Tests are not exams like the ones you have to take in school. Rather, tests are ways of testing code to see if it is performing the way we expect it to behave. Pretty easy, right?
For example, let's say we wanted to create a program that models an orange tree because nature is awesome. Well, what defines an orange tree to us? Is it safe to say that we need at least two objects, an orange object and a tree object? Can our tree mature and bear more fruit at a certain age? Could our oranges ripen and fall from the tree? Sounds like the list can become lengthy, right? Let's stop here for a few minutes and see what we need to fulfill these user stories.
# What We Need To Test?
Remember that we need to model an orange and a tree. So, we know that there are two objects that need to be created.
We could say that the tree will not bear fruit until it matures at one year of age and then it will bear X number of oranges. We would need a test to test if the tree ages and a test to see if the tree has created X number of oranges once it has matured.
How do we determine if an orange is ripe? Well, we can have an orange age and if it is at least 30 days old, then it is ripe and will fall from the tree. We would need to write a test to check for ripeness and a test to check if it falls at ripeness.
# Why Is It Important That We Test?
You could think of tests as a requirement list. Every time a change is made we want to test to see if our code still meets those requirements. For example, as our program becomes increasingly complex and we want to have our tree to have a certain lifespan, would it make sense for our tree to continue to create oranges after exceeding that lifespan?
A situation like our tree's lifespan is a prime example of why tests are important. As our program becomes more complex over time with new code added, the tests tell us that the new code negatively affected prior expectations of the behavior of the code. Tests can help keep the original behavior of the code and prevent new bugs from appearing.
# Test Driven Development
Test-driven development (TDD) is a development technique where you must first write a test that fails before you write new functional code. TDD is a great way to develop your program! Add a test, run all tests, write code, run tests, and then refactor code!