[<< wikibooks] The Way of the Java/Debugging
== Debugging ==
There are a few different kinds of errors that can occur
in a program, and it is useful to distinguish between them
in order to track them down more quickly.
Compile-time errors are produced by the compiler and usually
indicate that there is something wrong with the syntax of the
program.  Example: omitting the semi-colon at the end of a
Run-time errors are produced by the run-time system if something
goes wrong while the program is running.  Most run-time errors
are Exceptions.  Example: an infinite recursion eventually causes
a StackOverflowException.
Semantic errors are problems with a program that compiles and
runs, but doesn't do the right thing.  Example: an expression may
not be evaluated in the order you expect, yielding an unexpected

compile-time error
run-time error
semantic error

The first step in debugging is to figure out which kind of
error you are dealing with.  Although the following sections are
organized by error type, there are some techniques that are
applicable in more than one situation.

=== Compile-time errors ===

==== The compiler is spewing error messages. ====
error messages

If the compiler reports 100 error messages, that doesn't mean
there are 100 errors in your program.  When the compiler encounters
an error, it gets thrown off track for a while.  It tries to
recover and pick up again after the first error, but sometimes
it fails, and it reports spurious errors.
In general, only the first error message is reliable.  I suggest
that you only fix one error at a time, and then recompile the
program.  You may find that one semi-colon ``fixes 100 errors.
Of course, if you see several legitimate error messages, you
might as well fix more than one bug per compilation attempt.

==== I'm getting a weird compiler message and it won't go away. ====
First of all, read the error message carefully.  It is written
in terse jargon, but often there is a kernel of information
there that is carefully hidden.
If nothing else, the message will tell you where in the program the
problem occurred.  Actually, it tells you where the compiler was
when it noticed a problem, which is not necessarily where the error
is.  Use the information the compiler gives you as a guideline,
but if you don't see an error where the compiler is pointing,
broaden the search.
Generally the error will be prior to the location of the error
message, but there are cases where it will be somewhere else
entirely.  For example, if you get an error message at a method
invocation, the actual error may be in the method definition.
If you are building the program incrementally, you should have
a good idea about where the error is.  It will be in the last
line you added.
If you are copying code from a book, start by comparing 
your code to the book's code very carefully.  Check every character.
At the same time, remember that the book might be wrong, so
if you see something that looks like a syntax error, it might be.
If you don't find the error quickly, take a breath and look more
broadly at the entire program.  Now is a good time to go through the
whole program and make sure it is indented properly.  I won't say that
good indentation makes it easy to find syntax errors, but bad
indentation sure makes it harder.
Now, start examining the code for the common syntax errors.


Check that all parentheses and brackets are
balanced and properly nested.  All method definitions should be nested
within a class definition.  All program statements should be within a
method definition.
Remember that upper case letters are not the same as
lower case letters.
Check for semi-colons at the end of statements (and
no semi-colons after squiggly-braces).
Make sure that any strings in the code have matching
quotation marks (and that you use double-quotes, not single).
For each assignment statement, make sure that the type
on the left is the same as the type on the right.
For each method invocation, make sure that the arguments
you provide are in the right order, and have right type, and that the
object you are invoking the method on is the right type.
If you are invoking a fruitful method, make sure you
are doing something with the result.  If you are invoking a
void method, make sure you are not trying to do something
with the result.
If you are invoking an object method, make sure you are
invoking it on an object with the right type.  If you are invoking
a class method from outside the class where it is defined, make
sure you specify the class name.
Inside an object method you can refer to the instance
variables without specifying an object.  If you try that in
a class method, you will get a confusing message like, ``Static
reference to non-static variable.
If nothing works, move on to the next section...

==== I can't get my program to compile no matter what I do. ====
If the compiler says there is an error and you don't see it, that
might be because you and the compiler are not looking at the same
code.  Check your development environment to make sure the program
you are editing is the program the compiler is compiling.  If you
are not sure, try putting an obvious and deliberate syntax error
right at the beginning of the program.  Now compile again.  If
the compiler doesn't find the new error, there is probably something
wrong with the way you set up the project.
Otherwise, if you have examined the code thoroughly,
it is time for desperate measures.  You should start over with
a program that you can compile and then gradually add your code
Make a copy of the file you are working on.  If you are
working on Fred.java, make a copy called Fred.java.old.
Delete about half the code from Fred.java.  Try compiling
If the program compiles now, then you know the error is in
the other half.  Bring back about half of the code you deleted and
If the program still doesn't compile, the error must be in
this half.  Delete about half of the code and repeat.
Once you have found and fixed the error, start bringing back
the code you deleted, a little bit at a time.
This process is called ``debugging by bisection.  As an
alternative, you can comment out chunks of code instead of
deleting them.  For really sticky syntax problems, though,
I think deleting is more reliable---you don't have to worry
about the syntax of the comments, and by making the program
smaller you make it more readable.

bisection!debugging by
debugging by bisection

=== Run-time errors ===

==== My program hangs. ====
infinite loop
infinite recursion

If a program stops and seems to be doing nothing, we
say it is ``hanging.  Often that means that it is caught in
an infinite loop or an infinite recursion.
If there is a particular loop that you suspect is the
problem, add a print statement immediately before the loop
that says
``entering the loop and another immediately after that
says ``exiting the loop.
Run the program.  If you get the first message and not
the second, you've got an infinite loop.  Go to the section
titled ``Infinite loop.
Most of the time an infinite recursion will cause the program
to run for a while and then produce a StackOverflowException.
If that happens, go to the section
titled ``Infinite recursion.
If you are not getting a StackOverflowException, but you suspect
there is a problem with a recursive method, you can still use
the techniques in the infinite recursion section.
If neither of those things works, start testing other
loops and other recursive methods.
If none of those things works, then it is possible that
you don't understand the flow of execution in your program.
Go to the section titled ``Flow of execution.

===== Infinite loop =====
If you think you have an infinite loop and think you know
what loop is causing the problem, add a print statement at
the end of the loop that prints the values of the variables in
the condition, and the value of the condition.
For example,

   while (x > 0 && y < 0) 
       // do something to x
       // do something to y

       System.out.println ("x: " + x);
       System.out.println ("y: " + y);
       System.out.println ("condition: " + (x > 0 && y < 0));

Now when you run the program you will see three lines of output
for each time through the loop.  The last time through the
loop, the condition should be false.  If the loops keeps
going, you will be able to see the values of x and y
and you might figure out why they are not being updated correctly.

===== Infinite recursion =====
Most of the time an infinite recursion will cause the program
to run for a while and then produce a StackOverflowException.
If you suspect that method is causing an infinite recursion,
start by checking to make sure that there is a base case.
In other words, there should be some condition that will
cause the method to return without making a recursive
invocation.  If not, then you need to rethink the algorithm
and identify a base case.
If there is a base case, but the program doesn't seem to be
reaching it, add a print statement
at the beginning of the method that prints the parameters.
Now when you run the program you will see a few lines
of output every time the method is invoked, and you will
see the parameters.
If the parameters are not moving toward the base case, you
will get some ideas about why not.

===== Flow of execution =====
flow of execution

If you are not sure how the flow of execution is moving through
your program, add print statements to the beginning of each
method with a message like ``entering method foo, where
foo is the name of the method.
Now when you run the program it will print a trace of each
method as it is invoked.
It is often useful to print the parameters each method receives
when it is invoked.  When you run the program, check whether
the parameters are reasonable, and check for one of the
classic errors---providing parameters in the wrong order.

==== When I run the program I get an Exception. ====

If something goes wrong during run time, the Java run-time
system prints a message that includes the name of the
exception, the line of the program where the problem occurred,
and a stack trace.
The stack trace includes the method that is currently running,
and then the method that invoked it, and then the method that
invoked that, and so on.  In other words, it traces the
path of method invocations that got you to where you are.
The first step is to examine the place in the program where
the error occurred and see if you can figure out what happened.
[NullPointerException:] You tried to access an instance
variable or invoke a method on an object that is currently
null.  You should figure out what variable is null
and then figure out how it got to be that way.
Remember that when you declare a variable with an object type,
it is initially null, until you assign a value to it.
For example, this code causes a NullPointerException:
Point blank;
System.out.println (blank.x);
[ArrayIndexOutOfBoundsException:] The index you are using
to access an array is either negative or greater than
array.length-1.  If you can find the site where the
problem is, add a print statement immediately before it to
print the value of the index and the length of the array.
Is the array the right size?  Is the index the right value?
Now work your way backwards through the program and see where
the array and the index come from.  Find the nearest assignment
statement and see if it is doing the right thing.
If either one is a parameter, go to the place where the method
is invoked and see where the values are coming from.
[StackOverFlowException:] See ``Infinite recursion.

==== I added so many print statements I get inundated with output. ====
print statement

One of the problems with using print statements for debugging
is that you can end up buried in output.  There are two ways
to proceed: either simplify the output or simplify the program.
To simplify the output, you can remove or comment out print
statements that aren't helping, or combine them, or format
the output so it is easier to understand.
To simplify the program, there are several things you can do.  First,
scale down the problem the program is working on.  For example, if you
are sorting an array, sort a small array.  If the program takes
input from the user, give it the simplest input that causes the
Second, clean up the program.  Remove dead code and reorganize the
program to make it as easy to read as possible.  For example, if you
suspect that the error is in a deeply-nested part of the program,
try rewriting that part with simpler structure.  If you suspect a
large method, try splitting it into smaller methods and test them
Often the process of finding the minimal test case leads you
to the bug.  For example, if you find that a program works when
the array has an even number of elements, but not when it has
an odd number, that gives you a clue about what is going on.
Similarly, rewriting a piece of code can help you find subtle
bugs.  If you make a change that you think doesn't affect the
program, and it does, that can tip you off.

=== Semantic errors ===

==== My program doesn't work. ====
In some ways semantic errors are the hardest, because the
compiler and the run-time system provide no information about
what is wrong.  Only you know what the program was supposed to
do, and only you know that it isn't doing it.
The first step is to make a connection between the program
text and the behavior you are seeing.  You need a hypothesis
about what the program is actually doing.  One of the things
that makes this hard is that computers run so fast.
You will often wish that you could slow the program down to
human speed, but there is no straightforward way to do that,
and even if there were, it is not really a good way to debug.
Here are some questions to ask yourself:
Is there something the program was supposed to do, but
doesn't seem to be happening?  Find the section of the code
that performs that function and make sure it is executing when
you think it should.  Add a print statement to the beginning
of the suspect methods.
Is something happening that shouldn't?  Find code in
your program that performs that function and see if it is
executing when it shouldn't.
Is a section of code producing an effect that is not
what you expected?  Make sure that you understand the code in
question, especially if it involves invocations to built-in
Java methods.  Read the documentation for the methods you invoke.
Try out the methods by invoking the methods directly with
simple test cases, and check the results.
In order to program, you need to have a mental model of how
programs work.  If your program that doesn't do what you expect,
very often the problem is not in the program; it's in your mental

mental model

The best way to correct your mental model is to break the program
into its components (usually the classes and methods) and test
each component independently.  Once you find the discrepancy
between your model and reality, you can solve the problem.
Of course, you should be building and testing components as you
develop the program.  If you encounter a problem, there should be only
a small amount of new code that is not known to be correct.
Here are some common semantic errors that you might want to check for:
If you use the assignment operator,
=, instead of the equality operator, ==, in the
condition of an if, while or for statement,
you might get an expression that is sytactically legal, but
it doesn't do what you expect.
When you apply the equality operator, ==, to an
object, it checks shallow equality.  If you meant to check
deep equality, you should use the equals method (or
define one, for user-defined objects).
Some Java libraries expect user-defined objects to
define methods like equals.  If you don't define them
yourself, you will inherit the default behavior from the parent
class, which may not be what you want.
In general, inheritance can cause subtle semantic errors,
because you may be executing inherited code without realizing it.
Again, make sure you understand the flow of execution in your

==== I've got a big hairy expression and it doesn't do what I expect. ====
expression!big and hairy

Writing complex expressions is fine as long as they are readable,
but they can be hard to debug.  It is often a good idea to
break a complex expression into a series of assignments to
temporary variables.
For example:
rect.setLocation (rect.getLocation().translate

                    (-rect.getWidth(), -rect.getHeight()));

Can be rewritten as
int dx = -rect.getWidth();
int dy = -rect.getHeight();
Point location = rect.getLocation();
Point newLocation = location.translate (dx, dy);
rect.setLocation (newLocation);
The explicit version is easier to read, because the variable
names provide additional documentation, and easier to debug,
because we can check the types of the intermediate variables
and display their values.

temporary variable
order of evaluation

Another problem that can occur with big expressions is
that the order of evaluation may not be what you expect.
For example, if you are translating the expression

into Java, you might write

double y = x / 2 * Math.PI;
That is not correct, because multiplication and division have
the same precedence, and are evaluated from left to right.
So this expression computes .
A good way to debug expressions is to add parentheses to make
the order of evaluation explicit.
double y = x / (2 * Math.PI);
Any time you are not sure of the order of evaluation, use
parentheses.  Not only will the program be correct (in the sense
of doing what you intend); it will also be more readable for
other people who haven't memorized the rules of precedence.

==== I've got a method that doesn't return what I expect. ====
return statement

If you have a return statement with a complex expression,
you don't have a chance to print the return value before
returning.  Again, you can use a temporary variable.  For
example, instead of
public Rectangle intersection (Rectangle a, Rectangle b)  

   return new Rectangle (
       Math.min (a.x, b.x),
       Math.min (a.y, b.y),
       Math.max (a.x+a.width, b.x+b.width)-Math.min (a.x, b.x)
       Math.max (a.y+a.height, b.y+b.height)-Math.min (a.y, b.y) );

You could write
public Rectangle intersection (Rectangle a, Rectangle b)  

   int x1 = Math.min (a.x, b.x);
   int y2 = Math.min (a.y, b.y);
   int x2 = Math.max (a.x+a.width, b.x+b.width);
   int y2 = Math.max (a.y+a.height, b.y+b.height);
   Rectangle rect = new Rectangle (x1, y1, x2-x1, y2-y1);
   return rect;

Now you have the opportunity to display any of
the intermediate variables before returning.

==== My print statement isn't doing anything ====
print statement

If your use the println method, the output gets displayed
immediately, but if you use print (at least in some
environments) the output gets stored without being displayed until the
next newline character gets output.  If the program terminates without
producing a newline, you may never see the stored output.
If you suspect that this is happening to you, try changing
all the print statements to println.

==== I'm really, really stuck and I need help ====
First of all, try getting away from the computer for a few minutes.
Computers emit waves that affect the brain, causing the following
Frustration and/or rage.
Superstitious beliefs (``the computer hates me) and
magical thinking (``the program only works when I wear my
hat backwards).
Random walk programming (the attempt to program by writing
every possible program and choosing the one that does the right
If you find yourself suffering from any of these symptoms, get
up and go for a walk.  When you are calm, think about the program.
What is it doing?  What are some possible causes of that
behavior?  When was the last time you had a working program,
and what did you do next?
Sometimes it just takes time to find a bug.  I often find bugs
when I am away from the computer and I let my mind wander.  Some
of the best places to find bugs are trains, showers, and in bed,
just before you fall asleep.

==== No, I really need help. ====
It happens.  Even the best programmers occasionally get stuck.
Sometimes you work on a program so long that you can't see the
error.  A fresh pair of eyes is just the thing.
Before you bring someone else in, make sure you have exhausted
the techniques described here.  You program should be as simple
as possible, and you should be working on the smallest input
that causes the error.  You should have print statements in the
appropriate places (and the output they produce should be
comprehensible).  You should understand the problem well enough
to describe it concisely.
When you bring someone in to help, be sure to give
them the information they need.
What kind of bug is it?  Compile-time, run-time, or
If the bug occurs at compile-time or run-time, what is
the error message, and what part of the program does it indicate?
What was the last thing you did before this error occurred?
What were the last lines of code that you wrote, or what is
the new test case that fails?
What have you tried so far, and what have you learned? 
When you find the bug, take a second to think about what you
could have done to find it faster.  Next time you see something
similar, you will be able to find the bug more quickly.
Remember, in this class the goal is not to make the program
work.  The goal is to learn how to make the program work.
Program development plan
If you are spending a lot of time debugging, it is probably
because you do not have an effective program development
A typical, bad program development plan goes something like
Write an entire method.
Write several more methods.
Try to compile the program.
Spend an hour finding syntax errors.
Spend an hour finding run time errors.
Spend three hours finding semantic errors.
The problem, of course, is the first two steps.  If you
write more than one method, or even an entire method, before
you start the debugging process, you are likely to write more
code than you can debug.
If you find yourself in this situation, the only solution
is to remove code until you have a working program again, and
then gradually build the program back up.  Beginning programmers
are often unwilling to do this, because their carefully crafted
code is precious to them.  To debug effectively, you have to
be ruthless!

Here is a better program development plan:
Start with a working program that does something visible,

  like printing something.

Add a small number of lines of code at a time,

  and test the program after every change.

Repeat until the program does what it is supposed to do.
After every change, the program should produce some visible effect
that demonstrates the new code.  This approach to programming can save
a lot of time.  Because you only add a few lines of code at a time, it
is easy to find syntax errors.  Also, because each version of the
program produces a visible result, you are constantly testing your
mental model of how the program works.  If your mental model is
erroneous, you will be confronted with the conflict (and have a chance
to correct it) before you have written a lot of erroneous code.
One problem with this approach is that it is often
difficult to figure out a path from the starting place
to a complete and correct program.
I will demonstrate by developing a method called isIn
that takes a String and a Vector, and that returns a boolean:
true if the String appears in the list and false otherwise.
The first step is to write the shortest possible method
that will compile, run, and do something visible:
public static boolean isIn (String word, Vector v) 

   System.out.println ("isIn");
   return false;

Of course, to test the method we have to invoke it.  In
main, or somewhere else in a working program, we need
to create a simple test case.
We'll start with a case where the String appears in the vector
(so we expect the result to be true).
public static void main (String[] args) 

   Vector v = new Vector ();
   v.add ("banana");
   boolean test = isIn ("banana", v);
   System.out.println (test);

If everything goes according to plan, this code will compile,
run, and print the word isIn and the value false.
Of course, the answer isn't correct, but at this point we know
that the method is getting invoked and returning a value.
In my programming career, I have wasted way too much time debugging
a method, only to discover that it was never getting invoked.
If I had used this development plan, it never would have happened.
The next step is to check the parameters the method
public static boolean isIn (String word, Vector v) 

   System.out.println ("isIn looking for " + word);
   System.out.println ("in the vector " + v);
   return false;

The first print statement allows us to confirm that isIn
is looking for the right word.  The second statement prints a
list of the elements in the vector.
To make things more interesting, we might add a few more
elements to the vector:
public static void main (String[] args) 

   Vector v = new Vector ();
   v.add ("apple");
   v.add ("banana");
   v.add ("grapefruit");
   boolean test = isIn ("banana", v);
   System.out.println (test);

Now the output looks like this:
isIn looking for banana
in the vector [apple, banana, grapefruit]
Printing the parameters might seem silly, since we know what they
are supposed to be.  The point is to confirm that they are what we
think they are.

To traverse the vector, we can take advantage of the code
from Section vector.  In general, it is a great idea to
reuse code fragments rather than writing them from scratch.
public static boolean isIn (String word, Vector v) 

   System.out.println ("isIn looking for " + word);
   System.out.println ("in the vector " + v);

   for (int i=0; i