, or a Constructor Parameter Method.

Explicit Initialization

How do you initialize instance variables to their default value?

Make the code readable instead of changeable.

Implement a method “initialize” that sets all the values. Override the class method “new” to invoke it on new instances.

Lazy Initialization

How do you initialize instance variables to their default value?

Emphasize ease of change.

Force all accesses to variable to go through accessing method. Make Getting method check whether the variable has been initialized and initialize it if necessary.

Lazy Initialization

Timer>>count

count isNil

ifTrue: [count := self defaultCount]

^count

Default Value Method

How do you represent the default value of a variable?

Create a method that returns the value. Prepend “default” to the name of the variable as the name of the method.

Constant Method

How do you code a constant?

Create a method that returns the constant.

How do you get and set an instance variable’s value?

Direct Variable Access: Access and set the variable directly.

Indirect Variable Access: Use a Getting Method and a Setting Method

Collection Accessor Method

How do you provide access to an instance variable that holds a collection?

Provide methods that delegate to the collection. To name the methods, add the name of the collection to the collection name.

addEmployee:

includesEmployee:

Enumeration Method

How do you provide access to collection elements?

Implement a method that executes a Block for each element of the collection. Name the method by concatenating the name of the collection and “Do:”

employeesDo:

How to make a good program

Make it run the test casesMake it easy to understand, simple, follow

coding standards, and eliminate duplication.

It is OK to get the tests to run first, and then worry about making your program readable.

It is OK to make sure your program is always readable.

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How to make a good program

It is not OK to hand in programs that are hard to understand, that don’t follow standard patterns, and that have a lot of duplication.

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