Introduction
As a result, various testing types are being introduced to cover different aspects of software, such as inputs, outputs, user flows, functions, etc. Black box testing is one of the most popular testing methods. This article will help you understand what black box testing is, how it differs from white box testing, and the different box testing techniques you should know.
What Is Black Box Testing?
In black box testing, test cases are designed without referring to the application’s source code. Testers are only concerned with what goes in and what comes out of the system and check whether the system gives the expected output when a particular input is provided.
Black box testing is almost the complete opposite of white box testing, where testers get comprehensive access to the internal code of an application. This form of testing is also called behavioral testing due to its emphasis on the application behavior rather than its structure. However, using black box testing alone is not a good way to go since it focuses on the external functions of an application, not its internal structure. Hence, most organizations pair black-box testing with white-box testing (gray-box testing) to achieve a high percentage of test coverage.
Why Black Box Testing Matters in Software Quality
Some might wonder why black box testing is so popular among developers if it only covers inputs and outputs. It can be extremely beneficial and spot-on for testing some use cases.
- Quickly Validates User Requirements: Black box testing is a great way to ensure an application meets user requirements quickly. We can quickly verify applications’ behavior by analyzing outputs for specific input values.
- User-Focused Perspective: Black box testing is similar to how an end user interacts with the system. So, it is a great way to test how end users feel when they use the application.
- Improves Functional Quality: Black box testing focuses heavily on the functional aspects of software. It verifies that each feature works correctly under various conditions, from basic functionalities to edge cases.
- Regression Testing: Extremely useful in regression testing to ensure the application does not break due to new changes.
- Cost and Scalability: Black box testing is more cost-effective than white box testing. It does not require in-depth knowledge of the internal codebase, making it easier to scale testing across larger projects or multiple teams.
Techniques for Black Box Testing
Since you now know why testers prefer black box testing, let’s discuss the different techniques used for black box testing.
1. Equivalence Partitioning
The strategy of equivalence partitioning involves grouping test input parameters into one of the available classes (equivalence classes), in which it is assumed the application behaves the same way. Thus, tests are designed so that only one value from each class is used, decreasing the total number of test cases.
Example:
Consider a system that accepts student marks, which should be between 0 and 100. Any value outside this range is considered invalid. Equivalence classes could be:
- Valid class: 0 to 100
- Invalid class 1: Less than 1 (e.g., -5)
- Invalid class 2: Greater than 100 (e.g., 105)
function isValidInput(input) { return input >= 1 && input <= 100; } // Test cases based on equivalence partitioning console.log(isValidInput(50)); // True (valid class) console.log(isValidInput(-5)); // False (invalid class 1) console.log(isValidInput(105)); // False (invalid class 2)
Benefits:
- Reduces the number of test cases while maintaining test coverage.
- Tests representative values from each partition, ensuring effective validation.
- Organizes complex input patterns into manageable partitions, making test case design more straightforward.
2. Boundary Value Analysis
Boundary Value Analysis (BVA) tests the boundaries between equivalence partitions. Since errors often occur at the boundaries, testing the values at the edges of input ranges can catch a significant number of defects.
Example:
If the system accepts values between 1 and 100, boundary values would be:
- Lower boundary: -1, 0, 1
- Upper boundary: 99, 100, 101
function isWithinRange(value: number): boolean { return value >= 1 && value <= 100; } // Boundary value tests console.log(isWithinRange(1)); // True (lower boundary) console.log(isWithinRange(100)); // True (upper boundary) console.log(isWithinRange(0)); // False (below boundary) console.log(isWithinRange(101)); // False (above boundary)
Benefits:
- Effectively catches boundary-related errors.
- Simple technique with fewer test cases needed for coverage.
3. Decision Table Testing
Decision table testing is used when the software behavior depends on combinations of different inputs. It works by mapping all possible combinations of conditions and actions into a decision table, allowing the tester to cover all possible cases systematically.
Example:
Consider a student management system that grants different levels of access based on two conditions:
- If the user is an admin and puts the correct password, the system is accessible.
- If the user is not an admin or they put in an incorrect password, the system cannot be accessed.
function accessSystem(isAdmin: boolean, validPassword: boolean): string { if (isAdmin && validPassword) { return "Access Granted"; } return "Access Denied"; } // Test cases from decision table console.log(accessSystem(true, true)); // Access Granted console.log(accessSystem(true, false)); // Access Denied console.log(accessSystem(false, true)); // Access Denied console.log(accessSystem(false, false)); // Access Denied
Benefits:
- Ensures that all combinations of inputs are covered.
- Useful for systems with complex input conditions and rules.
4. State Transition Testing
State transition testing is used when the software has finite states, and the behavior depends on the current state and input conditions. It tests the transitions between states, ensuring the software responds correctly to inputs.
Example:
Consider a login system for an educational platform transitions between the following states:
- Logged out
- Logged in
- Locked (after three failed login attempts)
class LoginSystem { private state: string; private failures: number; constructor() { this.state = 'loggedOut'; this.failures = 0; } login(password: string): string { if (this.state === 'locked') { return "Account Locked"; } if (password === "correctPassword") { this.state = 'loggedIn'; return "Access Granted"; } else { this.failures++; if (this.failures >= 3) { this.state = 'locked'; return "Account Locked"; } return "Access Denied"; } } logout(): string { if (this.state === 'loggedIn') { this.state = 'loggedOut'; return "Logged Out"; } return "Not Logged In"; } } // Test cases based on state transitions const system = new LoginSystem(); console.log(system.login("wrongPassword")); // Access Denied console.log(system.login("wrongPassword")); // Access Denied console.log(system.login("wrongPassword")); // Account Locked console.log(system.login("correctPassword")); // Account Locked (locked state)
Benefits:
- Helps ensure proper handling of different system states.
- Useful for systems that depend on state changes, such as login systems or workflows.
5. Use Case Testing
Use case testing means testing the software’s operational aspects through predefined user actions or scenarios. Each use case defines the steps the user must take to use the application in the prescribed manner.
Example:
Consider an online shopping platform that needs to ensure students can complete the following use case:
- User logs in.
- User adds an item to the cart.
- User proceeds to the checkout.
- User completes the payment.
class ShoppingSystem { private cart: string[]; constructor() { this.cart = []; } login(username: string, password: string): string { if (username === "user" && password === "pass") { return "Login Successful"; } return "Login Failed"; } addToCart(item: string): string { this.cart.push(item); return ${item} added to cart ; } checkout(): string { if (this.cart.length > 0) { return "Proceed to Payment"; } return "Cart is empty"; } } // Use case testing const shoppingSystem = new ShoppingSystem(); console.log(shoppingSystem.login("user", "pass")); // Login Successful console.log(shoppingSystem.addToCart("Laptop")); // Laptop added to cart console.log(shoppingSystem.checkout()); // Proceed to Payment
Benefits:
- Ensures that key user interactions work as expected.
- Validates real-world user scenarios, making it highly relevant for end-user satisfaction.
Types of Black Box Testing
Apart from the techniques discussed above, many people categorize black box testing into three groups: functional testing, non-functional testing, and regression testing.
However, these are not black box techniques but rather the three main black box testing types that specify what aspect of the application you are testing. Each of these types consists of several techniques, including what we discussed earlier.
1. Functional Testing
This particular type of testing is used to verify an application with respect to user requirements. For instance, the user is supposed to log in successfully by using valid user credentials, whereas there should be an error message when an invalid user credential is used.
Four out of the five techniques we discussed belong to this type:
- Equivalence Partitioning
- Boundary Value Analysis
- Decision Table Testing
- Use Case Testing
2. Non Functional Testing
Non-functional testing measures aspects like application performance, usability, and security. For example, it may test the application’s capacity to support a certain number of concurrent users or response time under heavy load.
Non-functional testing has no direct mapping of the techniques mentioned above but relies on performance metrics, load tests, etc.
3. Regression Testing
The main aim of regression testing is to make sure that new changes or modifications do not affect the stable features of the application.
For example, consider a scenario in which you need to add a new payment setting to the shopping site. Regression testing makes sure the previous payment settings still work as expected, even with the new code changes.
Challenges and Limitations of Black Box Testing
Black box testing is an appropriate approach to evaluating the usability of a software application from the user’s perspective without any reference to the program’s source code. Despite its advantages, black box testing has its fair share of problems.
1. Limited Coverage
In black box testing, testers don’t know how the internal system works. This can lead to missing certain scenarios or conditions, leaving parts of the software untested. Some bugs remain hidden because the tests are not exhaustive, especially for sub-features or intricate logic that are not apparent outside.
Consider the below function as an example. It checks if a given number is an even number or not.
function processUser(age, isMember) { if (age >= 18 && isMember) { return "Access granted"; } else if (age >= 18 && !isMember) { return "Membership required"; } else { return "Access denied"; } }
In black box testing, a tester might check:
processUser(20, true); // Output: "Access granted" processUser(17, false); // Output: "Access denied"
But they may miss edge cases like:
processUser(18, false);
2. Redundant Testing
If the testers do not understand the source code, they will also make their own test cases based on their judgment and experience, creating some overlap within themselves where most of the tests are carried out more than once. This is quite unproductive as time and effort are wasted since resources are used to test similar things instead of exploring new areas that require attention.
For example, consider the function below, which checks whether a given number is an even number.
function isEven(number) { return number % 2 === 0; }
A black box tester might test:
isEven(2); // true isEven(4); // true isEven(8); // true
These tests all check the same path (even numbers), leading to redundant testing, while odd cases are not sufficiently covered:
isEven(3); // false isEven(5); // false
3. Changing Requirements
Since black box tests depend on application requirements, they need to be updated whenever there is a change in requirements. However, as the project scales, regularly updating test cases becomes an impossible task, allowing for bugs to reach production releases.
For example, Suppose you’re testing an e-commerce checkout function that initially allows only one payment method.
function checkout(paymentMethod) { if (paymentMethod === 'credit') { return "Payment processed"; } else { return "Payment method not supported"; } }
Later, the business changes the requirements to accept PayPal. If the black box test cases are not updated, it might miss the new requirement:
function checkout(paymentMethod) { if (paymentMethod === 'credit' || paymentMethod === 'paypal') { return "Payment processed"; } else { return "Payment method not supported"; } }
Old test case:
checkout('paypal'); // Output: "Payment method not supported"
This shows how tests must evolve with changing requirements, and missed updates can cause inaccurate results.
Conclusion
Black box testing is one of the most widely used software testing methods. Its main objective is to validate that the software will be accepted by the user and will operate as intended. Although it is beneficial in many aspects, which include checking the user need and mainly concentrating on the functionality, it also has its disadvantages, such as inadequate coverage, not being able to address the root of problems, duplication, and requiring changes when requirements are revised. Thus, many use black box testing combined with white box testing in order to provide a deep investigation of the software, such as its internal structure and external behavior.