Unit Testing in Java

Overview of Unit Testing

• Unit testing is a type of software testing that focuses on testing individual pieces of code (units) in isolation.
• It is used to verify that specific methods or classes are functioning correctly.
• Unit tests can be implemented using the JUnit framework.

Getting Started with JUnit

• JUnit needs to be added as a dependency in your Java project (e.g., using Maven or Gradle).
• Example of a simple method to test:
  • Class: SimpleCalculator
  • Method: add(int a, int b) that returns the sum of two integers.

Creating Unit Tests

• In IntelliJ, use the shortcut Ctrl + Shift + T to create a test for a class.
• The test class will automatically be named following a pattern (e.g., SimpleCalculatorTest).
• Test files are usually placed under source/test/java in the project structure.

Writing a Unit Test

1. Creating a Test Method
   • Label with @Test annotation (import from org.junit.jupiter.api).
2. Define the Test Scenario
   • Example: Testing that 2 + 2 equals 4.
3. Use Assertions
   • Use assertEquals(expected, actual) to verify results.
   • Example: assertEquals(4, calculator.add(2, 2));

Running Tests

• Run individual tests using the icons next to each test method in IntelliJ.
• Tests will indicate whether they pass or fail, and provide feedback on any failures.

Importance of Multiple Test Scenarios

• Always consider edge cases and multiple scenarios to ensure comprehensive testing.
• A single test case may pass while the code is incorrect if it does not cover all logic paths.
  • Example: If the addition method mistakenly performs multiplication, a test like assertEquals(4, calculator.add(2, 2)) will still pass.

Advanced Assertion Types

• assertNotEquals: Pass if two parameters are not equal.
• assertTrue / assertFalse: Pass based on boolean expression.
• assertNull / assertNotNull: Check for null values.

Testing More Complex Methods

• Example method: determineLetterGrade(int grade) which returns letter grades based on numeric grades.
• Create separate tests for each grade range (e.g., 59 should return F, 80 should return B).
• Utilize code coverage tools to ensure all lines of code are tested (aim for 100% coverage).

Handling Exceptions in Tests

• Use assertThrows to verify exceptions are thrown correctly.
  • Example: Test that passing a negative number throws IllegalArgumentException.

Benefits of Unit Testing

• Ensures code correctness through verifiable test scenarios.
• Facilitates safe code refactoring by providing confidence that existing functionality is preserved.

Conclusion

• Unit testing is a critical aspect of software development that improves code quality and reliability.
• Additional learning resources and tutorials are available to deepen understanding of Java and unit testing.