Last updated: Saturday 21st March 2015, 23:51 PT, AD

Debugging Java Programs

What is debugging?

Programming is a complex process, and because it is done by human beings,
it often leads to errors.  For whimsical reasons, programming errors are
called bugs and the process of tracking them down and
correcting them is called debugging.

Three kinds of errors can occur in a program: syntax errors, runtime errors, 
and semantic errors. It is useful to distinguish between them in order to track 
them down more quickly.

Syntax errors

Java can only execute a program if the program is syntactically correct;
otherwise, the process fails and returns an error message.
syntax refers to the structure of a program and the rules
about that structure.  For example, in English, a sentence must begin with a
capital letter and end with a period.  this sentence contains a
syntax error.  So does this one

If there is a single syntax error
anywhere in your program, Java will print an error message and quit,
and you will not be able to run your program. During the first few weeks
of your programming career, you will probably spend a lot of time tracking
down syntax errors.  As you gain experience, though, you will make fewer
errors and find them faster.

Runtime errors

The second type of error is a runtime error, so called because the error
does not appear until you run the program.  These errors are also called
exceptions because they usually indicate that something
exceptional (and bad) has happened.

Runtime errors are rare in the simple programs you will see in the first few 
chapters, so it might be a while before you encounter one.

Semantic errors

The third type of error is the semantic error.  If there is 
a semantic error in your program, it will run successfully, in the sense that
the computer will not generate any error messages, but it will not do the right 
thing.  It will do something else.  Specifically, it will do what you told it
to do.

The problem is that the program you wrote is not the program you wanted to
write.  The meaning of the program (its semantics) is wrong.  Identifying
semantic errors can be tricky because it requires you to work backward by
looking at the output of the program and trying to figure out what it is doing.

Experimental debugging

One of the most important skills you will acquire is debugging.  Although it 
can be frustrating, debugging is one of the most intellectually rich,
challenging, and interesting parts of programming.

In some ways, debugging is like detective work.  You are confronted with
clues, and you have to infer the processes and events that led to the results
you see.

Debugging is also like an experimental science.  Once you have an idea what
is going wrong, you modify your program and try again.  If your hypothesis was
correct, then you can predict the result of the modification, and you take a
step closer to a working program.  If your hypothesis was wrong, you have to
come up with a new one.  

For some people, programming and debugging are the same thing.  That is,
programming is the process of gradually debugging a program until it does what
you want.  The idea is that you should start with a program that does
something and make small modifications, debugging them as you go, so
that you always have a working program.

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