Unit, Integration & System Testing

These are the three classic levels of software testing, performed in sequence as components are progressively assembled.

1. Unit Testing

• What: tests the smallest testable unit — an individual function, method, class, or module — in isolation.
• What it addresses: verifies that the unit's internal logic is correct — branches, boundary values, error handling, expected outputs for given inputs.
• Who/how: done by developers during coding, using test drivers/stubs and frameworks (JUnit, pytest, NUnit). Typically white-box (statement/branch coverage).
• Goal: catch local logic errors as early as possible.

2. Integration Testing

• What: tests the interaction between integrated units/modules — the interfaces, data flow, and control flow across module boundaries.
• What it addresses: interface mismatches — wrong parameter passing, incompatible data formats, missing/duplicate functions, ordering/timing problems.
• Strategies:
  • Top-down — test high-level modules first, using stubs for lower ones.
  • Bottom-up — test low-level modules first, using drivers for higher ones.
  • Sandwich / mixed; and Big-bang — all modules integrated at once then tested.
• Goal: find interface/assembly errors that unit tests cannot.

3. System Testing

• What: tests the complete, fully integrated system as a whole against the requirements/SRS.
• What it addresses: end-to-end behaviour, functional requirements, and non-functional attributes — performance, security, usability, reliability, recovery, stress/load, compatibility.
• Who/how: by an independent test team in an environment close to production; primarily black-box.
• Goal: validate that the whole system satisfies its specification.

Comparison table

Aspect	Unit Testing	Integration Testing	System Testing
Scope	Single unit/module	Groups of interacting modules	Whole system
Focus	Internal logic	Interfaces & interaction	Requirements compliance
Test basis	Code design	Module interfaces	SRS/requirements
Nature	White-box	White/black-box	Black-box
Performed by	Developers	Developers/testers	Independent test team
Typical tools	JUnit, pytest	Test harnesses, drivers/stubs	Automation/performance tools
Example	calculateTax() returns right value	Module A correctly calls B's API	End-to-end login → order → invoice flow

Order: Unit → Integration → System (each building on the previous). After system testing come acceptance testing (user validates the system against business needs).