← VCE Software Development U3O1

Object descriptions

A design table for a class that lists its name, its attributes with data types, and its methods with their parameters and purpose.

Part of: VCE Software Development - U3O1 - Programming — Module 13, VCE Software Development - U3O2 - Analysis and Design — Module 97

Understanding Object descriptions in VCE Software Development U3O1

What it does: An object description pairs every attribute name with a data type, just as the concept says. The object name is the name of the class, usually a noun such as Student. A name ending in () is a method. A year level is a whole number such as 11, so Integer is the right data type. Boolean only stores True or False. An object description is a design tool. Programmers use it to plan a class before they write the code.

Common mistakes: A page count is a whole number, so the attribute's data type should be Integer, not Boolean. getMark() returns the mark attribute, which is a Float, so the method must return Float, not Boolean.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

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This module's key terms are also in a printable crossword (Modules 10–47) and word search (Modules 10–71).

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Learning
Duo
Which item is listed in an object description?
The object name, its attributes with data types, and its methodsOnly the final output shown on screen
Matches
Object name: Student Attributes: studentName, yearLevel
↓
The attributes have no data types listed. Each attribute needs a type, such as studentName (String) and yearLevel (Integer).
Applying
Meanings
Term
object description
Meaning
A design table for a class that lists its name, its attributes with data types, and its methods with their parameters and purpose.
Practice
Object: Student
Testing
Gaps
: Student
Check Answer
Spots
2Attribute: title (String)
3Attribute: pages (Boolean)

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Quick Questions

1. Which item is listed in an object description?
A. The object name, its attributes with data types, and its methods ✓
B. Only the final output shown on screen
An object description is a design table for a class. It lists the name, the attributes (with data types) and the methods.
2. Which attribute entry is written correctly in an object description?
A. studentName - print()
B. studentName - String ✓
An attribute is a piece of data, so it is listed with its name and a data type such as String, not with a function call.
3. What does the methods section of an object description show?
A. The data type of every attribute
B. What each method does and the parameters it needs ✓
Methods describe behaviour. The description lists each method's name, its parameters and its purpose. Data types belong with the attributes.
4. Which is a sensible object name for an object description?
A. Student ✓
B. calculateTotalMark()
The object name is the name of the class, usually a noun such as Student. A name ending in () is a method.
5. Which method row is complete in an object description?
A. setMark(newMark) - changes the student's mark to a new value ✓
B. setMark - Integer
A method row gives the name, its parameters and its purpose. 'Integer' on its own is a data type and does not say what the method does.

Match the Pairs

Object name: Student → Names the class or object being described, such as Student, at the top of the object description.
Methods: calculateAverage(marks) - returns the mean mark → Lists a behaviour of the object with its parameters and a short statement of what it does.
Each attribute in an object description is listed with its name and its data type. → Attributes: studentName (String), yearLevel (Integer)
An object description differs from a data dictionary because it also lists the methods the object can perform. → Methods: setYearLevel(newLevel) - changes the year level
Object name: Student Attributes: studentName, yearLevel → The attributes have no data types listed. Each attribute needs a type, such as studentName (String) and yearLevel (Integer).
Object name: Book Attributes: title (String), pages (Integer) Methods: (none listed) → The methods section is missing. The description should list methods such as getTitle() with their parameters and purpose.

Key Term

object description
A design table for a class that lists its name, its attributes with data types, and its methods with their parameters and purpose.

Fill in the Blanks

1. Object name: Student Attribute: studentName (String)
Hint: Type the label for the row that gives the class a name
2. Object name: Product Attribute: price (Float)
Hint: Type the data type suited to a price with cents
3. Object name: Book Attribute: isBorrowed (Boolean)
Hint: Type the data type for a true or false attribute
4. Object name: Student Method: addPoints(points) Purpose: adds to the points attribute
Hint: Type the parameter name that supplies the value to add
5. Object name: Account Attribute: balance (Float) Method: deposit(amount)
Hint: Type the label for a row describing behaviour, which a data dictionary does not include

Spot the Error

Question 1
A Object: Book
B Attribute: title (String)
C Attribute: pages (Boolean)
D Method: getTitle() -> String
Line C — A page count is a whole number, so the attribute's data type should be Integer, not Boolean.
Question 2
A Object: Student
B Attribute: name (String)
C Attribute: mark (Float)
D Method: getMark() -> Boolean
Line D — getMark() returns the mark attribute, which is a Float, so the method must return Float, not Boolean.
Question 3
A Object: speed (Integer)
B Attribute: speed (Integer)
C Method: accelerate(amount)
Line A — The first line of an object description gives the object's name (Car). Attributes and their data types come on later lines.
Question 4
A Object: Pet
B Attribute: age
C Attribute: name (String)
D Method: feed() - adds food
Line B — Every attribute in an object description needs a data type. It should be Attribute: age (Integer).
Question 5
A Object: Account
B Attribute: deposit(amount)
C Method: withdraw(amount)
D Method: getBalance() -> Float
Line B — deposit(amount) is an action that takes a parameter, so it is a method. Attributes store data and have a data type, so this line should start with Method:.

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