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Week 4 [Fri, Jan 27th] - Topics

Detailed Table of Contents

Guidance for the item(s) below:

In the tP, you'll be thrown into a codebase of about 6K . It would be hard to understand the design simply by reading the code. That's why the code base comes with a Developer Guide containing some design models i.e., the diagrams. That means, you should be able to interpret those models by the time you start the tP in a few weeks. 😨

Let's start getting ready for that today. First, let's go through a high-level explanation of models.

[W4.1] Design: Models


Design → Modelling → Introduction → What

Can explain models

A model is a representation of something else.

A class diagram is a model that represents a software design.

A model provides a simpler view of a complex entity because a model captures only a selected aspect. This omission of some aspects implies models are abstractions.

A class diagram captures the structure of the software design but not the behavior.

Multiple models of the same entity may be needed to capture it fully.

In addition to a class diagram (or even multiple class diagrams), a number of other diagrams may be needed to capture various interesting aspects of the software.


Design → Modelling → Introduction → How

Can explain how models are used

In software development, models are useful in several ways:

a) To analyze a complex entity related to software development.

Some examples of using models for analysis:

  1. Models of the can be built to aid the understanding of the problem to be solved.
  2. When planning a software solution, models can be created to figure out how the solution is to be built. An architecture diagram is such a model.

b) To communicate information among stakeholders. Models can be used as a visual aid in discussions and documentation.

Some examples of using models to communicate:

  1. You can use an architecture diagram to explain the high-level design of the software to developers.
  2. A business analyst can use a use case diagram to explain to the customer the functionality of the system.
  3. A class diagram can be reverse-engineered from code so as to help explain the design of a component to a new developer.

c) As a blueprint for creating software. Models can be used as instructions for building software.

Some examples of using models as blueprints:

  1. A senior developer draws a class diagram to propose a design for an OOP software and passes it to a junior programmer to implement.
  2. A software tool allows users to draw UML models using its interface and the tool automatically generates the code based on the model.
Model Driven Development extra



Design → Modelling → Introduction → UML models

Can identify UML models

The following diagram uses the class diagram notation to show the different types of UML diagrams.

Guidance for the item(s) below:

Now that we have a high-level understanding of the role played by models, let's start learning some UML models, starting with UML class diagrams (and object diagrams which are like a distant cousin of class diagrams).

Note that we are learning to interpret these models only, not draw them (that would come later), or design them. Hence, we will be going through these topics fairly rapidly.

[W4.2] Class/Object Diagrams: Basics

Video Q+


Design → Modelling → Modelling Structure → OO structures

Design → OOP → Classes → Basic

Can explain structure modeling of OO solutions

An OO solution is basically a network of objects interacting with each other. Therefore, it is useful to be able to model how the relevant objects are 'networked' together inside a software i.e. how the objects are connected together.

Given below is an illustration of some objects and how they are connected together. Note: the diagram uses an ad-hoc notation.

Note that these object structures within the same software can change over time.

Given below is how the object structure in the previous example could have looked like at a different time.

However, object structures do not change at random; they change based on a set of rules, as was decided by the designer of that software. Those rules that object structures need to follow can be illustrated as a class structure i.e. a structure that exists among the relevant classes.

Here is a class structure (drawn using an ad-hoc notation) that matches the object structures given in the previous two examples. For example, note how this class structure does not allow any connection between Genre objects and Author objects, a rule followed by the two object structures above.

UML Object Diagrams are used to model object structures and UML Class Diagrams are used to model class structures of an OO solution.

Here is an object diagram for the above example:

And here is the class diagram for it:


Design → Modelling → Modelling Structure → Class diagrams - basic

Can use basic-level class diagrams

Contents of the panels given below belong to a different chapter; they have been embedded here for convenience and are collapsed by default to avoid content duplication in the printed version.

UML Class Diagrams → Introduction → What

Classes form the basis of class diagrams.

UML Class Diagrams → Classes → What

UML Class Diagrams → Class-Level Members → What

Associations are the main connections among the classes in a class diagram.

OOP Associations → What

UML Class Diagrams → Associations → What

The most basic class diagram is a bunch of classes with some solid lines among them to represent associations, such as this one.

An example class diagram showing associations between classes.

In addition, associations can show additional decorations such as association labels, association roles, multiplicity and navigability to add more information to a class diagram.

UML Class Diagrams → Associations → Labels

UML Class Diagrams → Associations → Roles

OOP Associations → Multiplicity

UML Class Diagrams → Associations → Multiplicity

OOP Associations → Navigability

UML Class Diagrams → Associations → Navigability

Here is the same class diagram shown earlier but with some additional information included:


Which association notations are shown?

Explain Class Diagram

Draw Class Diagram for Box etc.


Design → Modelling → Modelling Structure → Object diagrams

Can use basic object diagrams

UML → Object Diagrams → Introduction

Object diagrams can be used to complement class diagrams. For example, you can use object diagrams to model different object structures that can result from a design represented by a given class diagram.

UML → Object Diagrams → Objects

UML → Object Diagrams → Associations


Draw an Object Diagram for Box etc.


Tools → UML → Object versus class diagrams

Can distinguish between class diagrams and object diagrams

Compared to the notation for class diagrams, object diagrams differ in the following ways:

  • Show objects instead of classes:
    • Instance name may be shown
    • There is a : before the class name
    • Instance and class names are underlined
  • Methods are omitted
  • Multiplicities are omitted. Reason: an association line in an object diagram represents a connection to exactly one object (i.e., the multiplicity is always 1).

Furthermore, multiple object diagrams can correspond to a single class diagram.

Both object diagrams are derived from the same class diagram shown earlier. In other words, each of these object diagrams shows ‘an instance of’ the same class diagram.

When the class diagram has an inheritance relationship, the object diagram should show either an object of the parent class or the child class, but not both.

Suppose Employee is a child class of the Person class. The class diagram will be as follows:

Now, how do you show an Employee object named jake?

  • This is not correct, as there should be only one object.

  • This is OK.

  • This is OK, as jake is a Person too. That is, we can show the parent class instead of the child class if the child class doesn't matter to the purpose of the diagram (i.e., the reader of this diagram will not need to know that jake is in fact an Employee).

Association labels/roles can be omitted unless they add value (e.g., showing them is useful if there are multiple associations between the two classes in concern -- otherwise you wouldn't know which association the object diagram is showing)

Consider this class diagram and the object diagram:

We can clearly see that both Adam and Eve lives in hall h1 (i.e., OK to omit the association label lives in) but we can't see if History is Adam's major or his minor (i.e., the diagram should have included either an association label or a role there). In contrast, we can see Eve is an English major.


[W4.3] Class Diagrams: Intermediate-Level

Video Q+


Tools → UML → Notes

Can use UML notes

UML notes can augment UML diagrams with additional information. These notes can be shown connected to a particular element in the diagram or can be shown without a connection. The diagram below shows examples of both.



Tools → UML → Constraints


Tools → UML → Class Diagrams → Associations as attributes

Can show an association as an attribute

An association can be shown as an attribute instead of a line.

Association multiplicities and the default value can be shown as part of the attribute using the following notation. Both are optional.

name: type [multiplicity] = default value

The diagram below depicts a multi-player Square Game being played on a board comprising of 100 squares. Each of the squares may be occupied with any number of pieces, each belonging to a certain player.

A Piece may or may not be on a Square. Note how that association can be replaced by an isOn attribute of the Piece class. The isOn attribute can either be null or hold a reference to a Square object, matching the 0..1 multiplicity of the association it replaces. The default value is null.

The association that a Board has 100 Squares can be shown in either of these two ways:

Show each association as either an attribute or a line but not both. A line is preferred as it is easier to spot.


Design → Modelling → Modelling Structure → Class diagrams - intermediate

Design → Modeling → Class Diagrams (Basic)

Can use intermediate-level class diagrams

A class diagram can also show different types of relationships between classes: inheritance, compositions, aggregations, dependencies.

Modeling inheritance

OOP → Inheritance → What

UML → Class Diagrams → Inheritance → What

Modeling composition

OOP → Associations → Composition

UML → Class Diagrams → Composition → What

Modeling aggregation

OOP → Associations → Aggregation

UML → Class Diagrams → Aggregation → What

Modeling dependencies

OOP → Associations → Dependencies

UML → Class Diagrams → Dependencies → What

A class diagram can also show different types of class-like entities:

Modeling enumerations

OOP → Classes → Enumerations

UML → Class Diagrams → Enumerations → What

Modeling abstract classes

OOP → Inheritance → Abstract Classes

UML → Class Diagrams → Abstract Classes → What

Modeling interfaces

OOP → Inheritance → Interfaces

UML → Class Diagrams → Interfaces → What


Statements about class diagram

Explain notations in the class diagram

Draw a Class Diagram for the code (StockItem, Inventory, Review, etc.)


Paradigms → OOP → Associations → Association classes

Can explain the meaning of association classes

An association class represents additional information about an association. It is a normal class but plays a special role from a design point of view.

A Man class and a Woman class are linked with a ‘married to’ association and there is a need to store the date of marriage. However, that data is related to the association rather than specifically owned by either the Man object or the Woman object. In such situations, an additional association class can be introduced, e.g. a Marriage class, to store such information.

Implementing association classes

There is no special way to implement an association class. It can be implemented as a normal class that has variables to represent the endpoint of the association it represents.

In the code below, the Transaction class is an association class that represents a transaction between a Person who is the seller and another Person who is the buyer.

class Transaction {
    //all fields are compulsory
    Person seller;
    Person buyer;
    Date date;
    String receiptNumber;
    Transaction(Person seller, Person buyer, Date date, String receiptNumber) {
        //set fields


Guidance for the item(s) below:

Switching to Java now, let's learn how to write Java GUIs. Fair warning: GUI programming is hard in any language, especially so in Java. Buckle down and get through it; there's no way around it.

[W4.4] Java: JavaFX


C++ to Java → Miscellaneous Topics → JavaFX

Can use JavaFX to build a simple GUI

JavaFX is a technology for building Java-based GUIs. Previously it was a part Java itself, but has become a third-party dependency since then. It is now being maintained by OpenJDK.

Refer to the JavaFX tutorial @SE-EDU/guides to learn how to get started with JavaFX.

Guidance for the item(s) below:

While we are on the topic of Java, also take note of this is a lesser-known Java 'syntactic sugar' feature that was introduced not long ago, in case you come across it one day or find some use for it in your coding.

[W4.5] Java: varargs

W4.5a :

C++ to Java → Miscellaneous Topics → Varargs

Can use Java varargs feature

Variable Arguments (Varargs) is a syntactic sugar type feature that allows writing a method that can take a variable number of arguments.

The search method below can be called as search(), search("book"), search("book", "paper"), etc.

public static void search(String ... keywords){
   // method body


[W4.6] Code Quality: Naming


Implementation → Code Quality → Naming → Introduction

Can explain the need for good names in code

Proper naming improves the readability of code. It also reduces bugs caused by ambiguities regarding the intent of a variable or a method.

There are only two hard things in Computer Science: cache invalidation and naming things. -- Phil Karlton


Implementation → Code Quality → Naming → Basic → Use nouns for things and verbs for actions

Can improve code quality using technique: use nouns for things and verbs for actions

Every system is built from a domain-specific language designed by the programmers to describe that system. Functions are the verbs of that language, and classes are the nouns.
-- Robert C. Martin, Clean Code: A Handbook of Agile Software Craftsmanship

Use nouns for classes/variables and verbs for methods/functions.


Name for a Bad Good
Class CheckLimit LimitChecker
Method result() calculate()

Distinguish clearly between single-valued and multi-valued variables.



Person student;
ArrayList<Person> students;


name = 'Jim'
names = ['Jim', 'Alice']


Implementation → Code Quality → Naming → Basic → Use standard words

Can improve code quality using technique: use standard words

Use correct spelling in names. Avoid 'texting-style' spelling. Avoid foreign language words, slang, and names that are only meaningful within specific contexts/times e.g. terms from private jokes, a TV show currently popular in your country.


Implementation → Code Quality → Naming → Intermediate → Use name to explain

Can improve code quality using technique: use name to explain

A name is not just for differentiation; it should explain the named entity to the reader accurately and at a sufficient level of detail.


Bad Good
processInput() (what 'process'?) removeWhiteSpaceFromInput()
flag isValidInput

If a name has multiple words, they should be in a sensible order.


Bad Good
bySizeOrder() orderBySize()

Imagine going to the doctor's and saying "My eye1 is swollen"! Don’t use numbers or case to distinguish names.


Bad Bad Good
value1, value2 value, Value originalValue, finalValue


Implementation → Code Quality → Naming → Intermediate → Not too long, not too short

Can improve code quality using technique: not too long, not too short

While it is preferable not to have lengthy names, names that are 'too short' are even worse. If you must abbreviate or use acronyms, do it consistently. Explain their full meaning at an obvious location.


Implementation → Code Quality → Naming → Intermediate → Avoid misleading names

Can improve code quality using technique: avoid misleading names

Related things should be named similarly, while unrelated things should NOT.

Example: Consider these variables

  • colorBlack: hex value for color black
  • colorWhite: hex value for color white
  • colorBlue: number of times blue is used
  • hexForRed: hex value for color red

This is misleading because colorBlue is named similar to colorWhite and colorBlack but has a different purpose while hexForRed is named differently but has a very similar purpose to the first two variables. The following is better:

  • hexForBlack hexForWhite hexForRed
  • blueColorCount

Avoid misleading or ambiguous names (e.g. those with multiple meanings), similar sounding names, hard-to-pronounce ones (e.g. avoid ambiguities like "is that a lowercase L, capital I or number 1?", or "is that number 0 or letter O?"), almost similar names.


Bad Good Reason
phase0 phaseZero Is that zero or letter O?
rwrLgtDirn rowerLegitDirection Hard to pronounce
right left wrong rightDirection leftDirection wrongResponse right is for 'correct' or 'opposite of 'left'?
redBooks readBooks redColorBooks booksRead red and read (past tense) sounds the same
FiletMignon egg If the requirement is just a name of a food, egg is a much easier to type/say choice than FiletMignon

[W4.7] Static Analysis


Quality Assurance → Quality Assurance → Static Analysis → What


Can explain static analysis

Static analysis: Static analysis is the analysis of code without actually executing the code.

Static analysis of code can find useful information such as unused variables, unhandled exceptions, style errors, and statistics. Most modern IDEs come with some inbuilt static analysis capabilities. For example, an IDE can highlight unused variables as you type the code into the editor.

The term static in static analysis refers to the fact that the code is analyzed without executing the code. In contrast, dynamic analysis requires the code to be executed to gather additional information about the code e.g., performance characteristics.

Higher-end static analysis tools (static analyzers) can perform more complex analysis such as locating potential bugs, memory leaks, inefficient code structures, etc.

Some example static analyzers for Java: CheckStyle, PMD, FindBugs

Linters are a subset of static analyzers that specifically aim to locate areas where the code can be made 'cleaner'.

Guidance for the item(s) below:

Next up are two techniques that can be used to improve code quality. You need to learn them as you will be encountering both in your iP soon.

[W4.8] Code reviews


Quality Assurance → Quality Assurance → Code Reviews → What


Can explain code reviews

Code review is the systematic examination of code with the intention of finding where the code can be improved.

Reviews can be done in various forms. Some examples below:

  • Pull Request reviews

    • Project Management Platforms such as GitHub and BitBucket allow the new code to be proposed as Pull Requests and provide the ability for others to review the code in the PR.
  • In pair programming

    • As pair programming involves two programmers working on the same code at the same time, there is an implicit review of the code by the other member of the pair.
  • Formal inspections

    • Inspections involve a group of people systematically examining project artifacts to discover defects. Members of the inspection team play various roles during the process, such as:

      • the author - the creator of the artifact
      • the moderator - the planner and executor of the inspection meeting
      • the secretary - the recorder of the findings of the inspection
      • the inspector/reviewer - the one who inspects/reviews the artifact

Advantages of code review over testing:

  • It can detect functionality defects as well as other problems such as coding standard violations.
  • It can verify non-code artifacts and incomplete code.
  • It does not require test drivers or stubs.


  • It is a manual process and therefore, error prone.


[W4.9] RCS: Managing Pull Requests I


Tools → Git and GitHub → Reviewing PRs


Can review PRs on GitHub

The PR review stage is a dialog between the PR author and members of the repo that received the PR, in order to refine and eventually merge the PR.

Given below are some steps you can follow when reviewing a PR.

1. Locate the PR:

  1. Go to the GitHub page of the repo.
  2. Click on the Pull requests tab.
  3. Click on the PR you want to review.

2. Read the PR description. It might contain information relevant to reviewing the PR.

3. Click on the Files changed tab to see the diff view.

4. Add review comments:

  1. Hover over the line you want to comment on and click on the icon that appears on the left margin. That should create a text box for you to enter your comment.
    • To give a comment related to multiple lines, click-and-drag the icon. The result will look like this:
  2. Enter your comment.
    • This page @SE-EDU/guides has some best practices PR reviewers can follow.
    • To suggest an in-line code change, click on this icon:

      The comment will look like this to the viewers:

  3. After typing in the comment, click on the Start a review button (not the Add single comment button. This way, your comment is saved but not visible to others yet. It will be visible to others only when you have finished the entire review.

  4. Repeat the above steps to add more comments.

5. Submit the review:

  1. When there are no more comments to add, click on the Review changes button (on the top right of the diff page).
  2. Type in an overall comment about the PR, if any. e.g.,
    Overall, I found your code easy to read for the most part except a few places
    where the nesting was too deep. I noted a few minor coding standard violations
    too. Some of the classes are getting quite long. Consider splitting into
    smaller classes if that makes sense.
    LGTM is often used in such overall comments, to indicate Looks good to me.
    nit (as in nit picking) is another such term, used to indicate minor flaws e.g., LGTM. Just a few nits to fix..
  3. Choose Approve, Comment, or Request changes option as appropriate and click on the Submit review button.