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ITEC200 Week02

Program Correctness and Efficiency

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Learning Objectives – Week 02 Program Correctness and Efficiency (Ch2)

Students can:

• Explain the differences between syntax errors, run-time errors and logical errors

• Explain and use the exception handling paradigm in Java (the Exception hierarchy, checked versus unchecked exceptions, try-catch-finally sequences, exception throwing)

• Describe various testing strategies and implement them effectively

• Use debugging tools and strategies effectively

• Use assertions and loop invariants to verify program segments

• Analyse algorithm efficiency using big-O notation

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Types of Errors

• Syntax errors: mistakes in the grammar of a language

• Run-time errors: errors picked up by the Java Virtual Machine during program execution (causes JVM to throw an Exception)

• Logical errors: program works but doesn’t behave as intended

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The Exception class hierarchy

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Methods inherited from Throwable

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Checked and Unchecked Exceptions

• Two categories of exceptions: checked and unchecked• Checked exception normally not due to programmer

error and is beyond the control of the programmer• Unchecked exception may result from

– Programmer error

– Serious external conditions that are unrecoverable

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Checked and Unchecked Exceptions

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Catching and Handling Exceptions

• When an exception is thrown, the normal sequence of execution is interrupted

• Default behavior– Program stops

– JVM displays an error message

• The programmer may override the default behavior by– Enclosing statements in a ‘try’ block

– Processing the exception in a ‘catch’ block

– Specify any final instructions in a ‘finally’ block

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Throwing Exceptions

• Instead of catching an exception in a lower-level method, it can be caught and handled by a higher-level method– Declare that the lower-level method may throw a checked

exception by adding a throws clause to the method header

– Can throw the exception in the lower-level method, using a throw statement

• The throws clause is useful if a higher-level module already contains a catch clause for this exception type

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Catching Exceptions Example

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Levels and Types of Testing

• Unit testing: checking the smallest testable piece of the software (a method or class)

• Integration testing: testing the interactions among units

• System testing: testing the program in context

• Acceptance testing: system testing designed to show that the program meets its functional requirements

• Black-box testing: tests the item based on its interfaces and functional requirements

• White-box testing: tests the software with the knowledge of its internal structure

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Testing Tools

• Stub: a substitute method that has the same header as the method it replaces, but its body only displays a message indicating that the stub was called

• Driver program: specifies the testing routine to be executed

• Test framework: a software productthat facilitates writing test cases, organizing the test cases into test suites, running the test suites, and reporting the results

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Debugging

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Reasoning about Programs: Assertions and Loop Invariants

• Assertions: logical statements about a program that are claimed to be true; generally written as a comment

• Preconditions and postconditions are assertions• A loop invariant is an assertion

– Helps prove that a loop meets it specification

– True before loop begins, at the beginning of each repetition of the loop body, and just after loop exit

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Assertions and Loop Invariants Example

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Efficiency of Algorithms

• Difficult to get a precise measure of the performance of an algorithm or program

• Can characterize a program by how the execution time or memory requirements increase as a function of increasing input size– Big-O notation

• A simple way to determine the big-O of an algorithm or program is to look at the loops and to see whether the loops are nested

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Efficiency of Algorithms (continued)

• Consider:

• First time through outer loop, inner loop is executed n-1 times; next time n-2, and the last time once.

• So we have – T(n) = 3(n – 1) + 3(n – 2) + … + 3 or

– T(n) = 3(n – 1 + n – 2 + … + 1)

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Efficiency of Algorithms (continued)

• We can reduce the expression in parentheses to: n x (n – 1)

2

• So, T(n) = 1.5n2 – 1.5n • We can therefore conclude that T(n) is O(n2)

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Efficiency of Algorithms (continued)

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Where to from here…

• Work through Chapter 2 of the Koffman & Wolfgang Text

• Conceptual Questions and Practical Exercises• Submit all preliminary work• Be prompt for your online class

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Acknowledgements

These slides were based upon the Objects, Abstraction, Data Structures and Design using Java Version 5.0 Chapter 2 PowerPoint presentation

by Elliot B. Koffman and Paul A. T. Wolfgang