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G54PRG ProgrammingLecture

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Amadeo Ascó Adam Moore

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Bugs & Debugging - Testing

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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

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Overview

• Types of Errors• When errors appear• Compiler Errors• Run-time errors• Fixing errors• Testing - Find Errors

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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;

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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

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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

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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

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Compiler Errors

Let see some using Eclipse!

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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

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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

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RuntimeExceptionArithmeticException e.g. 10 / 0

NullPointerExceptionIndexOutOfBoundsException...

Run-time errors

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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...

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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

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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);}...

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Run-time errors

Let see some using Eclipse!

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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

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Run-time errors

Let do some debugging using Eclipse!

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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

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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