Software Testing - State Transition Testing

State Transition Testing is a Black-Box testing technique used to test how a system behaves when it transitions from one state to another based on events, inputs, or conditions.

It is ideal for applications where the outcome depends on the current state of the system.

Common examples include:

  • ATM machines

  • Login systems

  • Elevators

  • Traffic signals

  • Online order workflows

What Is State Transition Testing?

State Transition Testing checks:

  • The different states the system can be in

  • The events/inputs that cause state changes

  • The valid and invalid transitions

  • The actions or outputs that occur during transitions

A state refers to a specific condition a system is in at a particular moment.

Where State Transition Testing Is Used

This technique is ideal when a system:

  • Has a finite number of states

  • Changes its behavior based on previous actions

  • Requires validation of state-dependent rules

Key Terms in State Transition Testing

1. State

The condition of the system at a given time
(e.g., "Locked", "Unlocked")

2. Event/Input

The action or trigger that causes a state change
(e.g., "Enter correct PIN")

3. Transition

Movement from one state to another
(e.g., Locked → Unlocked)

4. Action/Output

What the system does during the transition
(e.g., Display “Access Granted”)

State Transition Example 1: Login System

States:

  • Logged Out

  • Logged In

Events:

  • Correct password

  • Incorrect password

  • Too many failed attempts

State Transition Table

Current State Input Next State Output
Logged Out Correct Password Logged In Login Successful
Logged Out Incorrect Password Logged Out Error Message
Logged Out 3 Failed Attempts Locked Account Locked
Locked Any Input Locked Access Denied

Interpretation:

  • Correct password → login

  • Wrong password → remain logged out

  • After 3 fails → move to Locked state

State Transition Example 2: ATM Machine

States:

  • Idle

  • Card Inserted

  • PIN Entered

  • Transaction Processing

Transitions:

  • Insert card → goes from Idle → Card Inserted

  • Enter correct PIN → moves to PIN Entered

  • Select withdrawal → transitions to Transaction Processing

  • Remove card → returns to Idle

State Transition Example 3: Traffic Signal

States:

  • Red

  • Green

  • Yellow

Automatic Transitions:

Current State Event/Timer Next State
Red Timer Green
Green Timer Yellow
Yellow Timer Red

Useful for validating automatic system flows.

Types of State Transition Testing

1. Positive Testing

Verifies valid transitions
(e.g., Correct PIN → move to next screen)

2. Negative Testing

Checks invalid transitions
(e.g., Enter PIN when card is not inserted)

Tools Used for State Transition Testing

  • UML diagrams

  • State machine modeling tools

  • Test case management tools

  • Model-Based Testing tools

Benefits of State Transition Testing

  • Excellent for systems with sequential workflows

  • Captures issues related to incorrect state flow

  • Ensures system stability through different scenarios

  • Detects missing or incorrect transitions

  • Helps visualize flows using state diagrams

Limitations

  • Not useful for static systems with no state changes

  • Can become complex when states increase

  • Requires clear understanding of system behavior