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Week 9 [Mon, Aug 16th] - Topics

[W9.1] Class Diagrams: Intermediate-Level

[W9.1a] Design → Modelling → Modelling Structure → Class diagrams - intermediate

[W9.2] Logging

[W9.2a] Implementation → Error Handling → Logging → What
[W9.2b] Implementation → Error Handling → Logging → How

[W9.3] Assertions

[W9.3a] Implementation → Error Handling → Assertions → What
[W9.3b] Implementation → Error Handling → Assertions → How
[W9.3c] Implementation → Error Handling → Assertions → When

[W9.4] Design Principles

Abstraction

[W9.4a] Design → Design Fundamentals → Abstraction → What

Coupling

[W9.4b] Design → Design Fundamentals → Coupling → What
[W9.4c] Design → Design Fundamentals → Coupling → How
[W9.4d] Design → Design Fundamentals → Coupling → Types of coupling : OPTIONAL

Cohesion

[W9.4e] Design → Design Fundamentals → Cohesion → What
[W9.4f] Design → Design Fundamentals → Cohesion → How

Some Principles

[W9.4g] Principles → Single responsibility principle
[W9.4h] Principles → Separation of concerns principle

[W9.5] Testing: Intermediate Techniques

[W9.5a] Quality Assurance → Testing → Introduction → Testability
[W9.5b] C++ to Java → JUnit → JUnit: Intermediate
[W9.5c] Quality Assurance → Testing → Test-Driven Development → What : OPTIONAL

Guidance for the item(s) below:

This week, let us learn the remaining class diagram notations.

[W9.1] Class Diagrams: Intermediate-Level

W9.1a

Design → Modelling → Modelling Structure → Class diagrams - intermediate

Can use intermediate-level class diagrams

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

Modeling inheritance

Modeling composition

Modeling aggregation

Modeling dependencies

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

Modeling enumerations

Modeling abstract classes

Modeling interfaces

Which of these statements match the class diagram?

a. A Snake must belong to at least one Board.
b. A SnakeHeadSquare can contain only one Snake head.
c. A Square can contain a Snake head.
d. A Snake head can be in more than one SnakeHeadSquare.
e. The Board has exactly 5 Snakes.

~~(a)~~(b)(c)(d)(e)

Explanation:

(a) does not match because a Snake may or may not belong to a Board (multiplicity is 0..1)
(b) matches the diagram because the multiplicity given is 1
(c) matches the diagram because SnakeHeadSquare is a Square (due to inheritance)
(d) matches the diagram because the multiplicity given is *
(e) matches the diagram because the multiplicity given is 5

Explain the meaning of various class diagram notations in the following class diagram:

Consider the code below:

public interface Billable {
    void bill();
}

public abstract class Item
         implements Billable {
    public abstract void print();
}

public class StockItem extends Item {
    private Review review;
    private String name;

    public StockItem(
        String name, Rating rating) {

        this.name = name;
        this.review = new Review(rating);
    }

    @Override
    public void print() {
        //...
    }

    @Override
    public void bill() {
        //...
    }
}

public enum Rating {
    GOOD, OK, POOR
}

public class Review {
    private final Rating rating;

    public Review(Rating rating) {
        this.rating = rating;
    }
}

import java.util.List;

public class Inventory {
    private List<Item> items;

    public int getItemCount() {
        return items.size();
    }

    public void generateBill(Billable b) {
        // ...
    }

    public void add(Item s) {
        items.add(s);
    }
}

(a) Draw a class diagram to represent the code. Show all attributes, methods, associations, navigabilities, visibilities, known multiplicities, and association roles. Show associations as lines.
(b) Draw an object diagram to represent the situation where the inventory has one item named spanner and a review of POOR rating
i.e., new Inventory().add(new StockItem("spanner", new Review(Rating.POOR))).

Guidance for the item(s) below:

Given next are two techniques that help you locate problems in the code: logging, and assertions

[W9.2] Logging

W9.2a

Implementation → Error Handling → Logging → What

Can explain logging

Logging is the deliberate recording of certain information during a program execution for future reference. Logs are typically written to a log file but it is also possible to log information in other ways e.g. into a database or a remote server.

Logging can be useful for troubleshooting problems. A good logging system records some system information regularly. When bad things happen to a system e.g. an unanticipated failure, their associated log files may provide indications of what went wrong and actions can then be taken to prevent it from happening again.

A log file is like the flight data recorderblack box of an airplane; they don't prevent problems but they can be helpful in understanding what went wrong after the fact.

_{source: https://commons.wikimedia.org}

Why is logging like having the 'black box' in an airplane?

a. Because they can be used to find more info about how the system behaved before/during an ‘incident’.
b. Because they are hidden from the view of normal users.
c. Because they can both be in hardcopy form or digital form.
d. Because they are hard to open by normal users.

(a)

W9.2b

Implementation → Error Handling → Logging → How

Can use logging

Most programming environments come with logging systems that allow sophisticated forms of logging. They have features such as the ability to enable and disable logging easily or to change the logging how much information to recordintensity.

This sample Java code uses Java’s default logging mechanism.

First, import the relevant Java package:

import java.util.logging.*;

Next, create a Logger:

private static Logger logger = Logger.getLogger("Foo");

Now, you can use the Logger object to log information. Note the use of a INFO, WARNING etc.logging level for each message. When running the code, the logging level can be set to WARNING so that log messages specified as having INFO level (which is a lower level than WARNING) will not be written to the log file at all.

// log a message at INFO level
logger.log(Level.INFO, "going to start processing");
// ...
processInput();
if (error) {
    // log a message at WARNING level
    logger.log(Level.WARNING, "processing error", ex);
}
// ...
logger.log(Level.INFO, "end of processing");

Tutorials:

Java Logging API - Tutorial -- A tutorial by Lars Vogella
Java Logging Tutorial -- An alternative tutorial by Jakob Jenkov

A video tutorial by SimplyCoded:

Best Practices:

10 Tips for Proper Application Logging -- by Tomasz Nurkiewicz
What each logging level means -- conventions recommended by Apache Project

[W9.3] Assertions

W9.3a

Implementation → Error Handling → Assertions → What

Can explain assertions

Assertions are used to define assumptions about the program state so that the runtime can verify them. An assertion failure indicates a possible bug in the code because the code has resulted in a program state that violates an assumption about how the code should behave.

An assertion can be used to express something like when the execution comes to this point, the variable v cannot be null.

If the runtime detects an assertion failure, it typically takes some drastic action such as terminating the execution with an error message. This is because an assertion failure indicates a possible bug and the sooner the execution stops, the safer it is.

In the Java code below, suppose you set an assertion that timeout returned by Config.getTimeout() is greater than 0. Now, if Config.getTimeout() returns -1 in a specific execution of this line, the runtime can detect it as an assertion failure -- i.e. an assumption about the expected behavior of the code turned out to be wrong which could potentially be the result of a bug -- and take some drastic action such as terminating the execution.

int timeout = Config.getTimeout();

W9.3b

Implementation → Error Handling → Assertions → How

Can use assertions

Use the assert keyword to define assertions.

This assertion will fail with the message x should be 0 if x is not 0 at this point.

x = getX();
assert x == 0 : "x should be 0";
...

Assertions can be disabled without modifying the code.

java -enableassertions HelloWorld (or java -ea HelloWorld) will run HelloWorld with assertions enabled while java -disableassertions HelloWorld will run it without verifying assertions.

Java disables assertions by default. This could create a situation where you think all assertions are being verified as true while in fact they are not being verified at all. Therefore, remember to enable assertions when you run the program if you want them to be in effect.

Enable assertions in Intellij (how?) and get an assertion to fail temporarily (e.g. insert an assert false into the code temporarily) to confirm assertions are being verified.

Java assert vs JUnit assertions: They are similar in purpose but JUnit assertions are more powerful and customized for testing. In addition, JUnit assertions are not disabled by default. We recommend you use JUnit assertions in test code and Java assert in functional code.

Tutorials:

Java Assertions -- a simple tutorial from javatpoint.com
Programming with Assertions (first half) -- a more detailed tutorial from Oracle

Best practices:

Programming with Assertions (second half) -- from Oracle (also listed above as a tutorial) contains some best practices towards the end of the article.

W9.3c

Implementation → Error Handling → Assertions → When

Can use assertions optimally

It is recommended that assertions be used liberally in the code. Their impact on performance is considered low and worth the additional safety they provide.

Do not use assertions to do work because assertions can be disabled. If not, your program will stop working when assertions are not enabled.

The code below will not invoke the writeFile() method when assertions are disabled. If that method is performing some work that is necessary for your program, your program will not work correctly when assertions are disabled.

...
assert writeFile() : "File writing is supposed to return true";

Assertions are suitable for verifying assumptions about Internal Invariants, Control-Flow Invariants, Preconditions, Postconditions, and Class Invariants. Refer to [Programming with Assertions (second half)] to learn more.

Exceptions and assertions are two complementary ways of handling errors in software but they serve different purposes. Therefore, both assertions and exceptions should be used in code.

The raising of an exception indicates an unusual condition created by the user (e.g. user inputs an unacceptable input) or the environment (e.g., a file needed for the program is missing).
An assertion failure indicates the programmer made a mistake in the code (e.g., a null value is returned from a method that is not supposed to return null under any circumstances).

A Calculator program crashes with an ‘assertion failure’ message when you try to find the square root of a negative number.

a. This is a correct use of assertions.
b. The application should have terminated with an exception instead.
c. The program has a bug.
d. All statements above are incorrect.

(c)

Explanation: An assertion failure indicates a bug in the code. (b) is not acceptable because of the word "terminated". The application should not fail at all for this input. But it could have used an exception to handle the situation internally.

Which statements are correct?

a. Use assertions to indicate that the programmer messed up; use exceptions to indicate that the user or the environment messed up.
b. Use exceptions to indicate that the programmer messed up; use assertions to indicate that the user or the environment messed up.

(a)

Guidance for the item(s) below:

As you are still in the early stage of the project, this is a good time to learn some design principles that you can try to apply in the internal design of your product.

These principles build on top of the design fundamentals you learned earlier (i.e., abstraction, coupling, cohesion).

[W9.4] Design Principles

Abstraction

Guidance for the item(s) below:

Let's start by learning the three most fundamental design qualities upon which all other design principles are built.

W9.4a

Design → Design Fundamentals → Abstraction → What

Can explain abstraction

Abstraction is a technique for dealing with complexity. It works by establishing a level of complexity we are interested in, and suppressing the more complex details below that level.

The guiding principle of abstraction is that only details that are relevant to the current perspective or the task at hand need to be considered. As most programs are written to solve complex problems involving large amounts of intricate details, it is impossible to deal with all these details at the same time. That is where abstraction can help.

Data abstraction: abstracting away the lower level data items and thinking in terms of bigger entities

Within a certain software component, you might deal with a user data type, while ignoring the details contained in the user data item such as name, and date of birth. These details have been ‘abstracted away’ as they do not affect the task of that software component.

Control abstraction: abstracting away details of the actual control flow to focus on tasks at a higher level

print(“Hello”) is an abstraction of the actual output mechanism within the computer.

Abstraction can be applied repeatedly to obtain progressively higher levels of abstraction.

An example of different levels of data abstraction: a File is a data item that is at a higher level than an array and an array is at a higher level than a bit.

An example of different levels of control abstraction: execute(Game) is at a higher level than print(Char) which is at a higher level than an Assembly language instruction MOV.

Abstraction is a general concept that is not limited to just data or control abstractions.

Some more general examples of abstraction:

An OOP class is an abstraction over related data and behaviors.
An architecture is a higher-level abstraction of the design of a software.
Models (e.g., UML models) are abstractions of some aspect of reality.

Coupling

W9.4b

Design → Design Fundamentals → Coupling → What

Can explain coupling

Coupling is a measure of the degree of dependence between components, classes, methods, etc. Low coupling indicates that a component is less dependent on other components. High coupling (aka tight coupling or strong coupling) is discouraged due to the following disadvantages:

Maintenance is harder because a change in one module could cause changes in other modules coupled to it (i.e. a ripple effect).
Integration is harder because multiple components coupled with each other have to be integrated at the same time.
Testing and reuse of the module is harder due to its dependence on other modules.

In the example below, design A appears to have more coupling between the components than design B.

Discuss the coupling levels of alternative designs x and y.

Overall coupling levels in x and y seem to be similar (neither has more dependencies than the other). (Note that the number of dependency links is not a definitive measure of the level of coupling. Some links may be stronger than the others.). However, in x, A is highly-coupled to the rest of the system while B, C, D, and E are standalone (do not depend on anything else). In y, no component is as highly-coupled as A of x. However, only D and E are standalone.

Explain the link (if any) between regressions and coupling.

When the system is highly-coupled, the risk of regressions is higher too e.g. when component A is modified, all components ‘coupled’ to component A risk ‘unintended behavioral changes’.

Discuss the relationship between coupling and a measure of how easily a given component can be testedtestability.

Coupling decreases testability because if the Software Under TestSUT is coupled to many other components, it becomes difficult to test the SUT in isolation of its dependencies.

Choose the correct statements.

a. As coupling increases, testability decreases.
b. As coupling increases, the risk of regression increases.
c. As coupling increases, the value of automated regression testing increases.
d. As coupling increases, integration becomes easier as everything is connected together.
e. As coupling increases, maintainability decreases.

(a)(b)(c)~~(d)~~(e)

Explanation: High coupling means either more components are required to be integrated at once in a big-bang fashion (increasing the risk of things going wrong) or more drivers and stubs are required when integrating incrementally.

W9.4c

Design → Design Fundamentals → Coupling → How

Can reduce coupling

X is coupled to Y if a change to Y can potentially require a change in X.

If the Foo class calls the method Bar#read(), Foo is coupled to Bar because a change to Bar can potentially (but not always) require a change in the Foo class e.g. if the signature of Bar#read() is changed, Foo needs to change as well, but a change to the Bar#write() method may not require a change in the Foo class because Foo does not call Bar#write().

class Foo {
    ...
    new Bar().read();
    ...
}

class Bar {
    void read() {
        ...
    }
    
    void write() {
        ...
    }
}

Some examples of coupling: A is coupled to B if,

A has access to the internal structure of B (this results in a very high level of coupling)
A and B depend on the same global variable
A calls B
A receives an object of B as a parameter or a return value
A inherits from B
A and B are required to follow the same data format or communication protocol

Which of these indicate coupling between components A and B?

a. component A has access to the internal structure of component B.
b. components A and B are written by the same developer.
c. component A calls component B.
d. component A receives an object of component B as a parameter.
e. component A inherits from component B.
f. components A and B have to follow the same data format or communication protocol.

(a)~~(b)~~(c)(d)(e)(f)

Explanation: Being written by the same developer does not imply coupling.

W9.4d : OPTIONAL

Design → Design Fundamentals → Coupling → Types of coupling

Can identify types of coupling

Some examples of different coupling types:

Content coupling: one module modifies or relies on the internal workings of another module e.g., accessing local data of another module
Common/Global coupling: two modules share the same global data
Control coupling: one module controlling the flow of another, by passing it information on what to do e.g., passing a flag
Data coupling: one module sharing data with another module e.g. via passing parameters
External coupling: two modules share an externally imposed convention e.g., data formats, communication protocols, device interfaces.
Subclass coupling: a class inherits from another class. Note that a child class is coupled to the parent class but not the other way around.
Temporal coupling: two actions are bundled together just because they happen to occur at the same time e.g. extracting a contiguous block of code as a method although the code block contains statements unrelated to each other

Cohesion

W9.4e

Design → Design Fundamentals → Cohesion → What

Can explain cohesion

Cohesion is a measure of how strongly-related and focused the various responsibilities of a component are. A highly-cohesive component keeps related functionalities together while keeping out all other unrelated things.

Higher cohesion is better. Disadvantages of low cohesion (aka weak cohesion):

Lowers the understandability of modules as it is difficult to express module functionalities at a higher level.
Lowers maintainability because a module can be modified due to unrelated causes (reason: the module contains code unrelated to each other) or many modules may need to be modified to achieve a small change in behavior (reason: because the code related to that change is not localized to a single module).
Lowers reusability of modules because they do not represent logical units of functionality.

W9.4f

Design → Design Fundamentals → Cohesion → How

Can increase cohesion

Cohesion can be present in many forms. Some examples:

Code related to a single concept is kept together, e.g. the Student component handles everything related to students.
Code that is invoked close together in time is kept together, e.g. all code related to initializing the system is kept together.
Code that manipulates the same data structure is kept together, e.g. the GameArchive component handles everything related to the storage and retrieval of game sessions.

Suppose a Payroll application contains a class that deals with writing data to the database. If the class includes some code to show an error dialog to the user if the database is unreachable, that class is not cohesive because it seems to be interacting with the user as well as the database.

Compare the cohesion of the following two versions of the EmailMessage class. Which one is more cohesive and why?

// version-1
class EmailMessage {
    private String sendTo;
    private String subject;
    private String message;

    public EmailMessage(String sendTo, String subject, String message) {
        this.sendTo = sendTo;
        this.subject = subject;
        this.message = message;
    }

    public void sendMessage() {
        // sends message using sendTo, subject and message
    }
}

// version-2
class EmailMessage {
    private String sendTo;
    private String subject;
    private String message;
    private String username;

    public EmailMessage(String sendTo, String subject, String message) {
        this.sendTo = sendTo;
        this.subject = subject;
        this.message = message;
    }

    public void sendMessage() {
        // sends message using sendTo, subject and message
    }

    public void login(String username, String password) {
        this.username = username;
        // code to login
    }
}

Version 2 is less cohesive.

Explanation: Version 2 is handling functionality related to login, which is not directly related to the concept of ‘email message’ that the class is supposed to represent. On a related note, you can improve the cohesion of both versions by removing the sendMessage functionality. Although sending messages is related to emails, this class is supposed to represent an email message, not an email server.

Guidance for the item(s) below:

Given next are two design principles that we can apply when designing OOP systems. These aim to improve abstraction, coupling, cohesionthe three design qualities you learned earlier.

Some Principles

W9.4g

Principles → Single responsibility principle

Can explain single responsibility principle

Single responsibility principle (SRP): A class should have one, and only one, reason to change. _{-- Robert C. Martin}

If a class has only one responsibility, it needs to change only when there is a change to that responsibility.

Consider a TextUi class that does parsing of the user commands as well as interacting with the user. That class needs to change when the formatting of the UI changes as well as when the syntax of the user command changes. Hence, such a class does not follow the SRP.

Gather together the things that change for the same reasons. Separate those things that change for different reasons. _{―- Agile Software Development, Principles, Patterns, and Practices by Robert C. Martin}

An explanation of the SRP from www.oodesign.com
Another explanation (more detailed) by Patkos Csaba
A book chapter on SRP written by the father of the principle itself, Robert C Martin

W9.4h

Principles → Separation of concerns principle

Can explain separation of concerns principle

Separation of concerns principle (SoC): To achieve better modularity, separate the code into distinct sections, such that each section addresses a separate concern. -- Proposed by Edsger W. Dijkstra

A concern in this context is a set of information that affects the code of a computer program.

Examples for concerns:

A specific feature, such as the code related to the add employee feature
A specific aspect, such as the code related to persistence or security
A specific entity, such as the code related to the Employee entity

Applying Separation of ConcernsSoC reduces functional overlaps among code sections and also limits the ripple effect when changes are introduced to a specific part of the system.

If the code related to persistence is separated from the code related to security, a change to how the data are persisted will not need changes to how the security is implemented.

This principle can be applied at the class level, as well as at higher levels.

The n-tier architecture utilizes this principle. Each layer in the architecture has a well-defined functionality that has no functional overlap with each other.

This principle should lead to higher cohesion and lower coupling.

Can explain coupling

Maintenance is harder because a change in one module could cause changes in other modules coupled to it (i.e. a ripple effect).
Integration is harder because multiple components coupled with each other have to be integrated at the same time.
Testing and reuse of the module is harder due to its dependence on other modules.

In the example below, design A appears to have more coupling between the components than design B.

Discuss the coupling levels of alternative designs x and y.

Explain the link (if any) between regressions and coupling.

When the system is highly-coupled, the risk of regressions is higher too e.g. when component A is modified, all components ‘coupled’ to component A risk ‘unintended behavioral changes’.

Discuss the relationship between coupling and a measure of how easily a given component can be testedtestability.

Coupling decreases testability because if the Software Under TestSUT is coupled to many other components, it becomes difficult to test the SUT in isolation of its dependencies.

Choose the correct statements.

a. As coupling increases, testability decreases.
b. As coupling increases, the risk of regression increases.
c. As coupling increases, the value of automated regression testing increases.
d. As coupling increases, integration becomes easier as everything is connected together.
e. As coupling increases, maintainability decreases.

(a)(b)(c)~~(d)~~(e)

Can explain cohesion

Higher cohesion is better. Disadvantages of low cohesion (aka weak cohesion):

Lowers the understandability of modules as it is difficult to express module functionalities at a higher level.
Lowers maintainability because a module can be modified due to unrelated causes (reason: the module contains code unrelated to each other) or many modules may need to be modified to achieve a small change in behavior (reason: because the code related to that change is not localized to a single module).
Lowers reusability of modules because they do not represent logical units of functionality.

“Only the GUI class should interact with the user. The GUI class should only concern itself with user interactions”. This statement follows from,

a. A software design should promote separation of concerns in a design.
b. A software design should increase cohesion of its components.
c. A software design should follow single responsibility principle.

(a)(b)(c)

Explanation: By making ‘user interaction’ the GUI class’s sole responsibility, we increase its cohesion. This is also in line with the separation of concerns (i.e., we separated the concern of user interaction) and the single responsibility principle (the GUI class has only one responsibility).

Follow up notes for the item(s) above:

As you may have realized already, the two principles given above are somewhat similar, one is specific to OOP and applied at class level while the other is not specific to OOP and can be applied at any level.

To learn more principles, you can go to https://se-education.org/se-book/principles/.

Guidance for the item(s) below:

We started writing JUnit testing in the last week. The topics below helps you push a bit further in the same direction.

[W9.5] Testing: Intermediate Techniques

W9.5a

Quality Assurance → Testing → Introduction → Testability

Can explain testability

Testability is an indication of how easy it is to test an SUT. As testability depends a lot on the design and implementation, you should try to increase the testability when you design and implement software. The higher the testability, the easier it is to achieve better quality software.

W9.5b

C++ to Java → JUnit → JUnit: Intermediate

Can use intermediate features of JUnit

Skim through the JUnit 5 User Guide to see what advanced techniques are available. If applicable, feel free to adopt them.

W9.5c : OPTIONAL

Quality Assurance → Testing → Test-Driven Development → What

Can explain TDD

Test-Driven Development(TDD) advocates writing the tests before writing the SUT, while evolving functionality and tests in small increments. In TDD you first define the precise behavior of the SUT using test cases, and then write the SUT to match the specified behavior. While TDD has its fair share of detractors, there are many who consider it a good way to reduce defects. One big advantage of TDD is that it guarantees the code is testable.

A) In TDD, we write all the test cases before we start writing functional code.

True
False

B) Testing tools such as JUnit require us to follow TDD.

True
False

A) False

Explanation: No, not all. We proceed in small steps, writing tests and functional code in tandem, but writing the test before we write the corresponding functional code.

B) False

Explanation: They can be used for TDD, but they can be used without TDD too.