Black Box Testing

Black Box Testing

• Black-box testing also called behavioral testing,
• It focuses on the functional requirements of the software.
• Functional testing of a component of a system
• Examine behavior through inputs & the corresponding outputs.
• Input is properly accepted, output is correctly produced.
• Black-box testing attempts to find errors in the following categories:
• (1) Incorrect or missing functions
• (2) Interface errors
• (3) Errors in data structures or external database access
• (4) Behavior or performance errors
• (5) initialization and termination errors
• Black box testing is used during the later stages of testing after white box testing has been performed.
• Different Black box testing techniques
• (1) Graph Based Testing Methods
• (2) Equivalence Partitioning
• (3) Boundary Value Analysis
• (4) Orthogonal Array Testing

Black box testing techniques - (1) Graph-Based Testing Methods

• The first step in black-box testing is to understand the objects that are modeled in software and the relationships that connect these objects.
• Once this has been accomplished, the next step is to define a series of tests that verify “all objects have the expected relationship to one another”.

• Graph Representation
• A collection of nodes that represent objects,
• Links that represent the relationships between objects,
• Node weights that describe the properties of a node (e.g., a specific data value or state behavior),
• Link weights that describe some characteristic of a link.
• The symbolic representation of a graph is shown in Figure.
• Nodes are represented as circles connected by links that take a number of different forms.
• A directed link (represented by an arrow) indicates that a relationship moves in only one direction.
• A bidirectional link, also called a symmetric link, implies that the relationship applies in both directions.
• Parallel links are used when a number of different relationships are established between graph nodes.  

Black box testing techniques - (2) Equivalence Partitioning Method

• Equivalence partitioning is a black-box testing method that divides the input domain of a program into classes of data from which test cases can be derived.
• An equivalence class represents a set of valid or invalid states for input conditions.
• Equivalence classes guidelines
• If an input condition specifies a range,
• one valid and two invalid equivalence classes are defined
• Input range: 1 – 10 Eq classes: {1..10}, {x < 1}, {x > 10}
• If an input condition requires a specific value,
• one valid and two invalid equivalence classes are defined
• Input value: 250 Eq classes: {250}, {x < 250}, {x > 250}
• If an input condition specifies a member of a set,
• one valid and one invalid equivalence class are defined
• Input set: {-2.5, 7.3, 8.4} Eq classes: {-2.5, 7.3, 8.4}, {any other x}
• If an input condition is a Boolean value,
• one valid and one invalid class are define
• Input: {true condition} Eq classes: {true condition}, {false condition} 

Black box testing techniques - (3) Boundary Value Analysis Technique

• A greater number of errors occurs at the boundaries of the input domain.
• It is for this reason that boundary value analysis (BVA) has been developed as a testing technique
• Test both sides of each boundary
• Look at output boundaries for test cases
• Test min, min-1, max, max+1, typical values
• Example : 1 <= x <=100
• Valid : 1, 2, 99, 100
• Invalid : 0 and 101
• Guidelines for BVA
• If an input condition specifies a range bounded by values a and b, test cases should be designed with values a and b and just above and just below a and b.
• If an input condition specifies a number of values, test cases should be developed that exercise the minimum and maximum numbers. Values just above and below minimum and maximum are also tested.

Black box testing techniques - (4) Orthogonal Array Testing

• Orthogonal array testing can be applied to problems in which the input domain is relatively small but too large to accommodate complete testing.
• The orthogonal array testing method is particularly useful in finding region faults — an error category associated with faulty logic within a software component.
• Consider a system that has three input items, X, Y, and Z. Each of these input items has three discrete values associated with it. There are 3^3 = 27 possible test cases.
• Phadke suggests a geometric view of the possible test cases associated with X, Y, and Z illustrated in Figure…
• Used when the number of input parameters is small and the values that each of the parameters may take are clearly

• To illustrate the use of the L9 orthogonal array, consider the send function for a fax application.
• Four parameters, P1, P2, P3, and P4, are passed to the send function. For example : Function (p1,p2,p3,p4)
• Each takes on three discrete values P1 takes on values:
• P1 = 1, send it now 
• P1 := 2, send it one hour later
• P1 = 3, send it after midnight
• P2, P3, and P4 would also take on values of 1, 2, and 3, signifying other send functions.
• If a “one input item at a time” testing strategy were chosen, the following sequence of tests (P1, P2, P3, P4) would be specified: (1, 1, 1, 1), (2, 1, 1, 1), (3, 1, 1, 1), (1, 2, 1, 1), (1, 3, 1, 1), (1, 1, 2, 1), (1, 1, 3, 1), (1, 1, 1, 2), and (1, 1, 1, 3).

The L9orthogonal array testing approach enables you to provide good test coverage with far fewer test cases than the exhaustive strategy

Black Box Testing - Advantage & Disadvantages

• Advantage
• Find missing functionality
• Independent from code size and functionality.
• Disadvantages
• No systematic search for low level errors.
• Specification errors not found.