How Do I Write an Apex Unit Test?

This answer is not intended to teach you everything about writing unit tests, nor to specifically answer every question, but to provide a quick summary and links to the resources that will help you move forward and develop more specific questions that SFSE can assist with.Unit (and integration) testing is a big topic, but it starts with a small set of principles. If you've never written a unit test before, we strongly encourage you to complete the Unit Testing on the Lightning Platform Trailhead module and read at least the Month of Testing series. These materials and others are linked under Resources, below.Fundamentally, testing comprises three steps, all of which take place within the context of a unit test:Then, all code that is executed under step (2) is counted as covered under Salesforce's code coverage metrics. Code coverage is a side effect of high quality unit tests. Salesforce uses code coverage as a proxy to measure the presence of unit tests in your deployments.Unit testing principles are quite general, and most Apex code is not special in the sense of requiring unique approaches to create a successful test. Techniques for implementing tests that perform all three steps are taught in the resources we include below.

On Salesforce, all unit tests are executed in an isolated context. In this context, your code cannot see data in your organization, including ordinary records as well as Custom Settings. All data must be created via the unit test or @testSetup method.

Metadata records, including Users and Custom Metadata, are visible in test context.An older annotation, seeAllDatatrue, allows tests to see all data in the Salesforce org. Use of this annotation is strongly discouraged for new unit tests, and is considered a very bad practice. It's important instead to follow the first step above, by designing test data as input for your code. This practice makes unit tests self-contained and repeatable, and insulates them against fragility stemming from data changes.Unit tests that don't contain assertions are often called smoke tests. These tests have very limited value, because they show nothing other than that your code does not crash under a specific set of circumstances. They don't prove the code works or does what it's intended to do

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Salesforce Stack Exchange looks for detailed, specific questions that the community can help you with, and can't write tests on your behalf. We feel that working with the resources below can help you get started, and we encourage you to make an attempt to write your test and return to SFSE with your specific questions when you encounter challenges you can't resolve.