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Introduction Software is tested from two different perspectives: 1. Internal program logic is exercised using ―white box‖ test case design techniques. 2. Software requirements are exercised using ―black box‖ test case design techniques. In both cases, the intent is to find the maximum number of errors with the minimum amount of effort and time. Black-box testing alludes to tests that are conducted at the software interface. Although they are designed to uncover errors, black-box tests are used t
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  Introduction Software is tested from two different perspectives:1.   Internal program logic is exercised using ―white box‖ test case design techniques.  2.   Software requirements are exercised using ―black box‖ test case design techniques.  In both cases, the intent is to find the maximum number of errors with the minimum amount of effort and time.Black-box testing alludes to tests that are conducted at the software interface. Although they aredesigned to uncover errors, black-box tests are used to demonstrate that software functions areoperational, that input is properly accepted and output is correctly produced, and that theintegrity of external information (e.g., a database) is maintained. A black-box test examinessome fundamental aspects of a system with little regard for the internal logical structure of thesoftware. Black box Testing Techniques Black-box testing, also called behavioural testing, focuses on the functional requirements of thesoftware. That is, black-box testing enables the software engineer to derive sets of inputconditions that will fully exercise all functional requirements for a program. Black-box testing isnot an alternative to white-box techniques. Rather, it is a complementary approach that is likelyto uncover a different class of errors than white-box methods.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.   behaviour or performance errors, and5.   initialization and termination errors.Unlike white-box testing, which is performed early in the testing process, black box testing tendsto be applied during later stages of testing.Because black-box testing purposely disregards control structure, attention is focused on theinformation domain. Tests are designed to answer the following questions:    How is functional validity tested?    How is system behaviour and performance tested?    What classes of input will make good test cases?    Is the system particularly sensitive to certain input values?    How are the boundaries of a data class isolated?    What data rates and data volume can the system tolerate?     What effect will specific combinations of data have on system operation?There are various techniques in developing test cases for black box testing. I will be explainingtwo of the most common techniques, Equivalence Partitioning and BVA (Boundary ValueAnalysis) Equivalence Partitioning Equivalence partitioning is a black-box testing method that divides the input domain of aprogram into classes of data from which test cases can be derived. Test case design forequivalence partitioning is based on an evaluation of equivalence classes for an input condition.An equivalence class represents a set of valid or invalid states for input conditions. Typically, aninput condition is either a specific numeric value, a range of values, a set of related values, or aboolean condition. Equivalence classes may be defined according to the following guidelines:1.   If an input condition specifies a range, one valid and two invalid equivalence classes aredefined.2.   If an input condition requires a specific value, one valid and two invalid equivalenceclasses are defined.3.   If an input condition specifies a member of a set, one valid and one invalid equivalenceclass are defined.4.   If an input condition is boolean, one valid and one invalid class are defined.As an example, consider data maintained as part of an automated banking application.The user can access the bank using a personal computer, provide a six-digit password, andfollow with a series of typed commands that trigger various banking functions. During the log-onsequence, the software supplied for the banking application accepts data in the form:    area code  —  blank or three-digit number    prefix  —  three-digit number not beginning with 0 or 1    suffix  —  four-digit number    password  —  six digit alphanumeric string    commands  —  check, deposit, bill pay, and the likeThe input conditions associated with each data element for the banking application can bespecified as area code:    Input condition, Boolean  —  the area code may or may not be present.    Input condition, range  —  values defined between 200 and 999, with specific exceptions.    prefix: Input condition, range  —  specified value >200    Input condition, value  —  four-digit length    password: Input condition, Boolean  —  a password may or may not be present.    Input condition, value  —  six-character string.    command: Input condition, set  —  containing commands noted previously.  Applying the guidelines for the derivation of equivalence classes, test cases for each inputdomain data item can be developed and executed. Test cases are selected so that the largestnumber of attributes of an equivalence class are exercised at once. Boundary Value Analysis BVA extends equivalence partitioning by fo cusing on data at the ―edges‖ of an equivalence class.For reasons that are not completely clear, a greater number of errors tends to occur at theboundaries of the input domain rather than in the center . It is for this reason that boundaryvalue analysis (BVA) has been developed as a testing technique. Boundary value analysis leadsto a selection of test cases that exercise bounding values.Boundary value analysis is a test case design technique that complements equivalencepartitioning. Rather than selecting any element of an equivalence class, BVA leads to theselection of test cases at the edges of the class. Rather than focusing solely on input conditions,BVA derives test cases from the output domain as well.Guidelines for BVA are similar in many respects to those provided for equivalence partitioning:1.   If an input condition specifies a range bounded by values a and b, test cases should bedesigned with values a and b and values just above and just below a and b.2.   If an input condition specifies a number of values, test cases should be developed thatexercise the minimum and maximum numbers. Values just above and below minimumand maximum are also tested.3.   Apply guidelines 1 and 2 to output conditions. For example, assume that a temperaturevs. pressure table is required as output from an engineering analysis program. Test casesshould be designed to create an output report that produces the maximum (and minimum)allowable number of table entries.4.   If internal program data structures have prescribed boundaries (e.g., an array has adefined limit of 100 entries), be certain to design a test case to exercise the data structureat its boundary.Most software engineers intuitively perform BVA to some degree. By applying these guidelines,boundary testing will be more complete, thereby having a higher likelihood for error detection. Black Box Testing: Types and techniques of BBT I have covered what is White box Testing in previous article. Here I will concentrate on Black  box testing. BBT advantages, disadvantages and and How Black box testing is performed i.e theblack box testing techniques.  Black box testing treats the system as a “black  - box” , so it doesn’t explicitly use Knowledge of the internal structure or code. Or in other words the Test engineer need not know the internal working of the ―Black box‖ or application.   Main focus in black box testing is on functionality of the system as a whole. The term „behavioral testing‟ is also used for black box testing and white box testing is also sometimescalled „structural testing‟ . Behavioral test design is slightly different from black-box test design  because the use of internal knowledge isn’t strictly forbidden, but it’s still discouraged.Each testing method has its own advantages and disadvantages. There are some bugs that cannotbe found using only black box or only white box. Majority of the applicationa are tested by black box testing method. We need to cover majority of test cases so that most of the bugs will getdiscovered by blackbox testing.Black box testing occurs throughout the software development and Testing life cycle i.e in Unit,Integration, System, Acceptance and regression testing stages. Tools used for Black Box testing:  Black box testing tools are mainly record and playback tools. These tools are used for regressiontesting that to check whether new build has created any bug in previous working applicationfunctionality. These record and playback tools records test cases in the form of some scripts likeTSL, VB script, Java script, Perl. Advantages of Black Box Testing  - Tester can be non-technical.- Used to verify contradictions in actual system and the specifications.- Test cases can be designed as soon as the functional specifications are complete Disadvantages of Black Box Testing  - The test inputs needs to be from large sample space.- It is difficult to identify all possible inputs in limited testing time. So writing test cases is slowand difficult- Chances of having unidentified paths during this testing Methods of Black box Testing:   Graph Based Testing Methods:  Each and every application is build up of some objects. All such objects are identified and graphis prepared. From this object graph each object relationship is identified and test cases writtenaccordingly to discover the errors. Error Guessing:  This is purely based on previous experience and judgment of tester. Error Guessing is the art of guessing where errors can be hidden. For this technique there are no specific tools, writing thetest cases that cover all the application paths.
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