What is Test Driven Development Learn in Quick -15 Minutes

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Introduction to Test Driven Development

An agile approach to software development follows the four Agile manifests principles. It aims to deliver quickly the required code to customers at frequent internals enabling return on investment (ROI).

TDD provides a reliable solution. According to IEEE software prediction, TDD reduces 40% – 80% defects.

In “Test-Driven Development: By Example” Kent Beek clearly stated that “If you’re happily slamming some code together that more or fewer works and you’re happy never looking at the result again! TDD is not for you. TDD rests on a charmingly naive geekoid assumption that if you write better code, you will be more successful. TDD helps you to pay attention to the right issues at the right time so you make your design cleaner, you can refine your design as you learn.”

As per Kent Beek – “We never have enough time for testing. So, let’s just write the test first”. – This the prime philosophy behind TDD framework.

Quickly Code implicates-

  1. Code is thoroughly tested.
  2. Code meets the needs of the customer.
  3. Code can be changed easily.
  4. Code is clean well structured, self-documenting.

The technical practices prescribed by Extreme programming (XP) aid in creating quality code while adopting an Agile approach for software development. Test-driven development is one such practice.

Why Test-Driven Development?

As per Bob Martin – “The act of writing a unit test is more an act of design than of verification. It is also more an act of documentation than of verification. The act of writing a unit test closes a remarkable number of feedback loops, the least of which is the one pertaining to verification of function.”

Some facts of other development models

  1. Developers spend 10x more time just reading the code than writing.
  2. 620 million hours of the developers are lost for debugging an issue.
  3. $61 billion are costed for debugging alone.
  4. The average time to fix an issue is 3 hours.
  5. Bugs are 15x costly when it migrated to testing phase than implementation/design phase.
  6. A developer loses 17*3 hours on an average to maintain the code. Tests should be deterministic. Tests should run in a random order (link). Tests should have some percentage of tolerance limit. Test code should be treated as production code.
  7.  Tests should be written on a granular level. Design of tests should be in such a way that they can never be spitted again.
  8.  Test cases must be reviewed by others.
  9. Small fixes or code changes are 40x times error-prone than new development

History of TDD

Test-driven development was greatly inspired by a non-software quality control (QC) practice. Quality control in a non-software area is manual checks for every area of work before delivery.

  • 1976- Glen ford published “Software Reliability” that said developers should not test their own code.
  • 1990- Block box testing evolved so as the capture and replay tools.
  • 1991- SUnit- a test framework based was created.
  • 1994- Kent Back created a framework based on SUnit for small tale application.
  • 1998- The concept of XP (Extreme programming) evolved and it advocated writing a test first.
  • 2000- The mock objects were created.
  • 1998-2002 – Test first/ Test-driven concept flourished, especially after the mock objects.
  • 2003- Kent Back wrote in details on Test Driven Development.
  • 2010- TDD is a matured and heavily used technique.

The six steps of QC are

  1. “Add a check” translated to “Add a test”.
  2. “Run all checks” translated to “Run all tests”.
  3. “Do the groundwork” translated to “Write Down code that works”.
  4. “Run all checks” translated to “Run tests”.
  5. “Clean up the work” translated to “Refactor Code”.
  6. Repeat the process until finish.
Implementation phases of TDD
  • Beek and Astels proposed Test-Driven development in 2003.
  • As per Martin, Newkirk, Kess the TDD is all about the specification and not a process of validation.
  • Ron Jeffries defined the goal of TDD is to write Clean Code that actually works.
  • Kent Beck finally formalize the TDD concept as  TDD = ∑ TFD + Refactoring.
  • Kent Beek’s two rules for TDD –
    1. Developer should write new business code only when an automated test has failed.
    2. Developer should eliminate any duplication that they find.

Basic Principles of TDD

Test-driven development (TDD) is a technical practice consisting of soft development cycles where test cases covering the new functionality are written first. The code is written to pass the tests which are refactored without changing the behaviour of the code.

TDD is otherwise known as test-first development (TFD) and TDD is just the opposite of the traditional development process. Pair programming helps TDD to stay focus and object-oriented.

Developers write automated test cases (tests) before development. This means that development is driven by the tests. This ensures that no untested code goes into the software. Refactoring ensures that the code is “clean”.

The main philosophy of TDD, as mentioned by Kent Back, lies in Ron Jeffries’s phase “Clean Code that works”. TDD helps to design and develop the code in a predictable way by writing only code just enough to make the test cases pass.

Test Driven Development
Test-Driven Development

TDD works on a simple process of writing failing tests (written for new functionality) first. When code is written and refactored to make the test case pass, hypothetical coding is avoided. Only enough code to meet the functionality is written. It helps to create a simple workable code and greatly helps developers for generating a clean, clear and simple code. This code is easy to maintain.

In TDD Murphy’s law of debugging is the best way to actually debug a failing test.

Types of Test-driven development

Primarily TDD is of two types they are as follows:

  1. ATDD (Acceptance Test Driven Development)- In this technique, developers write a single acceptance test. Once the acceptance specification is fixed, developers write just enough code to pass the test. The goal of ATDD is to specify details, executable requirements just in time (JIT). ATDD is also called as Behaviour Driven Development (BDD).
  2. Developer TDD (DTDD) – Develop writes a single test (not a unit test) and developer himself write enough code to pass the test. The goal of DTDD is to specify the detailed, executable design a solution on a just in time (JIT). DTDD is also called TDD.

Test-Driven Development Concept

Test-driven development is a development approach where-

  1. A comprehensive suite of automated tests is maintained.
  2. No code goes into production unless it has associated tests.
  3. Tests are written first. Test cases can be written in several programming languages – Java, Ruby in several automation tools – Selenium, Watir, Windmill.
  4. Tests determine what code needs to be written.
Process Flow of Test Driven Development
Process Flow of Test Driven Development

In simple terms, TDD suggests developers write tests for new functionality or feature and then develop code to pass the tests. This post incorporates the design concepts from Agile modelling and Extreme programming which can be leveraged from Agile development projects.

Working principle of Test Driven Development

  • The developer first needs to understand the requirements to develop an automated test case and to achieve the desired result or feature.
  • Test cases are written first and are then added to the automation suites for execution.
  • When tests are written for new functionality and feature, code would not available for the same. Hence the tests tend to fail. These are called as failing tests.
  • The developer starts adding code to make the tests pass during execution.
  • When the set of tests passes, code is written for the new feature or developer refactors the new code as required to make it compliant with the acceptable standards and provides it for the next stages of testing.

These steps are repeated for any new functionality or feature to be developed.

In TDD way of developing a set of functions is often placed in a class or module. The best way to work with these classes and modules is to keep the unit small in size.

The advantages of smaller units –

  1. Easy and quick debugging – Small units will have lesser test cases. Hence even if an error is found, it is easy to debug and find the root cause of the issue in a short span of time.
  1. Quick self-documentation – Smaller units produce a small set of test cases, the report is then easy to read, understand and act upon

What are the Key Factors to adopt Test Driven Development?

Following are the key drivers for adopting Test-driven development –

  1. Design compliance is one of the visible benefits of test-driven development.
  2. Code maintainability improves.
  3. Acts as a safety net when a developer performs refactoring.
  4. TDD is adopted to deliver the work in a simple way to avoid complexity.
  5. TDD can be considered as both design and programming methodology which is based on a simple rule of writing production code to fix a failing test.
  6. TDD converts normal cycle of design code – test into a sequential activity cycle of the test – code – refactor in which developer writes a test first (initially failing) and then writes code to pass it and further refactor the code.
  7. TDD offers a simple and effective technique which helps to create high-quality software through the continuous design and test process and frequent refactoring. This way, the code becomes simple, less defective, better designed and of high quality. The developer gets confidence in writing code and becomes more productive.
  8. TDD allows us to write tests from the class’s public interface’s perspective as a result it looks after the class’s actual behaviour, implementation of the code takes a lower priority. Hence code created in TDD way produces easy-to-use (not easy to implement) interfaces.
    • Imagine the feature.
    • Divides the feature into smallest sub-tasks keeping the series of tests.
    • Write test cases for each of the sub-task. Use streams significantly.
    • Produce Red Bar.
    • Write production code.
    • Make the test-driven.
    • Refactor to update the code. [avoid complexity, one functionality one test]
  9. TDD should also include non-functional requirements like-
    • Security
    • Performance
    • Usability
    • Maintainability too.

How TDD is different from unit testing?

TDD is not what is similar to unit testing. TDD and unit testing use unit testing frameworks such as XUnit ( JUnit, NUnit etc), however, the purpose and why in which it is used is clearly different.

Here is the difference between unit test and test-driven development

Unit testTest-driven development (TDD)
The unit test is used to test code in isolation.TDD is used to imagine what functionality you need in production and express what is supposed to do you before the code is actually written.
The unit test helps in validating tests and boosting developer confidence about the accuracy of the code.TDD, in essence, is used for incremental design and evolutionary design
Unit testing is aimed at testing and verifying application program. Unit testing is not used to design the application.TDD is preliminary used to design the application and its features. Rather than completely designing application in one go, it is designed to test by test in the incremental fashion.
Unit test methods test a single unit at a time.TDD may test multiple units of code.
The unit test is a bottom-up approached that answers “what to do ?”TDD uses a top-down approach that answers “how to do?”
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How TDD is different from AMDD?

While TDD and AMDD both are a very new concept for a traditional developer. They may do mistakes while operating. Sometimes, it is seen they see these two as a challenge. But in the core, both model supports an evolutionary way of development.

TDD makes programming feedback shorter.AMDD makes modelling feedback loop shorter.
TDD emphasises more on detailed specification.AMDD emphasis more on better design thinking for large issues.
TDD empowers quality code.AMDD empowers quality communication.
TDD produces working software.AMDD produce a better working model by undertaking all stakeholders.
 TDD is all about coders. TDD helps writing smaller code with single responsibility instead of multiple responsibilitiesAMDD is all about different stakeholders with multiple tasks. The scope is very broad.
TDD provides feedback at a granular level.AMDD provides feedback on overall, in a verbal way.
TDD outputs clean design callable and testable code.AMDD outputs the design and architectural issues and their solution before the code starts.
TDD is a non-visual (mostly) process.AMDD is a visual process (mostly).

In a team, there are members who are a visual thinker and some believe working code. So those members who prefer text-oriented design may follow TDD and visual thinkers may follow AMDD. Using AMDD the developers may create different models where TDD developers convert those models to clean code.

At the end mixing these two approaches, we get a very high-quality working system with maintainable and Clean Code that can be delivered.

How TDD is different from BDD?



BDD is written in plain English. BDD is a user-centric approach.TDD is written in a programming language. TDD is a low-level approach.
BDD involves all stakeholders.Mostly developers are involved.
BDD focuses on the behaviour of an application keeping end-users in mind.TDD focuses on the implementation of the functionality.
Changes in requirements can be easily accommodated.TDD supports the changes but implementation is a challenge.
All stakeholders are always updated and always on the same page.Developers only concentrating on a particular piece are updated.
Works great for Medium-sized projects.Best suitable for small size projects but may scale up.

How TDD is different from Traditional testing?

Traditional testing and TDD can never be compared directly. They are two different things. However, when both testing techniques are applied to an application, that yields much more reliable software.

Traditional testingTDD
1. Traditional testing is involved at the last of SDLC.1. Testing is the driving factor hence involved in the beginning of the SDLC.
2. Find defects at later point of time.2. Find defects at the development time.
3. Defects are expense items.3. Defects are the mark of progress.
4. Traditional testing is move of validation of user requirement.4. TDD is a verification of the user requirements.
5. Focus is to design a test case and catch the defects.5. Focuses on production code.
6. 100% coverage is not possible.6.100% coverage can be achieved.

Types of TDD

TDD can be of two types –

  1. Inside out
  2. Outside in

Inside out

This theory is floated by the Detroit School of TDD, otherwise known as a classicist. In this approach, the focus is on the results. Testing starts with the smallest unit level and glows organically. Easy to group for beginners but later may need heavy refactoring. Inside out is also known as the bottom-up approach.

Feature of Inside out model –
  1. Can be used as a supplement of 4 rules of simple design.
  2. Very less or nothing upfront design. But requirements are broken into smaller pieces.
  3. Actual system design is done at the time of refactoring.
  4. All tests are state-based tests.
  5. Initially, it starts with a very small piece of code and during refactoring code may grow huge and can create multiple classes.
  6. Not dependent on test doubles like fake, mock etc.
The flowchart of inside out approach
The flowchart of inside out approach
Advantages of Inside out approach
  1. No over-engineering.
  2. Easy to absorb, understand and implement.
  3. No heavy design upfront.
  4. Excellent opportunity to explore the features.
  5. The process is free from Domain experts.
Drawbacks of Inside out approach
  1. Tests can expose state.
  2. Huge refactoring phase that leads to huge waiting time for the product to go to market.
  3. Units are often getting bigger during the refactoring state.
  4. If developers are not properly trained, they may skip, leave the refactoring phase incomplete.
  5. Developers may create unnecessary classes, methods, interfaces during the exploratory phase.
  6. The actual delivery of code can be delayed and can be treated a waste full experience.
  7. May lead to dissatisfied customers.

Outside in (Top Down / Mockist Approach / London School of TDD)

This theory is floated by the London School of TDD, otherwise known as Mockist. In this approach, the focus is on user behaviour. Testing starts with the outermost level and grows to the innermost level. This approach is dependent on knocking and stubbing. Tough to learn for beginners but ensures all business requirements are covered. Setting up mocks and stubs may cost extra efforts, time, resources.

In this approach, developers start development at a very high level of system architecture. Then they go to the downwards to the lower levels of implementation.

The lower level implementation is progressive in nature. This model is heavily dependent on test doubles like mock, fake pc to get the feel of the functionalities of lower-level components.

The model heavily dependent on the implementation paths (like, start, end) and the mocked portions.

The tests are written keeping user requirements at mind. Then the components are developed and integrated together. This approach also allows for using several APIs.

As the focus is on the complete system, developers get an idea on the different components of the system and how they will be integrated. They can also early predict how these components will react when integrated together.

This phenomenon gives tentative architecture and design approaches for the system upfront.

Features of outside in
  1. Outside-in TDD provides a proper direction from the beginning.
  2. Generally, it starts with an acceptance test in mind.
  3. Failing acceptance test triggers the information gathering phase (why faked, what components) etc.

Popular principles used in TDD-

The popular principles are –

  1. KISS- Keep it simple, stupid.
  2. YAGNI- You are not gonna need it.
  3. SOLID principle.
  4. Fake it till you make it.
  5. DRY -Don’t Repeat Yourself.
TDD tests follow the FIRST technique
  • F = Fast
  • I = Independent
  • R = Repeatable
  • S = Self-validating
  • T = Thorough.

TDD is focused on micro activities and focused on how these can be resolved.

3 rules of TDD
  1. Do not write any production code until it allows a failing test to pass.
  2. Only code is enough to fail a test.
  3. Only code is enough to help the failing test pass.
TDD cycle needs to be –
  1. Short
  2. Rhythmic
  3. Incremental
  4. Design focused
  5. Disciplined

TDD principles are based on Extreme programming –

Three Cs are –

  1. Card – A card talks about the intent of the customer’s request.
  2. Conversation – Once the card has been picked for development, it becomes an open ticket for the stakeholder. The ticket needs more conversation in order to resolve them.
  3. Concrete configuration examples – The value chain provides by a customer and recorded as an example.

In TDD, tests are being tested regularly.

Test-Driven Development and Test Coverage

While developing code using TDD, the code gets tested rigorously and thoroughly. We can set some targets for lines and branch coverage. For instance, let’s say for line coverage it should be above 80% for new changes developed and for branch coverage it should be >85%.

Tools such as Cobertura should be used to measure the coverage. To measure coverage (like J2EE environment), the process described below should be followed –

  1. Write unit tests in an iterative and incremental way.
  2. Write unit tests only for the Java cover as that is new or modified for that functional release.
  3. Change the unit tests to fit the existing code before altering the code.
  4. Alter functional class.
  5. Run unit tests. A unit test should run properly.
  6. Run code coverage checker such as Cobertura.
  7. Make sure that new class line coverage is > 80% and branch coverage > 85%.
  8. Refactor the Code only when the Code Coverage is achieved.

The colour terminology used in TDD-

  • Red – It indicates fails.
  • Green – It indicates a pass.
  • Orange – It indicates refactor.

Important terms of TDD

Code coverage

A systematic software testing method which describes degree by which source code of any software program is tested. Most common methods of Code coverage include statement/line coverage and branch/edge coverage.

Branch coverage testing

A test method which ensures that at least once every possible branch from each decision point is executed. This ensures that the available code is executed.

Line coverage

A testing method which ensures every line of code is tested. Line coverage report gives a percentage of lines of Code of a program executed to complete the test.

Unit testing

A method which ensures individual units of source code are fit for use. Unit is considered as a function or a procedure in procedural programming while in OOP, the unit is an entire interface.

Code refactoring

As defined by Martin Fowler, Code refactoring is a ‘ disciplined technique for restructuring and the existing body of code, altering its internal structure without changing its external behaviour’. Refactoring is done to improve code readability to reduce complexity and to improve Code maintainability.

Coupling of Code

Degree of coupling plays a significant role in software development. Too tight coupling (i.e if the software code is very much interdependent) may create issues during refactoring. The too-loose coupling may create issues in integrating. Then it too much of engineering.

Code re-usability

TDD from the mid of the project heavily depends on code re-usability. Code re-usability not only speed up the development process but also makes the delivery tester by providing a cleaner code. It reduces the

Mocking and Stubbing

The module may need to interact/interface with other modules and external resources (database/middle-tier). Mocks and stabs are used to test whether the written code works when interfaced with other module or external resources.

Mock objects are tempting to use. Developer tends to use Mock objects very frequently. But remember, Mock objects can introduce complexity interns of code and execution time. TDD principle says to run a test in second. Mock objects can violate this principle. Instead of Mock Developers should check if the Mock is really necessary or they can decouple this external dependency in another way.

Dummy– always provides a default return value.

Stub – Stub is a class that is hardcoded to return data from its methods and properties. It is used inside unit tests to test a clear method and derives the expected output for a known input. It provides different output based on the condition (added logic to dummy).

Spy- A spy is a process to capture parameters, state information from a live system to validate state.

Simulator– A component that provides approximately the target capabilities.

Mock -Mock is used to creating a simulated object in place of the actual project.


//using stub
public Customer createDummyCustomer () {
	Country country = new Country("India");
	City city = new City ("Delhi", country);
	Address address = new Address ("1234", "ABC street", city);
	Customer  customer = new Customer ("XYZ", "MNO", address);
@ Test
public void addCustomerTest(){
	Customer dummy = createDummyCustomer ();
	AddressBook addressBook = new AddressBook();
	addressBook.addCustomer (dummy):
	Assert.assertEquals(1, addressBook.getNumberOfCustomer());
//using mock
public void addCustomerWithDummyTest () }
	Customer dummy = mock(Customer.class);
	AddressBook addressBook = new AddressBook();
	Assert.assertEquals(1, addressBook.getNumberOfCustomer();

The test structure of Test-Driven Development –

As the tests are the heart of test-driven development, test design needs to be taken care of. Test cases should make sure that –

  1. They are complete in nature.
  2. They are independent.
  3. They are easy to read and understand.
  4. They should depict the flow clearly.

Each test case should have the following points covered –

  1. Set up
  2. Body of test
  3. Validation point
  4. Tear down or clean up

Set up -> Create the required predefined condition to execute the test case.

Body of test -> Actual steps to perform testing. It must include the inputs, outputs and target behaviour. A code that is hard to test in isolation is poorly designed and developed. The delivered code is costly to change.

Validation -> The steps to check or assert if the outcome of the test is correct.

Tear down/clean-up -> These steps are to return the application to its original state where the test starts. Tear down steps are very important to stop data or state population.

However, developers can create a common setup and clean-up logic for all their tests.

Avoid anti-patterns during test development –

  1. The test must start from a known and pre-configured state.
  2. Write independent tests and avoid interdependent tests. They all contribute to cascading false negatives. The root cause of defect detection is tough.
  3. The test should be executed fast and provide a report.

How to Apply Test Driven Development in Your Project?

TDD is an iterative and incremental procedure. TDD does not mean that developer can start working on code only after writing all the tests. A developer can first build the basic and high – level tests and once those are passed, the various other tests may be added. Duplication must be avoided while completing functionality and development.

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Step – 1  Add a test

TDD is such a technique that makes sure that committed source code is thoroughly tested. In traditional testing, a failing test ensures the existence of a bug. Similarly, in TDD, a failing test ensures a triggering point to write code to cover an area.

Adopting TDD means unlearning the traditional development process and start focusing on test-driven development. Self-learning is very tough.

This bottom-up approach actually gives confidence in development as TDD ensures 100% test coverage.

The art of TDD development
The art of TDD development

A well-written test can remove the necessity of documentation. The execution report can often replace the need for boring documentation. Interestingly developers do not like to read the documentation rather they want a clean and working code. TDD provides both. However, we need to note that tests or execution report is not certainly sufficient documentation. But they help to showcase the capabilities of the software.

New test gets added for every new story or task to be developed. This is the basic requirement for that particular user story. The test should be written in an automated way using various tools such as JUnit or NUnit. The developer must thoroughly understand the requirements of the story, before writing the test. This test will fail once the developer puts it on the automated framework.

TDD can be applied to older projects (like a legacy project).

TDD test cases can be very exhaustive and it can reduce the need for writing the same test cases. Testers can try the rare and common case. Save manual testing time and effort.

Phases of TDD
Phases of TDD

Step – 2  Run test

Next step is to run the test. This helps serve two purposes.

  1. Running of test assures that new Code has been added, which is a criterion for passing the test. This also makes sure that the functionality is not already developed as part of any other story.
  2. A failing test is the first attempt towards TDD efforts. Failure in the first attempt implies that TDD development is in progress.
red green refactor cycle
red green refactor cycle

Intermittent failures or sync issue failures are a very common challenge in TDD via automation. There are several reasons a sync issue may occur – (part of No element Exception found).

Make sure the tests are run two or three times to see what are actual failures. Intermittent and sync failures generally go away when they are executed two times consecutively. The remaining issues need proper care.

Step – 3  Make changes

Next, make changes in the test and add in such a way that the test passes. Passing test includes the process of writing code and making changes. To pass this test, Code written in the test should be isolated only for passing this particular test. No other code or functionality should be added.

  • The code development activity is an organic activity where the execution report aids the decision.
  • None other than developers themselves need to write test cases.
  • The development environment will provide some basic setup to compile code faster and execute test case faster. This is required to provide a rapid response for the smallest change a developer makes.
  • The components developed are consistent, cohesive and loosely coupled.
  • The tests will be very fast (small setup, a quick run, easy tear down)
  • The tests will be independent of each other so that they can be executed in any order and in isolation manner.
  • Test data should be easy to read and maintain.
  • Developers can use actual production like data as and when required they can use Delphix to mask and create production-like data.
  • Each test case is a small task of the overall goal.
  • TDD keeps test cases up to date.

Refactoring also involves-

  1. Omit of methods that are long.
  2. Lots of If/else statements along with switch cases.
  3. Logic simplifications.
  4. Change designs to upgrade.

TDD is a strict approach involving three pillars –

  1. Coding
  2. Testing
  3. Designing (refactoring)

    TDD approaches
    TDD approaches

Step – 4  Run all tests

In case the test does not pass in the first attempt the test must be run again. Failing in the test proves that it is not covering the functionality required by the story and it requires further changes.

The passing of tests assures that required functionality is covered. After passing the test, further changes need to be done to refactor the Code. Concept of refactoring can be described as cleaning of Code applying proper design pattern. This also assures that duplication of Code is absolutely avoided.

In one sprint we may have multiple stories. To keep adding more tests and new features of functionalities, the cycle of tests explained above can be repeated.

Tips to Make TDD better –

  1. Maintain the TDD test code just like application production code.
  2. End to end test may not always reveal the bug, developers must take care of the other related tests (Web Services).
  3. Do not decouple the application test code if that is not providing any significant advantages.
  4. MVC controllers should be tested integrated way. Developers must not write unit testing code for MVC.
  5. Include databases as far as possible. Removing the database from test code will not give significant benefits.

How to work on TDD on a legacy system?

As per feathers, “legacy code is some kind of Code that is without any / significant test and developers are afraid of change such code”.

In general legacy, codes are written way back to solve some problem at that time. The solution is only focused to solve the problem. As there are no tests around it, they are not designed for test-ability. Now it is the classic example of an egg-chicken paradox. In TDD way introduction of tests needs change in the code. Now to change the code in a proper way, test introduction is necessary.

Legacy code also has technical dept. It is like spraying salt on the wound. Developers fight to understand how the system actually works and what are the hidden side effect of the methods or functions.

So, while working with a legacy system with TDD, it is better to create a set of smoke test cases. These test cases will not prevent issues but can detect if something went wrong due to recent changes. It works as a safety net around developers.

Once the smoke tests are active, developers can start creating test cases against a feature and follow TDD process to upgrade tests as well as application code.

How to understand the project is in contract track for TDD implementation?

  1. Check the code coverage. A relatively high coverage does not talk about the perfect use of TDD. However, less than 80% of code coverage indicates that the team has not gained enough knowledge in TDD.
  2. Check the version log. A perfect TDD project should check-in test and product code almost at the same time.
  3. Check the process- developers should write the test first then code. They must write just enough code to make the failing tests pass.
  4. Test drive in design (object-oriented, functional, event drive, SOLID, YAGNI, KISS) through test drive in domains (GUI, Calculation, Computation. Persistent data access etc.).
  5. Check the road map creation- Team should be able to produce planned dates for granulate level features.
  6. Check the reusability factor- Team should be able to extract reusable components, elements, tests, code so that they can be clubbed with other areas.
  7. Check the bug fixing technique- Team should write enough test to cover the defect (when found).

Code visibility of test-driven development

In general, we write Unit testing test cases along with the code in the same project where the application code is being kept.

This allows test cases to access the application code. During TDD way of testing, developers need to make sure the classic oops practices like information hiding, encapsulation, concern separation etc. should also be maintained.

Private data and methods do not allow access from outside. In order to test this feature, developers need to work extra miles.  Popular ways to resolve these issues are via- reflection and inner class.

Any testing related substance should not be present in production code. Developers can use compiler directives to avoid them executing independent manner.

Test framework must accept the unit test output. For this test, anything protocol(TAP) has been created. It works in a language-independent manner.

TDD can extensively use Test Doubles (Dummy, stub, Spy, Simulation, Fake object, Mock object) to speed up the testing process.


How TDD helps to get better ROI?

Before ROI, we need to understand how TDD provides values to customers.

How TDD is providing value

  1. TDD can determine the requirement issues, early in the development cycle.
  2. TDD provides rapid valuable feedback thus software passes through many small changes.
  3. Effective refactoring reduces risk and impact.

With the below advantages, TDD can actually help an organization to obtain better ROI.

  1. Reduce cycle time of development.
  2. Quick, easy feedback.
  3. Rapid test.
  4. Supports constant changes.
  5. Self-certified code.
  6. Detects bugs early.
  7. Provides huge code coverage.

How to review projects Using TDD Test Diven Development Approach?

Following things can be reviewed to check if the project is really following a TDD approach:

Make sure that all the evidence listed here are available for every release and available to see in continuous integration dashboard.

  • > 10% Automated for new features (Unit testing)- Have a unit test for technology is added to Continuous Integration dashboard appropriate sections.
  • Unit tests should be written as standard (No expectations).
  • 80% line Coverage and 85% branch Coverage are achieved (On new code/ No expectations).

Opportunistic refactoring of legacy Code- Unit test will be written for untested legacy Code.

  • A unit test needs to be written for any method that is altered in a legacy class/ module of Code.
  • 80% line Coverage and 85% branch Coverage are achieved (No expectations).

Per release review of Unit test on each Component

  1. Report to detail the following –
  2. Are test being written using TDD?
  3. Has there been an increase in Testing for a Component?
  4. Defining where refactoring has occurred.

How TDD fits into Agile projects?

TDD’s main concepts follow Agile manifests and they abide by XP (Extreme Programming) principles.

Agile is a feedback-driven development. In TDD, the team gets rapid feedback. As a result, TDD can be easily fit into Agile space.

Secondly, In TDD, developers write tests from requirements. Not from the already developed code. The product owner or the requirement creator, testers and developers must collaborate for writing better tests. This is another principle of agile that everybody should be the same page, and write for the same goal.

Lastly, in TDD if attest pass and make the application ready, there is no need to test further from development end. An automated way to testing will pick it up from there. Developers can look into the new feature. Automation testing can push code to the next level or if it fails, it produces the same to the backlog.

What are the tools for Test-Driven Development?

There many tools which can help in developing a perfect test-driven development. Test written in TDD is automated unit tools. There are unit test writing tools available in almost all the languages (like JUnit for Java, Cunit for C, NUnit for Net, PHP Unit for PHP and UB Unit for VB).

Tool   NameDetails
JenkinsUsed as CI server
ANTFor automating using ANT script
Cheek StyleTo cheek adherence its Coding Standards
Find bugsThe static code analysis tool, this is used to select a wide range of problems by analysing the Java byte code.
PMDIt scans the source Code and further looks for potential problems, possible bugs, used code, complicated expressions and duplicate Code.
JdependJava package dependency analyser which generates design quality metrics
CoberturaCode Coverage tool. It is used for Java Coverage analysis.
XUnitFrameworkAny language’s unit test framework.

  1. csUnit- open source unit test tool for.NET project.
  2. PyUnit- Unit testing framework for Python project.
  3. PHPUnit- Unit testing framework for PHP project.
  4. NUnit- Unit testing framework for .Net project.
  5. Junit- Unit testing framework for Java project.
  6. DocTest- Unit testing framework for Python.
  7. Jest- for unit testing
JProbeUsed for enterprise-class Java profiling.
nvmfor Node.js application
ESlintfor linting
Prettierfor formatting
LogmonLog monitoring tool

How many tests should a developer write in TDD?

There is no specific role on the quantity of test case, a developer must write. But it tends to a minimum number or just enough test. Note- Every test case slow down the process and become challenging to maintain and refactor.

Secondly, TDD should not cover 100% of the test. However, in ideal condition, it should cover 100%. But TDD developers can omit the areas where data is coming from typically 3rd party APIs. They can essentially use test dongles.

TDD test cases should be short, to the point and focused on single responsibility of a function. The tests are the initial question that you ask yourself about the feature.

The misconception of TDD Implementation

The below section of the post talks about common misconceptions about TDD and different challenges faced while executing projects using TDD.

The common conceptions of TDD are as follows –

  1. TDD is a theoretical term and may not be beneficiate in actual practice.
  2. Independent or manual testing is not required with TDD. By using TDD, the user does not need independent manual testing is a myth. Independent/manual testing is always essential to obtain equal software. Myth – In TDD, we only need unit testing. Reality – TDD requires unit testing along with several other testing techniques.
  3. Myth – The unit tests covers 100% of the design specification. Reality – Unit testing may cover 50% to 60% of the design specification. But the acceptance testing team covers 100% design specification.
  4. The unit test makes 100% design specifications – unit test help in design specifications. As the developer thinks about the production code before they write the test in TDD and assume that it helps in creating detailed design effectively tests. Myth – Developers are creating 100% regression test suit.Reality – Actually no! even though it sounds great but they actually test a portion of UI test mostly happy paths and very less rainy paths. Actual testers cover all aspects of GUI testing and create a fully working regression suit.
  5. One can achieve 100% coverage using regression suits. It is observed in many agile projects that achieving 100% coverage is not possible because of the third party component or framework in the code, which might not have source code for test cases. As users might be working on legacy functionality or systems, there might be user interface which is hard to test.
  6. Myth – TDD outputs a set of test cases. Those test cases are sufficient to achieve quality. Reality – TDD produces a set of test cases but they are part of the actual testing effort. Other parts of testing should also be factored.
  7. Myth – TDD is for small project and does not scale enough to accommodate large projects. Reality – This is, however, true due to developers inability to take TDD into scale. The driving factors for this believe are –
    1. Even for a small project tests take longer time to execute.

    [Use latest CI-CD technique in a good machine].

    1. Every developer does not know how to test in TDD.

    [Skill issue – mindset issue – need coaching]

    1. Developers do not want to update TDD.

    [Create a team with a similar goal – better coaching].

The challenges Adopting and Executing of TDD Implementation

  1. Changing the developer’s mindset is challenging. Developers want to develop the Code and not necessarily like to test it. Hence in case of test-driven development, writing test cases first and then develop the code in an automated way, is found challenging by developers.
  2. It is a challenge to apply test-driven development approach for legacy Code bases.
  3. Applying TDD and writing automated tests for a multi-threaded environment is a challenge.
  4. Developers feel that writing a test first and then to develop the product may slow down the development process.
  5. It is believed that it is tough to follow TDD in high-pressure work conditions or stringent timeline.
  6. It is a challenge to apply TDD in a long-running project where the Codebase is already developed without using the test-driven approach. Applying TDD concepts to already developed untested Codebase is a challenge.
  7. Developers rarely estimate the effort required for TDD and this may pose to be a challenge.

Do you need a coach for TDD implementation in your project?

TDD is one of the toughest practices to implement? In this sense, TDD projects need coaches. Next question what type of coach is required?

Coaches can be of two types –

  1. Company certified internal coaches.
  2. Qualified external coaches.

Due to the cost factor in mind company tend to hire fewer numbers of external coaches. External coaches are necessary if the company does not have adequate internal coaches or setting up a fresh.

Internal coaches are on the other hand are assets to the company. They can reduce the dependency on external coaches and reduce the cost to a great portion.

The internal coaches can do wonder if

  1. The proper structure is in place or management is milling to support internal coaches to create one.
  2. Enough time is reserved for coaching.
  3. Developers are eager to learn a new way of development.
  4. Coaching skills are well appreciated and valued.

But most of the company treats the coaching, training and related skill development as a non-productive waste of time and resource. This is how the internal coaching for TDD fails.

But the combination of both internal and external coaches can do a significantly great job.

The effective term structure of coaches
The effective term structure of coaches

Common Mistakes of TDD

While operating a project in TDD, the teams often do the below mistakes-

  1. Write a lot of test at the beginning.
  2. Write long tests.
  3. Write trivial tests.
  4. Do not execute the tests frequently.
  5. A subset of developers follows TDD (partial adoption).
  6. A subset of developers writes tests and another set of developers write actual application code (Team division).
  7. Once the test is written, the code base is never maintained causing issues in long run.
  8. Design and write a class from the requirement specification then write test cases to cover the design or class. Very slow approach and learning curve go down.
  9. Write production code from the requirement specification then write test cases. – This is very tough to achieve.

Advantage of Test Driven Development

  1. Adds reliability to the development process. Once TDD is implemented in the project, if we write more tests, we become more productive. TDD test cases are directly proportional to productivity. Each line of code added is actually tested, it increases the developer’s confidence.
  2. More focused development-Test-driven development makes better sense as the main focus is to develop a better automation testing solution. Developers strive to improve their code without the fear associated with code changes. TDD can focus on customer’s need and can produce a better design.
  3. Easy Maintainance-Encourage programmers to maintain an exhaustive set of repeatable tests which may be run selectively or exhaustively with the help of tools.
  4. TDD can remove the fear factor of change requirements.
  5. Faster results – It provides immediate feedback to reduced revision errors. Developers rights only the code that is required resulting in faster results with clear, comprise code. TDD does not allow to take care of exceptional and error cases. Thus, actual development becomes faster.
  6. More Reliable code– Makes programming more than due to the reliability of the code.
  7. Less time in debugging-Remove / Reduce dependency on the debugger and no postponed of debugging.
  8. Simplicity – Clean work, modular and less complex code. It also offers shorter development cycles. TDD divide the project into smaller parts (sub-tasks) as a result developer can put smaller steps forward by completing smaller sub-tasks sequentially.
  9. Improved quality – Automation is the biggest advantage of adopting TDD. However, it reduces errors. TDD generates extensible, modularized, flexible and clean code. This helps easy integration of modules. Finally, TDD generates smaller, loosely coupled, cleaner classes and interfaces.
  10. Flexibility – Flexibility is inbuilt due to quick comments, short cycles and automation. TDD helps mocking hence dependency on third party plugin software is less.
  11. Rapid Respond- Has the ability to respond rapidly to all the changing requirements.
  12. TDD generates better software by reducing over-engineering.
  13. TDD poses as a safety net around the development team while adding new features to the application.
  14. TDD can lower the batch size (The volume of work a coder does in every step). TDD advocates to create an average size batch or a smaller size batch with more accuracy and consistency.
  15. TDD is more controlled development as result we get very fewer bugs.
  16. High coves ion – Every module provides a series of related capabilities that allows tests to maintain easily.

Disadvantages of Test Driven Development

  • When automation is introduced to manual testing, we may face much-unexpected resistance from the team.
  • All forms of TDD need stakeholders to have in-depth testing skills. This is very tough in a real-time scenario.
  • Unlike the GUI mode of TDD, test-driven database design (TDDD) is not effective due to underlying tool support (even though DB Unit is there).
  • TDD is not capable of scaling up that means it is not that much effective while designing a large project. (For we need to take help from Agile model-driven development (AMDD).
  • TDD does not do full testing (functional). As a result, a separate group of testers are always needed to create the regression suite – Extra sort to company.
  • Full hearted management support is essential for TDD projects to success. It is more of a top-down approach while adopting and practising TDD. But many times, management believes that writing test cases are waste of time, runaway and resources.
  • TDD may be affected due to the blind spot of the coders and heavily dependent on developers understanding.
  • Badly formatted, badly written tests (hardcoded values) are prone to error and failure. They are tough, expensive to maintain. These test cases are risk-prone, fragile.
  • TDD advocates a lot many test cases. These excessive number of test cases needs more time to write and execute. They need additional sets of data to perform. But due to these set up actual delivery of software may get hampered.
  • TDD has a relatively long learning curve (approximately 1.5 years to 2 years).
  • Starting a project in a TDD way and deliver the output is significantly slower than any other model.
  • TDD may need external coaches which is a significant cost factor for a company.
  • TDD does not allow easily to apply any standard practice.
  • TDD generates higher numbers of passing tests that may give a false positive or false sense of security.
  • Test rather poorly created tests become maintenance overhead later point of time.
  • The details of the test cases achieved during the tenure of TDD cannot be recreated so easily.

Where TDD can fail?

  1. If developers work on a non-TDD approach to work on a TDD project.
  2. If the developers lack in TDD knowledge or lack in TDD coaching.
  3. If the developers are solving complex scenarios.
  4. If the developers are working on a project where requirements are changing very frequently.

TDD can also fail if we assume the below written:

  • TDD is a great compliment to regular testing (automated and manual). But TDD is going to replace the traditional testing.
  • TDD can streamline the project from day one.
  • TDD helps to reduce bugs and coupling.
  • TDD is an iterative process.
  • TDD cannot automatically fix your algorithms. It can also not replace your logic.
  • TDD reduces the ‘total cost of ownership’ (TCO) to a great extent for a software project.

Related terms


Acceptance test-driven development –

Modern software developments are tool-assisted. Tools can help development with automatic test and task running.

The other benefit for the tool is that for execution, developers need not necessarily involve in execution. Tests can be executed automatically, in the background.

  • Represents another form of TDD
  • Translates set requirements into a set of executable acceptance tests and implements code against the test.
  • Good practice to start with TDD and stability increase, move to ATDD.
Advantages of ATDD-
  • TDD significantly reduces the new of Debugging.
  • Enhance the user experience.
  • Can decrease overall development time for a project.
Disadvantages of ATDD-
  1. TDD pushes back the big picture design.
  2. TDD is tough to apply in all cases.

Behaviour driven development (BDD)

  • Focuses or behaviour exhibited by the system rather than implementation.
  • Focuses on language and interactions used in the software development process.
  • Uses native language (like English) and the language of domain design to describe the written Code.


TDD generates very little amount of bloated code and provides a more stable code that requires very little refactoring later. On the other hand, significant time is consumed in traditional automation testing by test maintenance activity. But in TDD the self-healing capability makes it more efficient than the traditional way of testing. Finally, TDD can reduce regression testing effort.

TDD is not fully testing oriented and not fully designed oriented. TDD does not talk write some tasks and then build the production code to pass those test cases. TDD does not also talk about to do lots of testing. Scenario modelling can be a better fit for a complex project to implement TDD in a scaled-up manner.

TDD delivers better, a fault to learnt code. Every developer must practice TDD for the betterment of the project. TDD does not handle risks. If you run the test cases every time at once, you will be able to see the actual failures along with some extra failures that were never anticipated.

TDD works perfectly well with pair programming (If paired between a developer and a tester).

TDD is another developmental model where developers write the test first then the application code. If the toughest part (the setup) is achieved successfully, TDD can bring lots of advantages including cost-saving, delivering true value to customers etc. On the other hand, if TDD is not properly set up, it lets the project down. It becomes a waste of time and resource machine.

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