javac lab1a.java at the command line. If this went well, you
would get your command line back with no messages. The compiler creates
a file called, in this case, lab1a.class that contains the
Java byte code. All the interpreter needs to run your program is the
.class file - you could throw away the
.java file at this point if you never wanted to make
any changes to your program.
Things can go wrong if there are mistakes in your .java file. Here is a session with an error:
(maxwell@gamba.107) javac Newton.java
Newton.java:8: ';' expected.
return Math.cos(x)
^
1 error
(maxwell@gamba.108)
The compiler will list errors it found in the file in the order in
which it finds them. The format is
filename:line-number:
error-type, followed by an arrow pointing to the place in the
line where the error occurred. So, in the above error, there is a semi-colon
missing on line 8 in the file Newton.java. (If you don't know Java grammer
yet, don't worry, you will learn about it
later. You just need to know here that most lines in Java end
with a semi-colon.) The compiler shows that it
was expecting something at the end of the line. As you start out, you might
see other error messages - use the line number to find the spot in the file
and try to guess what might be wrong. Try undoing a recent
change and see if that fixes the problem. Also, an error early in the file
will often create a slough of errors later in the file, so start with
the first errors first, and try recompiling after you have fixed the
first few.
One final note: a file called myclass.java must contain a Java
class called myclass in it; otherwise the compiler will
fail. So don't rename .java files or you won't
be able to compile them.
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4 Running Java Programs |
David Maxwell,
who is still writing this, would like to
hear your comments and suggestions.