What code is easy to cover with tests? Name at least three criteria of well-testable code.

Well-tested code usually has the following characteristics:

1. Modularity and low coupling — code is divided into small, independent components or functions, making them easier to isolate and test.
2. Clean and clear interface — functions have well-defined inputs and outputs, without hidden dependencies.
3. No side effects — functions do not alter global state or external resources unnecessarily, which facilitates predictability and repeatability of tests.

Additionally, it is useful to:

• Use inversion of control and dependency injection to replace external components in tests.
• Implement good logging and error handling to simplify diagnostics.

An example of a function that is easy to cover with tests in Java:

public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
}

Such a function is easy to test because it:

• Does not depend on external state.
• Has a simple and clear interface.
• Returns predictable results.