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Unit Testing in Java: Best Practices and Examples

Akshit Kumar
Akshit Kumar
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Unit testing is an essential practice in software development. It allows developers to verify the correctness of individual units of code and ensures that they function as intended. In Java, unit testing is commonly done using frameworks such as JUnit, TestNG, or Mockito. In this blog post, we will explore the world of unit testing in Java, discussing best practices and providing examples to help you write effective and reliable test cases.

The Importance of Unit Testing:

Unit testing provides numerous benefits in software development. It helps catch bugs early in the development cycle, enhances code quality, facilitates code maintainability, and increases developer confidence. Well-designed unit tests serve as documentation and provide a safety net when making changes or refactoring code.

Setting Up Your Testing Environment:

To get started with unit testing in Java, you need to set up a robust testing environment. Choose a testing framework such as JUnit or TestNG and include the necessary dependencies in your project’s build configuration file (e.g., Maven or Gradle). Additionally, consider using mocking frameworks like Mockito to isolate dependencies during testing.

Anatomy of a Unit Test:

A unit test generally consists of three main sections: setup, execution, and verification. In the setup phase, you prepare the necessary preconditions for the test. The execution phase triggers the unit’s functionality, and the verification phase checks if the actual results match the expected results.

Writing Effective Test Cases:

a. Test-Driven Development (TDD):
Test-Driven Development (TDD) is a development technique where you write tests first and then implement the functionality to make those tests pass. It helps in designing modular, maintainable code from the outset and ensures maximum test coverage.

b. Boundary Value Analysis:
Boundary value analysis focuses on testing the boundaries between different input or output values. By selecting values at the lower and upper limits of each range, as well as the values just inside and outside those limits, you can uncover potential issues caused by edge cases or boundary conditions.

c. Equivalence Partitioning:
Equivalence partitioning involves dividing the input domain into classes of equivalent values. By selecting representative values from each class, you can reduce the number of test cases required while maintaining adequate coverage. For example, if your function accepts positive integers, you can choose test cases from the class of positive integers, avoiding unnecessary duplication.

d. Mocking and Stubbing:
When testing units with dependencies on external systems or complex objects, mocking and stubbing frameworks like Mockito or EasyMock can simulate those dependencies. This enables isolated testing and avoids unnecessary coupling.

Best Practices for Unit Testing:

a. Independent and Isolated Tests:
Each test case should be independent of others, meaning they should not rely on the state or execution order of other tests. Isolated tests reduce the chances of false positives or negatives and make debugging easier.
b. Test Naming Conventions:
Use meaningful and descriptive names for test methods to convey their purpose and expected behavior. This helps in quickly identifying failed tests and understanding their intent.

c. Arrange-Act-Assert Pattern:
Follow the Arrange-Act-Assert (AAA) pattern to structure your test cases. Clearly separate the setup, execution, and verification steps to enhance readability and maintainability.

d. Test Coverage:
Strive for high test coverage to ensure critical parts of your codebase are thoroughly tested. Tools like JaCoCo can measure test coverage and highlight areas that need additional tests.

e. Continuous Integration:
Incorporate your unit tests into a continuous integration (CI) system like Jenkins or Travis CI. Automating your tests in CI pipelines ensures that they are executed regularly and alerts you to any regressions introduced during development.

f. Handling Exceptions and Error Conditions:
When writing test cases, consider exceptional scenarios and error conditions. Validate that exceptions are thrown when expected, and ensure error handling is robust and appropriate.

Here are a few additional best practices for unit testing:-

  1. Use Repeatable Test Cases:
    Unit tests should be repeatable, meaning they produce the same results every time they are executed. This allows for consistent and reliable testing. Ensure that the test environment and inputs are controlled so that the tests are deterministic.
  2. One Assert per Test Case:
    Following the “one assert per test case” guideline helps keep the tests focused and easy to understand. Each test case should have a single logical assertion, verifying one specific behavior or outcome. This improves test readability and makes it easier to pinpoint failures.
  3. Keep Tests Fast:
    Unit tests should be fast to execute so that they can be run frequently during development and as part of the continuous integration process. Avoid time-consuming operations like network requests or complex setup and teardown steps whenever possible. Consider using mocking or stubbing to isolate and speed up the tests.
  4. Maintain Test Coverage:
    Aim for high test coverage to ensure that most, if not all, of your code is being tested. Coverage tools can help identify areas of your codebase that lack test coverage. Strive to cover critical paths, edge cases, and error conditions.
  5. Refactor Tests Along with Code:
    When making changes to the code being tested, make sure to update the corresponding unit tests to reflect the changes. Refactor the tests to ensure they remain relevant and maintainable, avoiding duplication or unnecessary complexity.
  6. Automate Tests:
    Automate the execution of unit tests to save time and effort. Use build automation tools or continuous integration (CI) systems to automatically run the tests whenever there are code changes. This helps catch issues early and ensures that all tests are executed consistently.
  7. Regularly Run Tests:
    Run unit tests frequently, preferably after every code change. This practice helps catch issues early in the development process and ensures that new code changes don’t introduce regressions. Integrating unit tests into your development workflow encourages a culture of testing and helps maintain code quality.

Example: Testing a Bank Account Class:

To illustrate the concepts discussed, let’s consider an example of testing a Bank Account class. Assume the Bank Account class has methods for depositing and withdrawing funds, and for checking the account balance.

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class BankAccountTest {
    void testDeposit() {
        BankAccount account = new BankAccount();
        assertEquals(100, account.getBalance(), "Deposit should increase the balance");
    void testWithdrawSufficientFunds() {
        BankAccount account = new BankAccount(200);
        assertEquals(100, account.getBalance(), "Withdrawal should decrease the balance");
    void testWithdrawInsufficientFunds() {
        BankAccount account = new BankAccount(50);
        assertThrows(InsufficientFundsException.class, () -> account.withdraw(100), 
                "Withdrawal with insufficient funds should throw InsufficientFundsException");
        assertEquals(50, account.getBalance(), "Balance should remain unchanged after failed withdrawal");

In the above example, we’ve written three test cases using JUnit. The first test case verifies that depositing funds increases the account balance. The second test case checks if withdrawing sufficient funds decreases the balance as expected. The third test case ensures that withdrawing with insufficient funds throws the appropriate exception and leaves the balance unchanged.


Unit testing is a vital part of the software development process. By following the best practices outlined in this blog post and utilizing appropriate testing frameworks, you can write effective unit tests in Java. We explored the importance of unit testing, how to set up the testing environment, techniques for writing test cases, and best practices to enhance the quality and reliability of your code. With the provided example of testing a Bank Account class, you can apply these principles to your own projects and improve the overall quality of your Java applications.

read more: Getting Started with Grafana: A Beginner’s Guide to Visualizing Your Data


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Akshit Kumar

Akshit Kumar

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