4.2 Black-box Test Techniques

4.2 Black-box Test Techniques

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4.2.1 Equivalence Partitioning

Equivalence partitioning divides data into partitions (also known as equivalence classes) in such a way that all the members of a given partition are expected to be processed in the same way. There are equivalence partitions for both valid and invalid values.

  • Valid values are values that should be accepted by the component or system. An equivalence partition containing valid values is called a “valid equivalence partition.”
  • Invalid values are values that should be rejected by the component or system. An equivalence partition containing invalid values is called an “invalid equivalence partition.”
  • Partitions can be identified for any data element related to the test object, including inputs, outputs, internal values, time-related values (e.g., before or after an event) and for interface parameters (e.g., integrated components being tested during integration testing).
  • Any partition may be divided into subpartitions if required.
  • Each value must belong to one and only one equivalence partition.
  • When invalid equivalence partitions are used in test cases, they should be tested individually, i.e., not combined with other invalid equivalence partitions, to ensure that failures are not masked.
  • Failures can be masked when several failures occur at the same time but only one is visible, causing the other failures to be undetected.

Coverage is measured as the number of equivalence partitions tested by at least one value, divided by the total number of identified equivalence partitions, normally expressed as a percentage. Equivalence partitioning is applicable at all test levels.

4.2.2 Boundary Value Analysis

Boundary value analysis (BVA) is an extension of equivalence partitioning, but can only be used when the partition is ordered, consisting of numeric or sequential data. The minimum and maximum values (or first and last values) of a partition are its boundary values (Beizer 1990).

4.2.3 Decision Table Testing

When creating decision tables, the tester identifies conditions (often inputs) and the resulting actions (often outputs) of the system. These form the rows of the table, usually with the conditions at the top and the actions at the bottom. Each column corresponds to a decision rule that defines a unique combination of conditions which results in the execution of the actions associated with that rule. The values of the conditions and actions are usually shown as Boolean values (true or false) or discrete values (e.g., red, green, blue), but can also be numbers or ranges of numbers. These different types of conditions and actions might be found together in the same table.

The common notation in decision tables is as follows:

For conditions:

  • Y means the condition is true (may also be shown as T or 1)
  • N means the condition is false (may also be shown as F or 0)
  • — means the value of the condition doesn’t matter (may also be shown as N/A)

For actions:

  • X means the action should occur (may also be shown as Y or T or 1)
  • Blank means the action should not occur (may also be shown as – or N or F or 0)

4.2.4 State Transition Testing

Components or systems may respond differently to an event depending on current conditions or previous history (e.g., the events that have occurred since the system was initialized). The previous history can be summarized using the concept of states.

A state transition diagram shows the possible software states, as well as how the software enters, exits, and transitions between states. A transition is initiated by an event (e.g., user input of value into a field). The event results in a transition. If the same event can result in two or more different transitions from the same state, that event may be qualified by a guard condition. The state change may result in the software taking an action

A state transition table shows all valid transitions and potentially invalid transitions between states, as well as the events, guard conditions, and resulting actions for valid transitions.

State transition diagrams normally show only the valid transitions and exclude the invalid transitions. Tests can be designed to cover a typical sequence of states, to exercise all states, to exercise every transition, to exercise specific sequences of transitions, or to test invalid transitions.

The concept of a state is abstract – it may represent a few lines of code
or an entire business process. Coverage is commonly measured as the number of identified states or transitions tested, divided by the total number of identified states or transitions in the test object, normally expressed as a percentage.

4.2.5 Use Case Testing

Tests can be derived from use cases, which are a specific way of designing interactions with software items, incorporating requirements for the software functions represented by the use cases. Use cases are associated with actors (human users, external hardware, or other components or systems) and subjects (the component or system to which the use case is applied).

Each use case specifies some behavior that a subject can perform in collaboration with one or more actors . A use case can be described by interactions and activities, as well as
preconditions, postconditions and natural language where appropriate. Interactions between the actors and the subject may result in changes to the state of the subject. Interactions may be represented graphically by work flows, activity diagrams, or business process models.

Coverage can be measured by the percentage of use case behaviors
tested divided by the total number of use case behaviors, normally expressed as a percentage.

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