Requirement 2 – adding two-player support

Now it's time to work on the specification of which player is about to play his turn.

There should be a way to find out which player should play next.

We can split this requirement into three tests:

The first turn should be played by player X
If the last turn was played by X, then the next turn should be played by O
If the last turn was played by O, then the next turn should be played by X

Until this moment, we haven't used any of the JUnit's asserts. To use them, we need to import the static methods from the org.junit.Assert class:

import static org.junit.Assert.*;

In their essence, methods inside the Assert class are very simple. Most of them start with assert. For example, assertEquals compares two objects—assertNotEquals verifies that two objects are not the same and assertArrayEquals verifies that two arrays are the same. Each of those asserts has many overloaded variations so that almost any type of Java object can be used.

In our case, we'll need to compare two characters. The first is the one we're expecting and the second one is the actual character retrieved from the nextPlayer method.

Now it's time to write those tests and the implementation.

Write the test before writing the implementation code .

The benefits of doing this are as follows—it ensures that testable code is written and ensures that every line of code gets tests written for it.

By writing or modifying the test first, the developer is focused on requirements before starting to work on a code. This is the main difference when compared to writing tests after the implementation is done. An additional benefit is that with tests first, we are avoiding the danger that the tests work as quality checking instead of quality assurance.

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Test-Driven Java Development（Second Edition）

Requirement 2 – adding two-player support