JavaExercism/hello-world/GETTING_STARTED.md

Getting Started

These exercises lean on Test-Driven Development (TDD), but they're not an exact match.

The following steps assume that you are in the base directory of the exercise.

You must have gradle installed.

Step 1

Run the test suite. It's written using the JUnit framework, and can be run with gradle:

$ gradle test

This will fail, complaining that there is no method called hello for the HelloWorld class.

To fix the error, open src/main/java/HelloWorld.java and add a hello method that returns a String. The method can return an empty string "" for now:

public class HelloWorld {
    public static String hello() {
        return "";
    }
}

Step 2

Run the tests again. It will give you a new error, since now the method exists, but the tests are expects the method to accept one argument as input, a name.

The errors will look something like:

$ gradle test
:compileJava
:processResources UP-TO-DATE
:classes
:compileTestJava
/exercism/exercises/java/hello-world/src/test/java/HelloWorldTest.java:9: error: method hello in class HelloWorld cannot be applied to given types;
        assertEquals("Hello, World!", HelloWorld.hello(""));
                                                ^
  required: no arguments
  found: String
  reason: actual and formal argument lists differ in length

  ...

4 errors
:compileTestJava FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':compileTestJava'.
> Compilation failed; see the compiler error output for details.

Fixing the Error

To fix it, open src/main/java/HelloWorld.java and add a name to the method the signature:

public class HelloWorld {
    public static String hello(String name) {
        return "";
    }
}

Now the hello method accepts a name as an input.

Understanding Test Failures

Test failures will either be compilation errors or test failures. So far, we've been dealing with compilation errors. Once our test and main code successful compile, gradle will run the actual test suite.

Step 3

Run the tests again. You'll notice that the gradle output passes the :compileTestJava phase and fails at the :test phase.

The errors will look something like:

$ gradle test
:compileJava
:processResources UP-TO-DATE
:classes
:compileTestJava
:processTestResources UP-TO-DATE
:testClasses
:test

HelloWorldTest > helloNoName FAILED
    org.junit.ComparisonFailure at HelloWorldTest.java:9

...

3 tests completed, 3 failed
:test FAILED

These are actual test failures. It means that both our main and test java code is running but the test is expecting one outcome, but getting another.

Open src/test/java/HelloWorldTest.java and go to line 9.

assertEquals("Hello, World!", HelloWorld.hello(""));

The test is expecting that hello returns "Hello, World!", when given an empty string ("") as input. The easiest way to make the test pass is to return "Hello, World!" instead of `"".

public class HelloWorld {
    public static String hello(String name) {
        return "Hello, World!";
    }
}

Wash, Rinse, Repeat

Run the tests again.

If it fails you're going to need to read the error message carefully to figure out what went wrong, fix the problem in the main code, and then try again until all the tests pass.

$ gradle test
:compileJava
:processResources UP-TO-DATE
:classes
:compileTestJava
:processTestResources UP-TO-DATE
:testClasses
:test

BUILD SUCCESSFUL

Submit

When everything is passing, you can submit your code with the following command:

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