Download - Bugs & Debugging - Testing
1
G54PRG ProgrammingLecture
1
Amadeo Ascó Adam Moore
10
Bugs & Debugging - Testing
2Amadeo Ascó , Adam Moore
Previously
• Design a program– Break the problem into smaller ones– It may help to do it several times to reduce the
problem enough– Outlines
• Javadoc– Industry standard for documenting Java code– Create documentation in HML format– Other format can be obtained
3Amadeo Ascó , Adam Moore
Overview
• Types of Errors• When errors appear• Compiler Errors• Run-time errors• Fixing errors• Testing - Find Errors
4Amadeo Ascó , Adam Moore
12345
12345
Types of Errors
• Syntax errors– Code that does not conform to the syntax of the programming language
– Detected a compiling time
// Declaring variablesint iCounter = 0int iValue;
iValue = ++iCounter;
5Amadeo Ascó , Adam Moore
Types of Errors
• Semantic errors
– Mistakes in the logic of the code
– Legal language code but not what intended
– Appear when running the code
• Exceptions
6Amadeo Ascó , Adam Moore
When errors appear
• Compile-time errors– Appear when the code is compiled
• Run-time errors– Appear when the code is running
• Makes the code to behave different than expected• Could take long time before effects are seen
– Logical errors are run-time errors– Some languages try to reduce the risk of this
errors, Java is one of them
7Amadeo Ascó , Adam Moore
Compiler Errors
• Eclipse will show them with a read circle and a cross
• Some information is provided– The point of the error may not be where the error
was introduced– Unbalanced braces cannot be detected until the
braces are closed – indexation helps to detect the point of the problem
– Previous error may create other errors, “spurious” errors
8Amadeo Ascó , Adam Moore
Compiler Errors
Let see some using Eclipse!
9Amadeo Ascó , Adam Moore
Run-time errors• Java does not allow some code to avoid
potential run-time errorsfloat fCostProduct1 = 12.50;float fCostProduct2 = 30.10;
int iTotal = fCostProduct1 + fCostProduct2;
• Potential run-time errors may be detected by Eclipse and are displayed with a yellow triangle with an exclamation mark– You should investigate them– Take the appropriate action to remove them
10Amadeo Ascó , Adam Moore
Run-time errors• Cause: Lack of validation
– An example would be when expected a number and provided a string
– Remember the exercise to convert from Celsius to Fahrenheit the data is inputted as an string from the console and converted to a number
– What happen if the user input a character not a digit? An exception is thrown
11Amadeo Ascó , Adam Moore
RuntimeExceptionArithmeticException e.g. 10 / 0
NullPointerExceptionIndexOutOfBoundsException...
Run-time errors
12Amadeo Ascó , Adam Moore
Run-time errorsOther causes:
– External factors• Out of memory• Insufficient i/o privileges...
– Internal factors• Arithmetic errors• Invalid operation, e.g. try to read beyond the end of a file• Try to access an object that doesn’t exist, e.g. null object• Attempt to read beyond the end of an array...
13Amadeo Ascó , Adam Moore
Run-time errors
• Logical errors– Causes a program to operate incorrectly– The program does not terminate abnormally or
crash– The behaviour of the program is different that
what was intended– Can be difficult to spot
14Amadeo Ascó , Adam Moore
123456789101112
123456789101112
Run-time errors// Example of a logical errorString strAnswer;
System.out.println("Press 'y' to continue or any other key” + ” to terminate: ");strAnswer = Scanner.nextLine();// Exit the program if the key 'y' is pressedif (strAnswer.charAt(0) == 'n') { // Terminate the application
System.exit(0);}...
15Amadeo Ascó , Adam Moore
Run-time errors
Let see some using Eclipse!
16Amadeo Ascó , Adam Moore
Fixing errors
1. Need to know an error exists – find errors2. Gather as much information as possible
– Logging messages helps
3. Establish where the error happens in the code– Debugging is the process of locating the errors
(bugs) in the code
4. Design the fix5. Implement the fix6. Test that the fix really fixes the problem
17Amadeo Ascó , Adam Moore
Run-time errors
Let do some debugging using Eclipse!
18Amadeo Ascó , Adam Moore
Testing - Find Errors
• Test to find errors (bugs) that may exist in the code
• Different type of tests are conducted at different stages of a project
• Tests intend to establish the compliance to the requirements
• Errors should be reproducible!– Requirement to be able to locate them and fix– Information gathered required to reproduce it
19Amadeo Ascó , Adam Moore
Testing - Find Errors• Create unit testing
– Test units of your code• Check if the unit does what it is expected• The units are methods
– Use JUnit in Java
• Run some simple examples• System test
– Test the complete, integrated system to evaluate the compliance of the system – behave as expected
• Customers – NOT GOOD