Tutorial 3. Strings

Contents

Instructions

Classes

• As with real-world classification, a class groups together objects that the programmer considers to be similar.
• Classes are arranged in a hierarchy that describes the behaviour of a set of objects of the same kind.
• The class hierarchy gives the description of a set of objects with similar characteristics and attributes (instance variables). See the comment for the class in the class browser.
• For example the Open University has created class Person, a subclass of Object.
  • The attributes (instance variables) of the class are Person name and address.
  • If a programmer required additional instance variables, such as telephoneNumber and/or dateOfBirth, the programmer could add these.
    • Note how spaces are not allowed in instance variables names. The capitalisation of the first letter, of what would be a separate word etc. is the norm. Examples noOfTickets, creditCardNo, postCode
• During the course you will create new classes.
• The M206 LearningWorks, and other programs have been set up with an existing "Library" of Classes installed.
• Classes usually have ---
  • Comments
  • Names, such as Object, Collection, Person, Date, Number, etc. Class names start with a capital letter.
  • Instances. Note that Abstract Classes do not have instances. See Abstract classes below.
    • An object that belongs to a given class is described as an instance of that class.
    • Instances of a class are created myFrog :=Frog new Would create an instance of the class Frog, the name of the instance myFrog is a temporary variable.
    • Instance names start with a lower case letter.
  • Instance variables, these are initially called attributes by the OU Course Team.
    • Do not confuse Instances with instance variables.
    • Instance variables refer to specific information about an instance.
      • Example The instance of the class Frog, myFrog has instance variables position and colour. The value of these instance variables may be 1 and Green respectably. These values can be changed using the 2 setter methods position: and colour: .
  • Class variables.
  • Methods
• Some classes are Abstract classes.
  • These are classes that that
    • Do not have instances.
    • Specify common behaviour for there subclasses.
    • Abstract classes have special methods in them.
      1. Base methods.
      2. Abstract methods.
      3. Template methods.
      Which are different ways of how messages are set up for subclasses.
  • They are discovered at the design stage, when it can be seen that several classes share common characteristics.
    • The OU course introduces the abstract class Amphibian, for the classes Frog, Toad and HoverFrog, in Chapter 21 Page 8.
    • This abstract class appears in LB 22 for the first time.

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Class Inheritance Hierarchy

Section of

The colours are used to explain the layout. Red Is the Object class. There is only 1 class at this level, the top of the root system, i.e. the base of the tree trunk. Green These are the subclasses of Object. Object is their superclass. There are approx. 125 classes at this level. In turn each one may have there own subclasses, but not all do, see the centre one. Yellow These classes each have a superclass, and also have one or more subclasses. Blue The diagrammatic root tip classes each have a superclass, but don't have any subclasses. Note that the green, central box could be coloured blue.

• The diagram only shows 4 levels.
• You will discover in the exercises in this tutorial, there are more levels than this.
• What is important for the classes lower down in this hierarchy is the direct route back to object.
  • This route determines theI nheritance relationship between classes and what messages that an instance of a class will understand, because the messages are inherited from classes higher in the hierarchy
  • Classes higher in the hierarchy have more general (common) characteristics.
  • Classes lower in the hierarchy have more specific characteristics and inherit the general characteristics from the classes higher in the hierarchy .
  • When a class inherits from another class it receives all that class's methods and variables, and is then able to define incrementally its own additional methods and variables.
  • Classes higher in the hierarchy can NOT use the more specific instance variables and methods that are in classes lower down the hierarchy.
  • They must be in the classes along this route. This is demonstrated in this tutorial.
  • Only 1 class at each level is on any specific route

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Subclass

• A new class may be defined in terms of an existing class.
• The new class, the subclass, requires the programmer only to specify what makes it different from the existing class, its superclass.
• Instances of a subclass can respond to the same messages (although not necessarily with the same response) and have the same attributes as the instances of the superclass.
• The subclass instances may have additional attributes (instance variables).
• The subclass may define additional messages. Hence the protocol of the subclass contains (has at least all the messages of) the protocol of the superclass. For example, in the OU ( Open University ) course " Computing an Object-oriented approach, HoverFrog is a subclass of Frog . The protocol of HoverFrog objects includes that of Frog objects (left , right , home , brown , green , jump , colour: , colour and sameColourAs: ) and also has a message to which Frog objects cannot respond (message selector hover:by: ). The HoverFrog and Frog objects respond identically to the messages they have in common that are listed above. HoverFrog objects have the attributes (instance variables) of Frog objects (colour and position ) and an additional attribute (instance variables) height.
• A subclass is any class which, when taking part in an inheritance hierarchy with another class, is the class to inherit functionality. All classes except Object are subclasses of some other class. A class can be both a subclass and a superclass if it is in the middle of an inheritance hierarchy, e.g. yellow boxes in the diagram.

Superclass

• In the Smalltalk programming language a subclass has only one superclass.
• Any class which, when taking part in an inheritance relationship with another class, is the class from which functionality is inherited.
• All classes except those at the 'root tips', blue boxes in the diagram, of the inheritance 'tree' are superclasses, even though they are also subclasses (except for Object).
• A superclasses can have 1 or many subclasses.

Strings

• Strings are enclosed in single quotes. E.g. 'Hello World '
• I am using the name strings as a general term. The class String is an Abstract class, see Classes above.
• You will learn the correct term during exercise 1a
• Think of a string as a "string" of individual characters.
• The string is actually an array of individual characters.
• Each individual character being an element of the array.
• They are very much like a string in any other programming language. But remember in Smalltalk the string is enclosed in single quotes.

1. Start up M206 LearningWorks program and select LB-12.
   • LB-12 is a download from the OU web page. If not available use either LB-13, or LB-14.
2. Type into the expression pane, a suitable comment, e.g. "Tutorial 3. Exercise 1a. String example."
3. Press the Enter key, to go to the next line.
4. Type in 'Hello World'
5. Evaluate. You can evaluate just the 'Hello World' or you can evaluate both lines. Because the "comment" is ignored in the evaluation.
6. 'Hello World' appears in the answer pane

This exercise shows that evaluating an object name displays a textual representation of itself in the answer pane. This may appear to break the rules because it does not appear that we have sent a message to an object. What we have done is send a message to Smalltalk itself, and the object name is the argument.
Later you will discover that the textual representation of the object can be programmed by code.
This is noticeable in instances of Frog, HoverFrog, and Toad, that the OU ( Open University ) use in their Computing an Object-oriented approach course.

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String class

• String is an abstract class that is the superclass for all string implementations such as ByteString and TwoByteString and also for Symbol class.
• Abstract classes can not have instances created .
• ByteString is an indirect subclass of String .
  • ByteString is known as a concrete class because it can have instances.
  • An example of a ByteString is 'Hello World'.
    • A sequence of characters between single quotes.
  • TwoByteString another concrete class, very similar to ByteString. The difference is that TwoByteString can hold more "exotic" characters that require more memory. These could be the Greek characters used in mathematics, Pi Omega etc.

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Workspace Comments

• The exercise being undertaken.
  • "Tutorial 3. Exercise 1a. String example."
  • "LB-12 Practical 1."
• The TMA being undertaken
  • "TMA 3. Question 2 a."
• Information
  • "The following code- Not working as expected - produces the wrong answer of ..... Correct answer is ......."
  • "The following code produces error message ............"

Inspecting

1. Using the "Inspect last message answer" button
2. Using the inspect message in your expression

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Inspector window

• An inspector is a tool used to look at the objects in the system.
• It displays an object and its attributes (instance variables).
• The name of the class of which the object is a member is displayed in the title bar and in a pane in the window.
• An inspector is created by pressing the Inspect button after selecting the name of the object whose attributes (instance variables)are to be viewed.
• An inspector opened on frog1 shows in the title bar that the object being inspected belongs to the Frog class.
• All instances of Frog , such as frog1 , have attributes (instance variables) for position and colour .
• These instance variable names are displayed alongside their current values.

• The title in the title bar is Inspector on either ByteString or Frog, the name of the class.
• The windows have varying number of information panes.
• Class pane.
  • Displays the class to which the object belongs.
  • Note: The class for 'Hello World' is ByteString. The term string is a more general term. When used in Smalltalk the user is normally referring to ByteString class.
  • The class pane is usually labelled, other panes are often not labelled.
• Unlabelled pane containing containing 'Hello World'
  • Displays the evaluation answer as a textual representation of it.
  • The textual representation for the Frog class has been programmed into a method in the class, if this had not been done the less informative default textual representation a Frog would have resulted.
• The bottom pane, labelled Elements.
  • Use the scroll bar, or enlarge the window to see all 11 elements.
    • The elements show the position of each character. in an array of characters. Arrays are later in the course.
  • If your count of the characters in 'Hello World' came to 10, then you probably did not count the space between Hello and World. The sixth element.
  • Look at the second column where you see for Element 1$H "16r0048"
    • $ Denotes a Character object follows.
    • 16r Stands for radix 16 i.e. a hexadecimal number follows.
    • 0048 48 is the hexadecimal value for H in the ASCII codes
• The bottom pane, for the Frog class displays a list of all the attributes (instance variables) and their current value.

• The title of the window is Inspector on ByteString class.
• That the information in this window is different from the previous one.
• Find in the bottom panes left column: superclass, subclass, organisation.
• Superclass.
  • The superclass is ByteEncodedString
  • A class can only have 1 superclass
  • This is the class that is higher in the class hierarchy.
  • You will inspect this soon.
• Subclasses.
  • The nil entry indicates that the ByteString class has no subclasses.
  • Subclasses is in the plural, so a class may have nil, one or many subclasses.
• Organisation
  • Within this line pick out at: put: size
    • These are messages that you will use later in this tutorial.

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Hierarchy - Exercises.

1. ByteString has no subclasses, therefore it must be at the very tip of a root in our hierarchy root system
2. ByteEncodedString is the superclass of ByteString. This class is hidden in the Class Browser
3. Object class is at the top of the hierarchy and I have likened this to the tree trunk.

Object 
        X
                X
                        X
                                X
                                        X
                                                X
                                                       ByteEncodedString
                                                               ByteString

1. Printout this page or write down the hierarchy list above.
2. In the Inspector on ByteString class that you still should have open from the previous exercise. (If required repeat the last 2 exercises.)
3. Double click on the line superclass, to bring up the Inspector on ByteString class
4. Read the class in the superclass line and enter it in the list replacing one of the Xs. Also check your progress on the Collection Class diagram, that you have printed out.
5. Look at the subclass line and note that this class has 4 subclasses. 3 off these subclasses are of no interest to you in this course, and are hidden from view in the class browser. But the 4^th one is ByteString.
6. Repeat steps 2 to 5, making sure that step 2 is carried out in the latest window that pops up, carry on until you reach the top of the root system i.e. Object..
   1. During these repeats, keep looking at the subclass line. You may have to enlarge the window to see all of the entries.
   2. Look for these classes:- List, Array, CharacterArray, OrderedCollection, Bag, and Set. You will be using all these classes, and some of their subclasses, in the M206 Open University (OU) course.
7. When you reach the Object class
   1. Maximise the window.
   2. Look at the subclass line, and note that you can't read all the entries.
   3. Double click this line.
   4. Maximise the new window, in this window note the Elements pane. Scroll down this pane and note how many subclasses the Object class has. In mine there are approx. 125. Remember most of these subclasses have many subclasses of there own, which in turn have further subclasses.
   5. You may now realise why a map of the entire system is not provided. From this list of 125 subclasses the OU M206 course concentrate on only a few, such as
   6. Element Number, Name
      1. 20 BlockClosure
      2. 23 Magnitude.
      3. 49 Boolean.
      4. 62 Dialog.
      5. 70 Collection. (See your printout).

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Sending messages to ByteString

• The names of messages do tend to inform you, to some degree, of what they will do.
• The comment in the method will tell you, but not in easily understood plain English.
• Note how all the classes , except one, in the Table Class column are in the hierarchy we have examined. You will find out why this one answers with an exception notifier.

  Object 
          Collection
                  SequenceableCollection
                          ArrayedCollection
                                  CharacterArray
                                           String
                                                  ByteEncodedString
                                                         ByteString
  

• Read the information in the table below.
  • In the Class column, Total class, the number after the class displayed is the total number of different classes that the method, that is referenced by the message selector is in.
    • E.g. The method asLowercase is in 3 classes. CharacterArray, Character and Text .
    • This information can be obtained from the Class Browser.
    • Looking at the Collection Class Hierarchy Diagram printout. Starting at ByteString, CharacterArray can be found on the hierarchy route, as you travel towards Object class, Therefore the message, asLowercase is understood by a ByteString object.
  • I have therefore selected the most suitable class for you, but as already stated, one line in the table should not be there, keep an eye out for it.

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Answers

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Solutions to Exercises

Object 
        Collection
                SequenceableCollection
                        ArrayedCollection
                                CharacterArray
                                         String
                                                ByteEncodedString
                                                       ByteString

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