Custom Salesforce Apex Test Class

I need a Salesforce Apex test class written specifically to test the functionality of my existing Apex classes.

Requirements:
- The test class should cover the creation and update functionality of my custom objects.
- The test class should ensure that data validation is being performed accurately.

It is important that the freelancer has experience with Salesforce Apex and can demonstrate previous experience in writing effective test classes.

Please provide samples of your previous Apex test classes if possible. Your understanding of Salesforce standard and custom objects will be a key factor in my decision.