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Web Based Programming Section 6

James King12 August 2003

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Key Skills

This section will teach how to: make your programs readable document you programs debug your programs

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Making code Readable

The key to readability is indentation it is difficult to see which bit of code is inside which if or loop if your presentation is messyIf you miss out a { or } it can be difficult to find the correct place to add one

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How not to Indent

public void nestit(){

for (int i=0; i<3; i=i+1){System.out.println("top of outer loop i

"+i);for (int j=0; j<3; j=j+1) {System.out.println("inside inner loop i "+i+" j "+j); } System.out.println("bottom of outer loop i "+i); }

There is a }

missing somewher

e

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One Way to indent (Pascal Method)

Each time you type a { all lines following are indented moreEach time you type a } all lines following are indented lessboth { and } are on separate lines by themselvesit is easy to match up { and } pairs like this...

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One way to Indent (Pascal Method)

public void nestit(){for (int i=0; i<3; i=i+1)

{System.out.println("top of outer loop i "+i);for (int j=0; j<3; j=j+1)

{System.out.println("inside inner loop i

"+i+" j "+j); } System.out.println("bottom of outer loop i "+i); }

There is a }

missing !!

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Indentation Style

We don’t mind which way you indent as long as you are consistent it is easy to see nesting it is easy to see where the blocks

start and end

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Comments and automatic documentation Generation

Key skills section

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Documentation - Comments

The simplest form of documentation for you code are comments. This allows you to add English descriptions to your codeThese can be placed almost anywhere in your codeComments can extend over multiple lines and are enclosed inside /* comment */Single line comments can be made with // comment

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Comments Example/* simple demonstration of the how the continue statement

works inside a loop */public void loopit() // takes no parameters { System.out.println("before loop"); for (int i=0; i<5; i=i+1) { System.out.println(" before if"+i); if (i==3) continue; System.out.println(" after if"+i); // bypassed if i is 3 // } oops I commented out the end of block marker by

accident... System.out.println("after loop"); }

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Automatic Documentation Generation

Java compiler can generate HTML web pages from your comments if you place them in certain places in a certain stylecomments go before the declarations

/** * about this procedure **/public void proc()

comments in the body of blocks are ignored

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Documentation Tags

In addition to describing the class inside the comment you can add a version and authors name using tags @version number @author name

You can document the parameters and return valves of a method @param variable description @return description

There are more tags...

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Documentation Example/** * description of the class * @author (your name) * @version (a version number or a date) */public class doc {

// description of the attributeprivate int x;/** * description of the constructor */public doc() {}/** * description of the method * @param y description of the parameter * @return description of the return value */public int sampleMethod(int y) {return x + y; } }

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HTML Documentation

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Debugging

Key skills section

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Debugging CodeMost compilers come with a debugger which usually allows you to Step though your program line by line Examine the value of each attribute and

variable in scope Run the code at full speed and stop at certain

points (breakpoint)

In addition BlueJ allows you to Call a method and optionally provide values

for the parameters it requires Create a new instance of a class

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Debuggers are useful in several situations

You want to understand how someone else's code works and it is too complex to run in your head (reverse engineer)You want to find out why your program is not doing what you expect it to and make it work correctly (debug)You want to see how Java runs your programs and how the ifs and loops work

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Debugging

Debugging is a practical activity so you will learn how to do this in the practical sessions.

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Catching and Generating Runtime Errors

Advanced Java Facilities

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Dealing with problems in a running program

The compiler tries to catch errors during byte code creation. However some errors only occur when the program is running

int i=1;i=i-1;int b=3/i;

3 divided by 0 is infinity.Infinity can not be stored in

a int

The Java interpreter generates a

ArithmeticException and stops running the program

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Catching ExceptionsTo avoid the program stopping we can catch the exception

try{int i=1;i=i-1;int b=3/i;}

catch (Exception e){System.err.println(“CRASH”);}

Any exception generated anywhere in this block of

code will cause execution to jump to the catch block,

skipping any remaining code. If there is no exception the

catch block is skipped

In either case execution

eventually gets here

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Information about the Exception

The e in catch (Exception e) acts just like a parameter and is actually an object. It contains information about the exception

e.printStackTrace(); displays on the console screen the class and

line number the exception was generated at

e.getMessage(); returns a string with details of why the

exception occurred

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Catching different types of exceptions

try{}

Catch (ArithmeticException f){}

catch (Exception e){}

This will catch only ArithmeticExcepti

on and any subclasses

Since all Exceptions are subclasses of

Exception this will pick up all other

exceptions

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Cleaning up the mess

Sometimes regardless of if an exception is generated or not you want some code to be executed

try{}

catch (Exception e){}

finally{}

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Exceptions

Java built in methods and classes may generate exceptions if used incorrectlyThe Java compiler expects you to either catch these possible exceptions in the method they could be created in or leave it to the method that called that method to handle them…

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Throws

Java expects each method to catch any exceptions it could generateIf you don’t want to do this you must add the uncaught exceptions to the method definition in a throws list

void problem2() throws Exception{throw new Exception();}