11.6: Some Hints for Using Collections

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A common mistake with add: The following error is one of the most frequent Smalltalk mistakes.

| collection |
collection := OrderedCollection new add: 1; add: 2.
collection
>>> 2

Here the variable collection does not hold the newly created collection but rather the last number added. This is because the method add: returns the element added and not the receiver.

The following code yields the expected result:

| collection |
collection := OrderedCollection new.
collection add: 1; add: 2.
collection
>>> an OrderedCollection(1 2)

You can also use the message yourself to return the receiver of a cascade of messages:

| collection |
collection := OrderedCollection new add: 1; add: 2; yourself
>>> an OrderedCollection(1 2)

Removing an element of the collection you are iterating on Another mistake you may make is to remove an element from a collection you are currently iterating over.

| range |
range := (2 to: 20) asOrderedCollection.
range do: [ :aNumber | aNumber isPrime
                        ifFalse: [ range remove: aNumber ] ].
range
>>> an OrderedCollection(2 3 5 7 9 11 13 15 17 19)

This result is clearly incorrect since 9 and 15 should have been filtered out!

The solution is to copy the collection before going over it.

| range |
range := (2 to: 20) asOrderedCollection.
range copy do: [ :aNumber | aNumber isPrime
                                ifFalse: [ range remove: aNumber ] ].
range
>>> an OrderedCollection(2 3 5 7 11 13 17 19)

Redefining = but not hash A difficult error to spot is when you redefine = but not hash. The symptoms are that you will lose elements that you put in sets or other strange behaviour. One solution proposed by Kent Beck is to use bitXor: to redefine hash. Suppose that we want two books to be considered equal if their titles and authors are the same. Then we would redefine not only = but also hash as follows:

Book >> = aBook
    self class = aBook class ifFalse: [ ^ false ].
    ^ title = aBook title and: [ authors = aBook authors ]

Book >> hash
    ^ title hash bitXor: authors hash

Another nasty problem arises if you use a mutable object, i.e., an object that can change its hash value over time, as an element of a Set or as a key to a Dictionary. Don’t do this unless you love debugging!