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Program Correctness and Efficiency Chapter 2

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Program Correctness and Efficiency. Chapter 2. Chapter Objectives. To understand the differences between the three categories of program errors To understand the effect of an uncaught exception and why you should catch exceptions - PowerPoint PPT Presentation

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Page 1: Program Correctness and Efficiency

Program Correctness and Efficiency

Program Correctness and Efficiency

Chapter 2Chapter 2

Page 2: Program Correctness and Efficiency

Chapter ObjectivesChapter Objectives

To understand the differences between the three categories of program errors

To understand the effect of an uncaught exception and why you should catch exceptions

To become familiar with the Exception hierarchy and the difference between checked and unchecked exceptions

To learn how to use the try-catch-finally sequence to catch and process exceptions

To understand what it means to throw an exception and how to throw an exception in a method

To understand the differences between the three categories of program errors

To understand the effect of an uncaught exception and why you should catch exceptions

To become familiar with the Exception hierarchy and the difference between checked and unchecked exceptions

To learn how to use the try-catch-finally sequence to catch and process exceptions

To understand what it means to throw an exception and how to throw an exception in a method

Page 3: Program Correctness and Efficiency

Chapter Objectives (continued)

Chapter Objectives (continued)

To understand the different testing strategies and when and how they are performed

To learn how to write special methods to test other methods and classes

To become familiar with debugging techniques and debugger programs

To be introduced to the process of program verification and the use of assertions and loop invariants

To understand the meaning of big-O notation and how it is used to analyze an algorithm’s efficiency

To understand the different testing strategies and when and how they are performed

To learn how to write special methods to test other methods and classes

To become familiar with debugging techniques and debugger programs

To be introduced to the process of program verification and the use of assertions and loop invariants

To understand the meaning of big-O notation and how it is used to analyze an algorithm’s efficiency

Page 4: Program Correctness and Efficiency

Program Defects and “Bugs”Program Defects and “Bugs”

A program may be efficient, but is worthless if it produces a wrong answer

Defects often appear in software after it is delivered Testing can never demonstrate the complete

absence of defects In some situations it is very difficult to test a

software product completely in the environment in which it is used

Debugging: removing defects

A program may be efficient, but is worthless if it produces a wrong answer

Defects often appear in software after it is delivered Testing can never demonstrate the complete

absence of defects In some situations it is very difficult to test a

software product completely in the environment in which it is used

Debugging: removing defects

Page 5: Program Correctness and Efficiency

Syntax ErrorsSyntax Errors

Syntax errors are mistakes in the grammar of a language

The Java compiler detects syntax errors during compilation and requires you to correct them before successfully compiling the program

Some common syntax errors include: Omitting or misplacing braces Performing an incorrect type of operation on a

primitive type value Invoking an instance method not defined Not declaring a variable before using it Providing multiple declarations of a variable

Syntax errors are mistakes in the grammar of a language

The Java compiler detects syntax errors during compilation and requires you to correct them before successfully compiling the program

Some common syntax errors include: Omitting or misplacing braces Performing an incorrect type of operation on a

primitive type value Invoking an instance method not defined Not declaring a variable before using it Providing multiple declarations of a variable

Page 6: Program Correctness and Efficiency

Run-time Errors or ExceptionsRun-time Errors or Exceptions

Run-time errors Occur during program execution Occur when the JVM detects an operation that it

knows to be incorrect Cause the JVM to throw an exception

Examples of run-time errors include Division by zero Array index out of bounds Number format and Input mismatch error Null pointer exceptions

Run-time errors Occur during program execution Occur when the JVM detects an operation that it

knows to be incorrect Cause the JVM to throw an exception

Examples of run-time errors include Division by zero Array index out of bounds Number format and Input mismatch error Null pointer exceptions

Page 7: Program Correctness and Efficiency

Run-time Errors or Exceptions (continued)

Run-time Errors or Exceptions (continued)

Page 8: Program Correctness and Efficiency

Logic ErrorsLogic Errors

A logic error occurs when the programmer or analyst Made a mistake in the design of a class or

method Implemented an algorithm incorrectly

Most logic errors do not cause syntax or run-time errors and are thus difficult to find

Sometimes found through testing Sometimes found during real-world operation of the

program

A logic error occurs when the programmer or analyst Made a mistake in the design of a class or

method Implemented an algorithm incorrectly

Most logic errors do not cause syntax or run-time errors and are thus difficult to find

Sometimes found through testing Sometimes found during real-world operation of the

program

Page 9: Program Correctness and Efficiency

The Exception Class HierarchyThe Exception Class Hierarchy

When an exception is thrown, one of the Java exception classes is instantiated

Exceptions are defined within a class hierarchy that has the class Throwable as its superclass

Classes Error and Exception are subclasses of Throwable

RuntimeException is a subclass of Exception

When an exception is thrown, one of the Java exception classes is instantiated

Exceptions are defined within a class hierarchy that has the class Throwable as its superclass

Classes Error and Exception are subclasses of Throwable

RuntimeException is a subclass of Exception

Page 10: Program Correctness and Efficiency

The Class ThrowableThe Class Throwable

Throwable is the superclass of all exceptions All exception classes inherit the methods of

throwable

Throwable is the superclass of all exceptions All exception classes inherit the methods of

throwable

Page 11: Program Correctness and Efficiency

The Class Throwable (continued)

The Class Throwable (continued)

Page 12: Program Correctness and Efficiency

Checked and Unchecked Exceptions

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

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

Page 13: Program Correctness and Efficiency

Checked and Unchecked Exceptions

Checked and Unchecked Exceptions

Page 14: Program Correctness and Efficiency

Catching and Handling Exceptions

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

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

Page 15: Program Correctness and Efficiency

Uncaught ExceptionsUncaught Exceptions

When an exception occurs that is not caught, the program stops and the JVM displays an error message and a stack trace

The stack trace shows the sequence of method calls, starting at the method that threw the exception and ending at main

When an exception occurs that is not caught, the program stops and the JVM displays an error message and a stack trace

The stack trace shows the sequence of method calls, starting at the method that threw the exception and ending at main

Page 16: Program Correctness and Efficiency

The try-catch-finally SequenceThe try-catch-finally Sequence

Avoid uncaught exceptions Write a try-catch sequence to catch an exception Handle it rather than relying on the JVM

Catch block is skipped if all statements within the try block execute without error

Avoid uncaught exceptions Write a try-catch sequence to catch an exception Handle it rather than relying on the JVM

Catch block is skipped if all statements within the try block execute without error

Page 17: Program Correctness and Efficiency

Handling Exceptions to Recover from Errors

Handling Exceptions to Recover from Errors

Exceptions provide the opportunity to Recover from errors Report errors

User error is a common source of error and should be recoverable

Catch block within the first catch clause having an appropriate exception class executes, others are skipped

Compiler displays an error message if it encounters an unreachable catch clause

Exceptions provide the opportunity to Recover from errors Report errors

User error is a common source of error and should be recoverable

Catch block within the first catch clause having an appropriate exception class executes, others are skipped

Compiler displays an error message if it encounters an unreachable catch clause

Page 18: Program Correctness and Efficiency

The finally BlockThe finally Block

When an exception is thrown, the flow of execution is suspended and there is no return to the try block

There are situations in which allowing a program to continue after an exception could cause problems

The code in the finally block is executed either after the try block is exited or after a catch clause is exited

The finally block is optional

When an exception is thrown, the flow of execution is suspended and there is no return to the try block

There are situations in which allowing a program to continue after an exception could cause problems

The code in the finally block is executed either after the try block is exited or after a catch clause is exited

The finally block is optional

Page 19: Program Correctness and Efficiency

Throwing ExceptionsThrowing 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

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

Page 20: Program Correctness and Efficiency

Throwing Exceptions (continued)

Throwing Exceptions (continued)

Can use a throw statement in a lower-level method to indicate that an error condition has been detected

Once the throw statement executes, the lower-level method stops executing immediately

Can use a throw statement in a lower-level method to indicate that an error condition has been detected

Once the throw statement executes, the lower-level method stops executing immediately

Page 21: Program Correctness and Efficiency

Catching Exceptions ExampleCatching Exceptions Example

Page 22: Program Correctness and Efficiency

Programming StyleProgramming Style You can always avoid handling exceptions by declaring that

they are thrown, or throwing them and letting them be handled farther back in the call chain

It is usually best to handle the exception instead of passing it along

The following are recommended guidelines: If an exception is recoverable in the current method,

handle the exception in the current method If a checked exception is likely to be caught in a

higher-level method, declare that it can occur using a throws clause

It is not necessary to use a throws clause with unchecked exceptions

You can always avoid handling exceptions by declaring that they are thrown, or throwing them and letting them be handled farther back in the call chain

It is usually best to handle the exception instead of passing it along

The following are recommended guidelines: If an exception is recoverable in the current method,

handle the exception in the current method If a checked exception is likely to be caught in a

higher-level method, declare that it can occur using a throws clause

It is not necessary to use a throws clause with unchecked exceptions

Page 23: Program Correctness and Efficiency

Testing ProgramsTesting Programs

There is no guarantee that a program that is syntax and run-time error free will also be void of logic errors

The “best” situation is a logic error that occurs in a part of the program that always executes; otherwise, it may be difficult to find the error

The worst kind of logic error is one that occurs in an obscure part of the code (infrequently executed)

There is no guarantee that a program that is syntax and run-time error free will also be void of logic errors

The “best” situation is a logic error that occurs in a part of the program that always executes; otherwise, it may be difficult to find the error

The worst kind of logic error is one that occurs in an obscure part of the code (infrequently executed)

Page 24: Program Correctness and Efficiency

Structured WalkthroughsStructured Walkthroughs

Most logic errors arise during the design phase and are the result of an incorrect algorithm

Logic errors may also result from typographical errors that do not cause syntax or run-time errors

One form of testing is hand-tracing the algorithm before implementing

Structured walkthrough: designer must explain the algorithm to other team members and simulate its execution with other team members looking on

Most logic errors arise during the design phase and are the result of an incorrect algorithm

Logic errors may also result from typographical errors that do not cause syntax or run-time errors

One form of testing is hand-tracing the algorithm before implementing

Structured walkthrough: designer must explain the algorithm to other team members and simulate its execution with other team members looking on

Page 25: Program Correctness and Efficiency

Levels and Types of TestingLevels and Types of Testing

Testing: exercising a program under controlled conditions and verifying the results

Purpose is to detect program defects after all syntax errors have been removed and the program compiles

No amount of testing can guarantee the absence of defects in sufficiently complex programs

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

Integration testing: testing the interactions among units

Testing: exercising a program under controlled conditions and verifying the results

Purpose is to detect program defects after all syntax errors have been removed and the program compiles

No amount of testing can guarantee the absence of defects in sufficiently complex programs

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

Integration testing: testing the interactions among units

Page 26: Program Correctness and Efficiency

Levels and Types of Testing (continued)

Levels and Types of Testing (continued)

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

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

Page 27: Program Correctness and Efficiency

Preparations for TestingPreparations for Testing

A test plan should be developed early in the design phase

Aspects of a test plan include deciding how the software will be tested, when the tests will occur, who will do the testing, and what test data will be used

If the test plan is developed early, testing can take place concurrently with the design and coding

A good programmer practices defensive programming and includes code to detect unexpected or invalid data

A test plan should be developed early in the design phase

Aspects of a test plan include deciding how the software will be tested, when the tests will occur, who will do the testing, and what test data will be used

If the test plan is developed early, testing can take place concurrently with the design and coding

A good programmer practices defensive programming and includes code to detect unexpected or invalid data

Page 28: Program Correctness and Efficiency

Testing Tips for Program Systems

Testing Tips for Program Systems

Most of the time, you will test program systems that contain collections of classes, each with several methods

If a method implements an interface, its specification should document input parameters and expected results

Carefully document each method parameter and class attribute using comments as you write the code

Leave a trace of execution by displaying the method name as you enter it

Display values of all input parameters upon entry to a method

Most of the time, you will test program systems that contain collections of classes, each with several methods

If a method implements an interface, its specification should document input parameters and expected results

Carefully document each method parameter and class attribute using comments as you write the code

Leave a trace of execution by displaying the method name as you enter it

Display values of all input parameters upon entry to a method

Page 29: Program Correctness and Efficiency

Testing Tips for Program Systems (continued)

Testing Tips for Program Systems (continued)

Display the values of any class attributes that are accessed by this method

Display the values of all method outputs after returning from a method

Plan for testing as you write each module rather than after the fact

Display the values of any class attributes that are accessed by this method

Display the values of all method outputs after returning from a method

Plan for testing as you write each module rather than after the fact

Page 30: Program Correctness and Efficiency

Developing the Test DataDeveloping the Test Data

Test data should be specified during the analysis and design phases for the different levels of testing: unit, integration, and system

In black-box testing, we are concerned with the relationship between the unit inputs and outputs There should be test data to check for all

expected inputs as well as unanticipated data In white-box testing, we are concerned with

exercising alternative paths through the code Test data should ensure that all if statement

conditions will evaluate to both true and false

Test data should be specified during the analysis and design phases for the different levels of testing: unit, integration, and system

In black-box testing, we are concerned with the relationship between the unit inputs and outputs There should be test data to check for all

expected inputs as well as unanticipated data In white-box testing, we are concerned with

exercising alternative paths through the code Test data should ensure that all if statement

conditions will evaluate to both true and false

Page 31: Program Correctness and Efficiency

Testing Boundary ConditionsTesting Boundary Conditions

When hand-tracing through an algorithm or performing white-box testing, you must exercise all paths

Check special cases called boundary conditions

When hand-tracing through an algorithm or performing white-box testing, you must exercise all paths

Check special cases called boundary conditions

Page 32: Program Correctness and Efficiency

Why do Testing?Why do Testing?

Normally testing is done by The programmer Other members of the software team who did not

code the module being tested Final users of the software product

Do not rely on programmers for testing as they are often blind to their own oversights

Companies also have quality assurance organizations that verify that the testing process is performed correctly

In extreme programming, programmers work in pairs where one writes the code and the other writes the tests

Normally testing is done by The programmer Other members of the software team who did not

code the module being tested Final users of the software product

Do not rely on programmers for testing as they are often blind to their own oversights

Companies also have quality assurance organizations that verify that the testing process is performed correctly

In extreme programming, programmers work in pairs where one writes the code and the other writes the tests

Page 33: Program Correctness and Efficiency

StubsStubs

It may be difficult to test a method or class that interacts with other methods or classes

The replacement of a method that has not yet been implemented or tested is called a stub

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

It may be difficult to test a method or class that interacts with other methods or classes

The replacement of a method that has not yet been implemented or tested is called a stub

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

Page 34: Program Correctness and Efficiency

DriversDrivers

A driver program declares any necessary object instances and variables, assigns values to any of the method’s inputs, calls the method, and displays the values of any outputs returned by the method

You can put a main method in a class to serve as the test driver for that class’s methods

A driver program declares any necessary object instances and variables, assigns values to any of the method’s inputs, calls the method, and displays the values of any outputs returned by the method

You can put a main method in a class to serve as the test driver for that class’s methods

Page 35: Program Correctness and Efficiency

Using a Test FrameworkUsing a Test Framework

A test framework is a software product that facilitates writing test cases, organizing the test cases into test suites, running the test suites, and reporting the results

A test framework often used for Java products is JUnit, an open-source product that can be used in a stand-alone mode and is available from junit.org

A test framework is a software product that facilitates writing test cases, organizing the test cases into test suites, running the test suites, and reporting the results

A test framework often used for Java products is JUnit, an open-source product that can be used in a stand-alone mode and is available from junit.org

Page 36: Program Correctness and Efficiency

“BUGS”“BUGS”

IMPORTANT NOTICE! YOU PUT EVERY BUG THE PROGRAM YOU WROTE! They DID NOT “Crawl In” from outside.

IMPORTANT NOTICE! YOU PUT EVERY BUG THE PROGRAM YOU WROTE! They DID NOT “Crawl In” from outside.

Page 37: Program Correctness and Efficiency

LOOK FOR BUGS IN CORNERSLOOK FOR BUGS IN CORNERS

Boundary conditions. Boundary conditions.

Page 38: Program Correctness and Efficiency

Debugging a ProgramDebugging a Program

Debugging is the major activity performed by programmers during the testing phase

Testing determines if there is an error, debugging determines the cause of it

Debugging is like detective work Inspect carefully the information displayed by

your program Insert additional diagnostic output statements in

the method to determine more information

Debugging is the major activity performed by programmers during the testing phase

Testing determines if there is an error, debugging determines the cause of it

Debugging is like detective work Inspect carefully the information displayed by

your program Insert additional diagnostic output statements in

the method to determine more information

Page 39: Program Correctness and Efficiency

Using a DebuggerUsing a Debugger

Debuggers often are included with IDEs A debugger can execute your program incrementally

rather than all at once Single-step execution executes in increments as

small as one program statement Breakpoints are used to traverse large portions of

code before stopping The actual mechanics of using a debugger depend

on the IDE that you are using

Debuggers often are included with IDEs A debugger can execute your program incrementally

rather than all at once Single-step execution executes in increments as

small as one program statement Breakpoints are used to traverse large portions of

code before stopping The actual mechanics of using a debugger depend

on the IDE that you are using

Page 40: Program Correctness and Efficiency

Using a Debugger (continued)Using a Debugger (continued)

Page 41: Program Correctness and Efficiency

Reasoning about Programs: Assertions and Loop Invariants

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

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

Page 42: Program Correctness and Efficiency

Assertions and Loop Invariants Example

Assertions and Loop Invariants Example

Page 43: Program Correctness and Efficiency

Efficiency of AlgorithmsEfficiency 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

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

Page 44: Program Correctness and Efficiency

Efficiency of Algorithms (continued)

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)

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)

Page 45: Program Correctness and Efficiency

Efficiency of Algorithms (continued)

Efficiency of Algorithms (continued)

We can reduce the expression in parentheses to: n x (n – 1) 2

So, T(n) = 1.5n2 – 1.5n This polynomial is zero when n is 1. For values

greater than 1, 1.5n2 is always greater than 1.5n2 – 1.5n

Therefore, we can use 1 for n0 and 1.5 for c to conclude that T(n) is O(n2)

We can reduce the expression in parentheses to: n x (n – 1) 2

So, T(n) = 1.5n2 – 1.5n This polynomial is zero when n is 1. For values

greater than 1, 1.5n2 is always greater than 1.5n2 – 1.5n

Therefore, we can use 1 for n0 and 1.5 for c to conclude that T(n) is O(n2)

Page 46: Program Correctness and Efficiency

Efficiency of Algorithms (continued)

Efficiency of Algorithms (continued)

Page 47: Program Correctness and Efficiency

Chapter ReviewChapter Review

Three kinds of defects can occur in programs Syntax errors Run-time errors Logic errors

All exceptions in the Exception class hierarchy are derived from a common superclass called Throwable

The default behavior for exceptions is for the JVM to catch them by printing an error message and a call stack trace and then terminating the program

Three kinds of defects can occur in programs Syntax errors Run-time errors Logic errors

All exceptions in the Exception class hierarchy are derived from a common superclass called Throwable

The default behavior for exceptions is for the JVM to catch them by printing an error message and a call stack trace and then terminating the program

Page 48: Program Correctness and Efficiency

Chapter Review (continued)Chapter Review (continued)

Two categories of exceptions: checked and unchecked

A method that can throw a checked exception must either catch it or declare that it is thrown

“throw” statement throws an unchecked exception Program testing is done at several levels starting

with the smallest testable piece of a program called a unit

Integration testing: once units are individually tested, they can then be tested together

System testing: once the whole program is put together, it is tested as a whole

Two categories of exceptions: checked and unchecked

A method that can throw a checked exception must either catch it or declare that it is thrown

“throw” statement throws an unchecked exception Program testing is done at several levels starting

with the smallest testable piece of a program called a unit

Integration testing: once units are individually tested, they can then be tested together

System testing: once the whole program is put together, it is tested as a whole

Page 49: Program Correctness and Efficiency

Chapter Review (continued)Chapter Review (continued)

Acceptance programming involves testing in an operational manner demonstrating its functionality

Black-box testing tests the item based on its functional requirements without knowledge of its internal structure

White-box testing tests the item using knowledge of its internal structure

Test drivers and stubs are tools used in testing Test drivers exercise a method or class Stubs stand in for called methods

Big-O notation determines the efficiency of a program

Acceptance programming involves testing in an operational manner demonstrating its functionality

Black-box testing tests the item based on its functional requirements without knowledge of its internal structure

White-box testing tests the item using knowledge of its internal structure

Test drivers and stubs are tools used in testing Test drivers exercise a method or class Stubs stand in for called methods

Big-O notation determines the efficiency of a program