Hello World

Welcome to Hello World on Exercism's Java Track. If you need help running the tests or submitting your code, check out HELP.md.

Instructions

The classical introductory exercise. Just say "Hello, World!".

"Hello, World!" is the traditional first program for beginning programming in a new language or environment.

The objectives are simple:

Write a function that returns the string "Hello, World!".
Run the test suite and make sure that it succeeds.
Submit your solution and check it at the website.

If everything goes well, you will be ready to fetch your first real exercise.

Since this is your first Java exercise, we've included a detailed Tutorial that guides you through your solution. Check it out for tips and assistance!

Introduction
Exercise Structure
Solving "Hello, World!"
Submitting your first iteration
Next Steps

Welcome to the first exercise on the Java track!

This document is a step-by-step guide to solving this exercise. The instructions should work equally well on Windows, macOS and Linux.

Even if you are already familiar with Java basics, stepping through this guide may be helpful as it will help you understand the output from the tool, Gradle, that we use to compile and test our code, as well as introduce some of the patterns common to all exercises in this track.

When you fetch a new Java exercise, you will receive:

one or more test files (always). These live in the src/test/java directory and define a set of tests that our solution must satisfy before we can safely declare that it is working.
one or more starter files (initially). If provided, these live in the src/main/java directory and define shells of the classes you will need in order to solve the current problem.

Before proceeding any further, make sure you have completed the required setup steps described by the links below:

Step 1: Run the tests against the starter solution

Use Gradle to run the tests:

$ gradle test

This command does a lot and displays a bunch of stuff. Let's break it down...

:compileJava
:processResources UP-TO-DATE
:classes

Each line that begins with a colon (like :compileJava) is Gradle telling us that it's starting that task. The first three tasks are about compiling the source code of our solution. This exercise contains starter files with just enough code for the solution to compile without requiring modification.

When a task is successful, it generally does not output anything. This is why :compileJava and :classes do not produce any additional output. :processResources reports that it had nothing to do.

So far, so good...

The next three tasks are about compiling source code of the tests.

:compileTestJava
:processTestResources UP-TO-DATE
:testClasses

... with both sets of source code successfully compiled, Gradle turns to running the task you asked it to: executing the tests against the solution.

:test
GreeterTest > testThatGreeterReturnsTheCorrectGreeting FAILED
    java.lang.UnsupportedOperationException: Delete this statement and write your own implementation.
        at getGreeting(Greeter.java:4)
        at GreeterTest.testThatGreeterReturnsTheCorrectGreeting(GreeterTest.java:9)
1 tests completed, 1 failed
:test FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':test'.
> There were failing tests. See the report at: file:///home/<username>/hello-world/build/reports/tests/index.html
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED
Total time: 12.361 secs

Seeing the word "FAILED" might give you the impression you've done something wrong. You haven't. This output is just a very verbose way of telling us that the included test did not pass. This is expected - we haven't written any code to help it pass yet!

Let's focus in on the important bits:

GreeterTest > testThatGreeterReturnsTheCorrectGreeting FAILED
    java.lang.UnsupportedOperationException: Delete this statement and write your own implementation.

...is read: "Within the test class named GreeterTest, the test method testThatGreeterReturnsTheCorrectGreeting did not pass because an UnsupportedOperationException was thrown with the message Delete this statement and write your own implementation.."

The next line of the stack trace tells us where this UnsupportedOperationException was thrown:

        at getGreeting(Greeter.java:4)

Looks like it was on line 4 in the Greeter.java file. Let's update that line as instructed, and try running the tests again.

Step 2: Replace the `UnsupportedOperationException`

In your favorite text editor, open src/main/java/Greeter.java. You should find the source of the exception encountered above on line 4:

throw new UnsupportedOperationException("Delete this statement and write your own implementation.");

Exceptions are often used in Java to draw the attention of a developer when something goes wrong. In our case, nothing is wrong per se; we just haven't written our solution yet! The use of an UnsupportedOperationException in this situation is designed to remind us of exactly this fact. It effectively says: "Your Code Goes Here!".

Delete the contents of line 4 and replace it with the following:

return null;

Now run gradle test again.

You should see a new error this time. Don't worry though, that still represents forward progress!

GreeterTest > testThatGreeterReturnsTheCorrectGreeting FAILED
    java.lang.AssertionError: expected:<Hello, World!> but was:<null>

...is read: "Within the test class named GreeterTest, the test method testThatGreeterReturnsTheCorrectGreeting did not pass because the solution did not satisfy an assertion."

Apparently, our test was expecting to see the string "Hello, World!", but it received the value null instead.

The last line of the stack trace tells us exactly where this "unsatisfied assertion" lives:

        at testThatGreeterReturnsTheCorrectGreeting(GreeterTest.java:9)

Looks like the mismatch was discovered on line 9 in the test file.

Knowing these two facts:

that the return value was not what was expected, and
that the failure was on line 9 of the test file,

we can turn this failure into success.

Step 3: Fix the test!

Open src/test/java/GreeterTest.java and go to line 9. It reads:

assertEquals("Hello, World!",new Greeter().getGreeting());

The test is expecting that the method getGreeting() returns "Hello, World!". Instead, getGreeting() is returning null. Let's fix that.

Open src/main/java/Greeter.java. It should look like this:

class Greeter {
    String getGreeting() {
        return null;
    }
}

Remove the return null that we previously added, and instead return the string our test was expecting:

    String getGreeting(){
        return"Hello, World!";
        }

Save the file and run the tests again:

$ gradle test
:compileJava
:processResources UP-TO-DATE
:classes
:compileTestJava
:processTestResources UP-TO-DATE
:testClasses
:test
GreeterTest > testThatGreeterReturnsTheCorrectGreeting PASSED
BUILD SUCCESSFUL
Total time: 11.717 secs

Success! Our solution passes the only test case supplied. We're good to go!

With a working solution that we've reviewed, we're ready to submit it to exercism.io.

$ exercism submit src/main/java/Greeter.java

From here, there are a number of paths you can take.

Regardless of what you decide to do next, we sincerely hope you learn and enjoy being part of this community. If at any time you need assistance do not hesitate to ask for help:

http://exercism.io/languages/java/help

Cheers!

Move on to the next exercise

There are many more exercises you can practice with. Grab the next one! (example for two-fer)

$ exercism download --exercise=two-fer --track=java

Become a mentor

The heart of Exercism are the conversations about coding practices. It's definitely fun to practice, but engaging with others both in their attempts and your own is how you get feedback. That feedback can help point out what you're doing well and where you might need to improve.

Some submissions will be nearly identical to yours; others will be completely different. Seeing both kinds can be instructive and interesting.

Mentors review submitted solutions for the track they've chosen to mentor to help users learn as much as possible. To read more about mentoring and to sign up see: http://mentoring.exercism.io/.

Contribute to Exercism

The entire of Exercism is Open Source and is a labor of love for over 100 maintainers and many more contributors.

A starting point to jumping in can be found here:

https://github.com/exercism/docs/tree/master/contributing-to-language-tracks

Source

Created by

@counterleft

Contributed to by

@c-thornton
@di72nn
@FridaTveit
@imwithsam
@javaeeeee
@jmrunkle
@jtigger
@katrinleinweber
@kytrinyx
@lemoncurry
@matthewmorgan
@mohamedFalah
@msomji
@muzimuzhi
@otraczyk
@PTrottier
@rcraggs
@sjwarner-bp
@SleeplessByte
@Smarticles101
@sshine
@stkent
@Zaldrick

Based on

This is an exercise to introduce users to using Exercism - http://en.wikipedia.org/wiki/%22Hello,_world!%22_program

Metallifax/hello-world