Detailed Table of Contents
Guidance for the item(s) below:
Let's learn about a few more Git techniques, starting with branching. Although these techniques are not really needed for the iP, we force you to use them in the iP so that you have more time to practice them before they are really needed in the tP.
Can explain branching
Branching is the process of evolving multiple versions of the software in parallel. For example, one team member can create a new branch and add an experimental feature to it while the rest of the team keeps working on another branch. Branches can be given names e.g. master
, release
, dev
.
A branch can be merged into another branch. Merging usually results in a new commit that represents the changes done in the branch being merged.
Merge conflicts happen when you try to merge two branches that had changed the same part of the code and the RCS cannot decide which changes to keep. In those cases, you have to ‘resolve’ the conflicts manually.
Exercises
Project Management → Revision Control → Branching
Can use Git branching
Git supports branching, which allows you to do multiple parallel changes to the content of a repository.
A Git branch is simply a named label pointing to a commit. The HEAD
label indicates which branch you are on. Git creates a branch named master
by default. When you add a commit, it goes into the branch you are currently on, and the branch label (together with the HEAD
label) moves to the new commit.
Given below is an illustration of how branch labels move as branches evolve.
master
) and there is only one commit on it.master
and the HEAD
labels have moved to the new commit.fix1
has been added. The repo has switched to the new branch too (hence, the HEAD
label is attached to the fix1
branch).c
) has been added. The current branch label fix1
moves to the new commit, together with the HEAD
label.master
branch.d
) has been added. The master
label has moved to that commit.fix1
branch and added a new commit (e
) to it.master
branch and the fix1
branch has been merged into the master
branch, creating a merge commit f
. The repo is currently on the master
branch.Follow the steps below to learn how to work with branches. You can use any repo you have on your computer (e.g. a clone of the samplerepo-things) for this.
0. Observe that you are normally in the branch called master
.
git status
on branch master
1. Start a branch named feature1
and switch to the new branch.
Click on the Branch
button on the main menu. In the next dialog, enter the branch name and click Create Branch
.
Note how the feature1
is indicated as the current branch.
You can use the branch
command to create a new branch and the checkout
command to switch to a specific branch.
git branch feature1
git checkout feature1
One-step shortcut to create a branch and switch to it at the same time:
git checkout –b feature1
2. Create some commits in the new branch. Just commit as per normal. Commits you add while on a certain branch will become part of that branch.
Note how the master
label and the HEAD
label moves to the new commit (The HEAD
label of the local repo is represented as in SourceTree).
3. Switch to the master
branch. Note how the changes you did in the feature1
branch are no longer in the working directory.
Double-click the master
branch.
git checkout master
4. Add a commit to the master branch. Let’s imagine it’s a bug fix.
To keep things simple for the time being, this commit should not involve the same content that you changed in the feature1
branch. To be on the safe side, this commit can change an entirely different file.
5. Switch back to the feature1
branch (similar to step 3).
6. Merge the master
branch to the feature1
branch, giving an end-result like the following. Also note how Git has created a merge commit.
Right-click on the master
branch and choose merge master into the current branch
. Click OK
in the next dialog.
git merge master
The objective of that merge was to sync the feature1
branch with the master
branch. Observe how the changes you did in the master
branch (i.e. the imaginary bug fix) is now available even when you are in the feature1
branch.
Instead of merging master
to feature1
, an alternative is to rebase the feature1
branch. However, rebasing is an advanced feature that requires modifying past commits. If you modify past commits that have been pushed to a remote repository, you'll have to force-push the modified commit to the remote repo in order to update the commits in it.
7. Add another commit to the feature1
branch.
8. Switch to the master
branch and add one more commit.
9. Merge feature1
to the master branch, giving and end-result like this:
Right-click on the feature1
branch and choose Merge...
.
git merge feature1
10. Create a new branch called add-countries
, switch to it, and add some commits to it (similar to steps 1-2 above). You should have something like this now:
Avoid this common rookie mistake!
Always remember to switch back to the master
branch before creating a new branch. If not, your new branch will be created on top of the current branch.
11. Go back to the master
branch and merge the add-countries
branch onto the master
branch (similar to steps 8-9 above). While you might expect to see something like the following,
... you are likely to see something like this instead:
That is because Git does a fast forward merge if possible. Seeing that the master
branch has not changed since you started the add-countries
branch, Git has decided it is simpler to just put the commits of the add-countries
branch in front of the master
branch, without going into the trouble of creating an extra merge commit.
It is possible to force Git to create a merge commit even if fast forwarding is possible.
Tick the box shown below when you merge a branch:
Use the --no-ff
switch (short for no fast forward):
git merge --no-ff add-countries
Here's how to push a branch to a remote repo:
Here's how to push a branch named add-intro
to your own fork of a repo named samplerepo-pr-practice
:
Normally: git push {remote repository} {branch}
. Examples:
git push origin master
pushes the master
branch to the repo named origin
(i.e., the repo you cloned from)git push upstream-repo add-intro
pushes the add-intro
branch to the repo named upstream-repo
If pushing a branch you created locally to the remote for the first time, add the -u
flag to get the local branch to track the new upstream branch:
e.g., git push -u origin add-intro
See git-scm.com/docs/git-push for details of the push
command.
Can use Git to resolve merge conflicts
Merge conflicts happen when you try to combine two incompatible versions (e.g., merging a branch to another but each branch changed the same part of the code in a different way).
Here are the steps to simulate a merge conflict and use it to learn how to resolve merge conflicts.
0. Create an empty repo or clone an existing repo, to be used for this activity.
1. Start a branch named fix1
in the repo. Create a commit that adds a line with some text to one of the files.
2. Switch back to master
branch. Create a commit with a conflicting change i.e. it adds a line with some different text in the exact location the previous line was added.
3. Try to merge the fix1
branch onto the master
branch. Git will pause mid-way during the merge and report a merge conflict. If you open the conflicted file, you will see something like this:
COLORS
------
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
white
4. Observe how the conflicted part is marked between a line starting with <<<<<<
and a line starting with >>>>>>
, separated by another line starting with =======
.
Highlighted below is the conflicting part that is coming from the master
branch:
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
This is the conflicting part that is coming from the fix1
branch:
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
5. Resolve the conflict by editing the file. Let us assume you want to keep both lines in the merged version. You can modify the file to be like this:
COLORS
------
blue
black
green
red
white
6. Stage the changes, and commit.
Guidance for the item(s) below:
Let's learn how to create a pull request (PRs) on GitHub; you need to create one for your project this week.
Can create PRs on GitHub
Suppose you want to propose some changes to a GitHub repo (e.g., samplerepo-pr-practice) as a pull request (PR). Here is a scenario you can try in order to learn how to create PRs:
1. Fork the repo onto your GitHub account.
2. Clone it onto your computer.
3. Commit your changes e.g., add a new file with some contents and commit it.
master
branchadd-intro
(remember to switch to the master
branch before creating a new branch) and add your commit to it.4. Push the branch you updated (i.e., master
branch or the new branch) to your fork, as explained here.
5. Initiate the PR creation:
Go to your fork.
Click on the Pull requests tab followed by the New pull request button. This will bring you to the 'Comparing changes' page.
Set the appropriate target repo and the branch that should receive your PR, using the base repository
and base
dropdowns. e.g.,
base repository: se-edu/samplerepo-pr-practice base: master
Normally, the default value shown in the dropdown is what you want but in case your fork has , the default may not be what you want.
Indicate which repo:branch contains your proposed code, using the head repository
and compare
dropdowns. e.g.,
head repository: myrepo/samplerepo-pr-practice compare: master
6. Verify the proposed code: Verify that the diff view in the page shows the exact change you intend to propose. If it doesn't, as necessary.
7. Submit the PR:
Click the Create pull request button.
Fill in the PR name and description e.g.,
Name: Add an introduction to the README.md
Description:
Add some paragraph to the README.md to explain ...
Also add a heading ...
If you want to indicate that the PR you are about to create is 'still work in progress, not yet ready', click on the dropdown arrow in the Create pull request button and choose Create draft pull request
option.
Click the Create pull request button to create the PR.
Go to the receiving repo to verify that your PR appears there in the Pull requests
tab.
The next step of the PR life cycle is the PR review. The members of the repo that received your PR can now review your proposed changes.
You can update the PR along the way too. Suppose PR reviewers suggested a certain improvement to your proposed code. To update your PR as per the suggestion, you can simply modify the code in your local repo, commit the updated code to the same master
branch, and push to your fork as you did earlier. The PR will auto-update accordingly.
Sending PRs using the master
branch is less common than sending PRs using separate branches. For example, suppose you wanted to propose two bug fixes that are not related to each other. In that case, it is more appropriate to send two separate PRs so that each fix can be reviewed, refined, and merged independently. But if you send PRs using the master
branch only, both fixes (and any other change you do in the master
branch) will appear in the PRs you create from it.
To create another PR while the current PR is still under review, create a new branch (remember to switch back to the master
branch first), add your new proposed change in that branch, and create a new PR following the steps given above.
It is possible to create PRs within the same repo e.g., you can create a PR from branch feature-x
to the master
branch, within the same repo. Doing so will allow the code to be reviewed by other developers (using PR review mechanism) before it is merged.
Problem: merge conflicts in ongoing PRs, indicated by the message This branch has conflicts that must be resolved. That means the upstream repo's master
branch has been updated in a way that the PR code conflicts with that master
branch. Here is the standard way to fix this problem:
master
branch from the upstream repo to your local repo.git checkout master
git pull upstream master
master
branch (that you updated in the previous step) onto the PR branch, in order to bring over the new code in the master
branch to your PR branch.git checkout pr-branch # assuming pr-branch is the name of branch in the PR
git merge master
master
branch.
Resolve the conflict manually (this topic is covered elsewhere), and complete the merge.master
branch, the merge conflict alert in the PR will go away automatically. Guidance for the item(s) below:
As your project gets bigger and changes become more frequent, it's natural to look for ways to automate the many steps involved in going from the code you write in the editor to an executable product. This is a good time to start learning about that aspect too.
Can explain build automation tools
Build automation tools automate the steps of the build process, usually by means of build scripts.
In a non-trivial project, building a product from its source code can be a complex multi-step process. For example, it can include steps such as: pull code from the revision control system, compile, link, run automated tests, automatically update release documents (e.g. build number), package into a distributable, push to repo, deploy to a server, delete temporary files created during building/testing, email developers of the new build, and so on. Furthermore, this build process can be done ‘on demand’, it can be scheduled (e.g. every day at midnight) or it can be triggered by various events (e.g. triggered by a code push to the revision control system).
Some of these build steps such as compiling, linking and packaging, are already automated in most modern IDEs. For example, several steps happen automatically when the ‘build’ button of the IDE is clicked. Some IDEs even allow customization of this build process to some extent.
However, most big projects use specialized build tools to automate complex build processes.
Some popular build tools relevant to Java developers: Gradle, Maven, Apache Ant, GNU Make
Some other build tools: Grunt (JavaScript), Rake (Ruby)
Some build tools also serve as dependency management tools. Modern software projects often depend on third party libraries that evolve constantly. That means developers need to download the correct version of the required libraries and update them regularly. Therefore, dependency management is an important part of build automation. Dependency management tools can automate that aspect of a project.
Maven and Gradle, in addition to managing the build process, can play the role of dependency management tools too.
Resources
Working With Gradle in Intellij IDEA (6 minutes)
Exercises
Can explain continuous integration and continuous deployment
An extreme application of build automation is called continuous integration (CI) in which integration, building, and testing happens automatically after each code change.
A natural extension of CI is Continuous Deployment (CD) where the changes are not only integrated continuously, but also deployed to end-users at the same time.
Some examples of CI/CD tools: Travis, Jenkins, Appveyor, CircleCI, GitHub Actions
Guidance for the item(s) below:
Next, we have a few more Java topics that you need as you move from a 'programming exercise' mode to a 'production code' mode.
Can explain JavaDoc
JavaDoc is a tool for generating API documentation in HTML format from comments in the source code. In addition, modern IDEs use JavaDoc comments to generate explanatory tooltips.
An example method header comment in JavaDoc format (adapted from Oracle's Java documentation)
/**
* Returns an Image object that can then be painted on the screen.
* The url argument must specify an absolute {@link URL}. The name
* argument is a specifier that is relative to the url argument.
* <p>
* This method always returns immediately, whether or not the
* image exists. When this applet attempts to draw the image on
* the screen, the data will be loaded. The graphics primitives
* that draw the image will incrementally paint on the screen.
*
* @param url an absolute URL giving the base location of the image
* @param name the location of the image, relative to the url argument
* @return the image at the specified URL
* @see Image
*/
public Image getImage(URL url, String name) {
try {
return getImage(new URL(url, name));
} catch (MalformedURLException e) {
return null;
}
}
Generated HTML documentation:
Tooltip generated by Intellij IDE:
Can write JavaDoc comments
In the absence of more extensive guidelines (e.g., given in a coding standard adopted by your project), you can follow the two examples below in your code.
A minimal JavaDoc comment example for methods:
/**
* Returns lateral location of the specified position.
* If the position is unset, NaN is returned.
*
* @param x X coordinate of position.
* @param y Y coordinate of position.
* @param zone Zone of position.
* @return Lateral location.
* @throws IllegalArgumentException If zone is <= 0.
*/
public double computeLocation(double x, double y, int zone)
throws IllegalArgumentException {
// ...
}
A minimal JavaDoc comment example for classes:
package ...
import ...
/**
* Represents a location in a 2D space. A <code>Point</code> object corresponds to
* a coordinate represented by two integers e.g., <code>3,6</code>
*/
public class Point {
// ...
}
Resources
Can read/write text files using Java
You can use the java.io.File
class to represent a file object. It can be used to access properties of the file object.
This code creates a File
object to represent a file fruits.txt
that exists in the data
directory relative to the current working directory and uses that object to print some properties of the file.
import java.io.File;
public class FileClassDemo {
public static void main(String[] args) {
File f = new File("data/fruits.txt");
System.out.println("full path: " + f.getAbsolutePath());
System.out.println("file exists?: " + f.exists());
System.out.println("is Directory?: " + f.isDirectory());
}
}
full path: C:\sample-code\data\fruits.txt
file exists?: true
is Directory?: false
If you use backslash to specify the file path in a Windows computer, you need to use an additional backslash as an escape character because the backslash by itself has a special meaning. e.g., use "data\\fruits.txt"
, not "data\fruits.txt"
. Alternatively, you can use forward slash "data/fruits.txt"
(even on Windows).
You can read from a file using a Scanner
object that uses a File
object as the source of data.
This code uses a Scanner
object to read (and print) contents of a text file line-by-line:
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class FileReadingDemo {
private static void printFileContents(String filePath) throws FileNotFoundException {
File f = new File(filePath); // create a File for the given file path
Scanner s = new Scanner(f); // create a Scanner using the File as the source
while (s.hasNext()) {
System.out.println(s.nextLine());
}
}
public static void main(String[] args) {
try {
printFileContents("data/fruits.txt");
} catch (FileNotFoundException e) {
System.out.println("File not found");
}
}
}
i.e., contents of the data/fruits.txt
5 Apples
3 Bananas
6 Cherries
You can use a java.io.FileWriter
object to write to a file.
The writeToFile
method below uses a FileWrite
object to write to a file. The method is being used to write two lines to the file temp/lines.txt
.
import java.io.FileWriter;
import java.io.IOException;
public class FileWritingDemo {
private static void writeToFile(String filePath, String textToAdd) throws IOException {
FileWriter fw = new FileWriter(filePath);
fw.write(textToAdd);
fw.close();
}
public static void main(String[] args) {
String file2 = "temp/lines.txt";
try {
writeToFile(file2, "first line" + System.lineSeparator() + "second line");
} catch (IOException e) {
System.out.println("Something went wrong: " + e.getMessage());
}
}
}
Contents of the temp/lines.txt
:
first line
second line
Note that you need to call the close()
method of the FileWriter
object for the writing operation to be completed.
You can create a FileWriter
object that appends to the file (instead of overwriting the current content) by specifying an additional boolean parameter to the constructor.
The method below appends to the file rather than overwrites.
private static void appendToFile(String filePath, String textToAppend) throws IOException {
FileWriter fw = new FileWriter(filePath, true); // create a FileWriter in append mode
fw.write(textToAppend);
fw.close();
}
The java.nio.file.Files
is a utility class that provides several useful file operations. It relies on the java.nio.file.Paths
file to generate Path
objects that represent file paths.
This example uses the Files
class to copy a file and delete a file.
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
public class FilesClassDemo {
public static void main(String[] args) throws IOException{
Files.copy(Paths.get("data/fruits.txt"), Paths.get("temp/fruits2.txt"));
Files.delete(Paths.get("temp/fruits2.txt"));
}
}
The techniques above are good enough to manipulate simple text files. Note that it is also possible to perform file I/O operations using other classes.
Can use Java packages
You can organize your types (i.e., classes, interfaces, enumerations, etc.) into packages for easier management (among other benefits).
To create a package, you put a package statement at the very top of every source file in that package. The package statement must be the first line in the source file and there can be no more than one package statement in each source file. Furthermore, the package of a type should match the folder path of the source file. Similarly, the compiler will put the .class
files in a folder structure that matches the package names.
The Formatter
class below (in <source folder>/seedu/tojava/util/Formatter.java
file) is in the package seedu.tojava.util
. When it is compiled, the Formatter.class
file will be in the location <compiler output folder>/seedu/tojava/util
:
package seedu.tojava.util;
public class Formatter {
public static final String PREFIX = ">>";
public static String format(String s){
return PREFIX + s;
}
}
Package names are written in all lower case (not camelCase), using the dot as a separator. Packages in the Java language itself begin with java
. or javax
. Companies use their reversed Internet domain name to begin their package names.
For example, com.foobar.doohickey.util
can be the name of a package created by a company with a domain name foobar.com
To use a public from outside its package, you must do one of the following:
The Main
class below has two import statements:
import seedu.tojava.util.StringParser
: imports the class StringParser
in the seedu.tojava.util
packageimport seedu.tojava.frontend.*
: imports all the classes in the seedu.tojava.frontend
packagepackage seedu.tojava;
import seedu.tojava.util.StringParser;
import seedu.tojava.frontend.*;
public class Main {
public static void main(String[] args) {
// Using the fully qualified name to access the Processor class
String status = seedu.tojava.logic.Processor.getStatus();
// Using the StringParser previously imported
StringParser sp = new StringParser();
// Using classes from the tojava.frontend package
Ui ui = new Ui();
Message m = new Message();
}
}
Note how the class can still use the Processor
without importing it first, by using its fully qualified name seedu.tojava.logic.Processor
Importing a package does not import its sub-packages, as packages do not behave as hierarchies despite appearances.
import seedu.tojava.frontend.*
does not import the classes in the sub-package seedu.tojava.frontend.widget
.
If you do not use a package statement, your type doesn't have a package -- a practice not recommended (except for small code examples) as it is not possible for a type in a package to import a type that is not in a package.
Optionally, a static import can be used to import static members of a type so that the imported members can be used without specifying the type name.
The class below uses static imports to import the constant PREFIX
and the method format()
from the seedu.tojava.util.Formatter
class.
import static seedu.tojava.util.Formatter.PREFIX;
import static seedu.tojava.util.Formatter.format;
public class Main {
public static void main(String[] args) {
String formatted = format("Hello");
boolean isFormatted = formatted.startsWith(PREFIX);
System.out.println(formatted);
}
}
Formatter
class
Note how the class can use PREFIX
and format()
(instead of Formatter.PREFIX
and Formatter.format()
).
When using the commandline to compile/run Java, you should take the package into account.
If the seedu.tojava.Main
class is defined in the file Main.java
,
<source folder>
, the command is:javac seedu/tojava/Main.java
<compiler output folder>
, the command is:java seedu.tojava.Main
Resources
Can use JAR files
Java applications are typically delivered as JAR (short for Java Archive) files. A JAR contains Java classes and other resources (icons, media files, etc.).
An executable JAR file can be launched using the java -jar
command e.g., java -jar foo.jar
launches the foo.jar
file.
The IDE or build tools such as Gradle can help you to package your application as a JAR file.
See the tutorial Working with JAR files @se-edu/guides to learn how to create and use JAR files.
Guidance for the item(s) below:
As you know, one of the objectives of the iP is to raise the quality of your code. We'll be learning about various ways to improve the code quality in the next few weeks, starting with coding standards.
Guidance for the item(s) below:
Please watch the lecture video given below as it has some extra points not given in the text version.
Can explain the importance of code quality
Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live. -- Martin Golding
Production code needs to be of high quality. Given how the world is becoming increasingly dependent on software, poor quality code is something no one can afford to tolerate.
Can explain the need for following a standard
One essential way to improve code quality is to follow a consistent style. That is why software engineers follow a strict coding standard (aka style guide).
The aim of a coding standard is to make the entire code base look like it was written by one person. A coding standard is usually specific to a programming language and specifies guidelines such as the locations of opening and closing braces, indentation styles and naming styles (e.g. whether to use Hungarian style, Pascal casing, Camel casing, etc.). It is important that the whole team/company uses the same coding standard and that the standard is generally not inconsistent with typical industry practices. If a company's coding standard is very different from what is typically used in the industry, new recruits will take longer to get used to the company's coding style.
IDEs can help to enforce some parts of a coding standard e.g. indentation rules.
Exercises
Guidance for the item(s) below:
As promised last week, let's learn some more sophisticated ways of testing.
Can explain the need for early developer testing
Delaying testing until the full product is complete has a number of disadvantages:
Therefore, it is better to do early testing, as hinted by the popular rule of thumb given below, also illustrated by the graph below it.
The earlier a bug is found, the easier and cheaper to have it fixed.
Such early testing of partially developed software is usually, and by necessity, done by the developers themselves i.e. developer testing.
Exercises
Can explain test drivers
A test driver is the code that ‘drives’ the for the purpose of testing i.e. invoking the SUT with test inputs and verifying if the behavior is as expected.
PayrollTest
‘drives’ the Payroll
class by sending it test inputs and verifies if the output is as expected.
public class PayrollTest {
public static void main(String[] args) throws Exception {
// test setup
Payroll p = new Payroll();
// test case 1
p.setEmployees(new String[]{"E001", "E002"});
// automatically verify the response
if (p.totalSalary() != 6400) {
throw new Error("case 1 failed ");
}
// test case 2
p.setEmployees(new String[]{"E001"});
if (p.totalSalary() != 2300) {
throw new Error("case 2 failed ");
}
// more tests...
System.out.println("All tests passed");
}
}
Can explain test automation tools
JUnit is a tool for automated testing of Java programs. Similar tools are available for other languages and for automating different types of testing.
This is an automated test for a Payroll
class, written using JUnit libraries.
@Test
public void testTotalSalary() {
Payroll p = new Payroll();
// test case 1
p.setEmployees(new String[]{"E001", "E002"});
assertEquals(6400, p.totalSalary());
// test case 2
p.setEmployees(new String[]{"E001"});
assertEquals(2300, p.totalSalary());
// more tests...
}
Most modern IDEs have integrated support for testing tools. The figure below shows the JUnit output when running some JUnit tests using the Eclipse IDE.
Can explain unit testing
Unit testing: testing individual units (methods, classes, subsystems, ...) to ensure each piece works correctly.
In OOP code, it is common to write one or more unit tests for each public method of a class.
Here are the code skeletons for a Foo
class containing two methods and a FooTest
class that contains unit tests for those two methods.
class Foo {
String read() {
// ...
}
void write(String input) {
// ...
}
}
class FooTest {
@Test
void read() {
// a unit test for Foo#read() method
}
@Test
void write_emptyInput_exceptionThrown() {
// a unit tests for Foo#write(String) method
}
@Test
void write_normalInput_writtenCorrectly() {
// another unit tests for Foo#write(String) method
}
}
import unittest
class Foo:
def read(self):
# ...
def write(self, input):
# ...
class FooTest(unittest.TestCase):
def test_read(self):
# a unit test for read() method
def test_write_emptyIntput_ignored(self):
# a unit test for write(string) method
def test_write_normalInput_writtenCorrectly(self):
# another unit test for write(string) method
Resources
Can use simple JUnit tests
When writing JUnit tests for a class Foo
, the common practice is to create a FooTest
class, which will contain various test methods.
Suppose we want to write tests for the IntPair
class below.
public class IntPair {
int first;
int second;
public IntPair(int first, int second) {
this.first = first;
this.second = second;
}
public int intDivision() throws Exception {
if (second == 0){
throw new Exception("Divisor is zero");
}
return first/second;
}
@Override
public String toString() {
return first + "," + second;
}
}
Here's a IntPairTest
class to match (using JUnit 5).
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.fail;
public class IntPairTest {
@Test
public void testStringConversion() {
assertEquals("4,7", new IntPair(4, 7).toString());
}
@Test
public void intDivision_nonZeroDivisor_success() throws Exception {
assertEquals(2, new IntPair(4, 2).intDivision());
assertEquals(0, new IntPair(1, 2).intDivision());
assertEquals(0, new IntPair(0, 5).intDivision());
}
@Test
public void intDivision_zeroDivisor_exceptionThrown() {
try {
assertEquals(0, new IntPair(1, 0).intDivision());
fail(); // the test should not reach this line
} catch (Exception e) {
assertEquals("Divisor is zero", e.getMessage());
}
}
}
Notes:
@Test
annotation.Assert.assertEquals(expected, actual)
methods to compare the expected output with the actual output. If they do not match, the test will fail. JUnit comes with other similar methods such as Assert.assertNull
and Assert.assertTrue
.testStringConversion
but when writing test methods, sometimes another convention is used: whatIsBeingTested_descriptionOfTestInputs_expectedOutcome
e.g., intDivision_zeroDivisor_exceptionThrown
catch
block. But if it is not thrown as expected, the test will reach Assert.fail()
line and will fail as a result.Adding JUnit 5 to your IntelliJ Project -- by Kevintroko@YouTube
Resources
Quality Assurance → Testing → Unit Testing → What →
Can use stubs to isolate an SUT from its dependencies
A proper unit test requires the unit to be tested in isolation so that bugs in the cannot influence the test i.e. bugs outside of the unit should not affect the unit tests.
If a Logic
class depends on a Storage
class, unit testing the Logic
class requires isolating the Logic
class from the Storage
class.
Stubs can isolate the from its dependencies.
Stub: A stub has the same interface as the component it replaces, but its implementation is so simple that it is unlikely to have any bugs. It mimics the responses of the component, but only for a limited set of predetermined inputs. That is, it does not know how to respond to any other inputs. Typically, these mimicked responses are hard-coded in the stub rather than computed or retrieved from elsewhere, e.g. from a database.
Consider the code below:
class Logic {
Storage s;
Logic(Storage s) {
this.s = s;
}
String getName(int index) {
return "Name: " + s.getName(index);
}
}
interface Storage {
String getName(int index);
}
class DatabaseStorage implements Storage {
@Override
public String getName(int index) {
return readValueFromDatabase(index);
}
private String readValueFromDatabase(int index) {
// retrieve name from the database
}
}
Normally, you would use the Logic
class as follows (note how the Logic
object depends on a DatabaseStorage
object to perform the getName()
operation):
Logic logic = new Logic(new DatabaseStorage());
String name = logic.getName(23);
You can test it like this:
@Test
void getName() {
Logic logic = new Logic(new DatabaseStorage());
assertEquals("Name: John", logic.getName(5));
}
However, this logic
object being tested is making use of a DataBaseStorage
object which means a bug in the DatabaseStorage
class can affect the test. Therefore, this test is not testing Logic
in isolation from its dependencies and hence it is not a pure unit test.
Here is a stub class you can use in place of DatabaseStorage
:
class StorageStub implements Storage {
@Override
public String getName(int index) {
if (index == 5) {
return "Adam";
} else {
throw new UnsupportedOperationException();
}
}
}
Note how the StorageStub
has the same interface as DatabaseStorage
, but is so simple that it is unlikely to contain bugs, and is pre-configured to respond with a hard-coded response, presumably, the correct response DatabaseStorage
is expected to return for the given test input.
Here is how you can use the stub to write a unit test. This test is not affected by any bugs in the DatabaseStorage
class and hence is a pure unit test.
@Test
void getName() {
Logic logic = new Logic(new StorageStub());
assertEquals("Name: Adam", logic.getName(5));
}
In addition to Stubs, there are other type of replacements you can use during testing, e.g. Mocks, Fakes, Dummies, Spies.
Resources
Exercises
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.