Jun 16, 2014 IAT 265 1

IAT 265

Debugging

Dialectical Materialism

g Dialectical materialism is a strand of Marxism, synthesizing Hegel's dialectics, which proposes that

g Every economic order grows to a state of maximum efficiency, while simultaneously developing internal contradictions and weaknesses that contribute to its systemic decay

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Dialectics

g Thus, programming is a dialectic process:– ENbugging– Debugging

g Karl Marx said so!

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How do I know my program is broken?

g Compiler Errors – easy to fix!

g Runtime Exceptions– more difficult to fix, but at least you're

using java and these get reportedg Your program just doesn't do the right

thing.

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Compiler Errors

g Errors dealing with language syntaxg Simple logical errors

– Whatever the compiler can possibly catch.

g Generally, the line number stated has the error on it– Sometimes the fix is elsewhere

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How to fix compiler errors?

g Start at the top of the error listg Some errors cause others

– Wrong variable declaration causes errors in usage of that variable

g Use the line number! g If that line looks OK, check the line

above– maybe missed a brace/semicolon or

other necessary syntax element.

Count Brackets and Braces

{ qwdkj { dw wqdlk lqwd { n,mnwq } } }

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1

2

3

2

1

0

Braces match if the last == 0!

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Compile Time Errors

g Some errors aren't necessarily errors.– For example:

String foo; //assume we initialize this somewhere elsepublic void blah(){

Object bar;try{

bar = foo.toString();}catch(Exception e){

println(“Oh no!!”);return;

} println(bar.toString()); //lets call this line 101

}– Will give you something like:

line 101: variable bar might not be initialized! (or something like that)

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print your variables

g println()– Use println often– Print everything: array values, pointer

values, array index, objects etc– Each println should label itself with class

name and line number– Java: Be sure to use System.out.flush();

to ensure you are getting all data

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Learn to read your code

g Keep a notepad around to keep track of variable values.– Use comments to document complex

code– Keep one step to one line. – Format your code! Indentations help

readability– Keep your code neat: save your

mental effort for understanding, not reading

Always the Same Placeg My keys are always the same place:

– Right front pocketg My Java variables are always the

same place– Top of method or top of class

g Why?– I always know where to look for

variables!

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Always the Same Place

g For loops: always formatted the same

g Switch: always formatted the sameg Variables: I reuse the same names

for( int i = 0 ; i < arr.size() ; i++ ) { ... }g Doing it the same way every time

Means:– You don’t have to read the whole for

loop

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Always the Same Placeg Here’s what you see:

for( int i = 0 ; i < arr.size() ; i++ ) {}

g Here’s what I see:for( int i = 0 ; i < arr.size() ; i++ ) {}

g Here’s what I see when something’s missing:for int i = 0 ; i < arr.size() ; i++ ) {}Jun 16, 2014 IAT 265 13

Always the Same Place

g Doing something the same way allows me to notice when something is different

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Runtime Exceptions

There are two types of Runtime Exceptions

Checked and Unchecked

Checked exceptions:

Java makes you deal with these in your code

Things that you would expect to fail: I/O mainly

Unchecked exceptions

Java does not require you to catch these

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Checked Exceptions

g IOException (FileNotFoundException)g Input and output is typically hard to

write because you have to deal with the real world’s complexities

g Java requires that you put these in Try/Catch Blocks– Processing manages some of this

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Unchecked Exceptions

g Exceptions that only the programmer can anticipate– Extremely hard for a compiler to

determineg NullPointerException (NPE) and

ArrayIndexOutOfBoundsException (AIOBE)

g Caused by semantic errors– uninitialized variable, bad loop logic…

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Exceptions

g On exception, you get a stack traceg Find the first line of the stack trace

that occurs in your program. g That line is where the exception

occurred, not necessarily where the fix is.– On that line, did you get an NPE? – Is there some object that you're calling a

method on? Is that object Null? – For AIOBE, check index values

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Things to remember

g In java Objects are passed by reference and primitives are passed by value. public void doStuff(String a) { a = a + “bar”; }

public void doMoreStuff(int a) { a = a+5; }

public static void main(...){String temp = “foo”;int temp2 = 5;doStuff(temp);doMoreStuff(temp2);System.out.println (temp);System.out.println (temp2);

}prints out:

foobar5

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The #1 debugging tip

g TEST YOUR CODE OFTEN!– Catching your small errors early will help

you avoid the big complicated errors later.

– If you write a chunk of code that you can test, test it.

– You'll regret not spending 5 minutes writing a simple test case when you spend hours trying to find out it has a bug later.

Wrong and Right

g Buildg Buildg Buildg Buildg Buildg Buildg Test

g Buildg Testg Buildg Testg Buildg Testg Buildg Test

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