Java Reference
In-Depth Information
It's possible for you to introduce an error before you even start writing your pro-
gram, if you choose the wrong name for its file.
Common Programming Error
File Name Does Not Match Class Name
As mentioned earlier, Java requires that a program's class name and file name
match. For example, a program that begins with
public class Hello
must be
stored in a file called
Hello.java
.
If you use the wrong file name (for example, saving it as
WrongFileName.java
),
you'll get an error message like this:
WrongFileName.java:1: class Hello is public, should be
declared in a file named Hello.java
public class Hello {
∧
1 error
The file name is just the first hurdle. A number of other errors may exist in your
Java program. One of the most common syntax errors is to misspell a word. You may
have punctuation errors, such as missing semicolons. It's also easy to forget an entire
word, such as a required keyword.
The error messages the compiler gives may or may not be helpful. If you don't
understand the content of the error message, look for the caret marker (
) below the
line, which points at the position in the line where the compiler became confused.
This can help you pinpoint the place where a required keyword might be missing.
∧
Common Programming Error
Misspelled Words
Java (like most programming languages) is very picky about spelling. You need
to spell each word correctly, including proper capitalization. Suppose, for exam-
ple, that you were to replace the
println
statement in the “hello world” pro-
gram with the following:
System.out.pruntln("Hello, world!");
When you try to compile this program, it will generate an error message similar
to the following:
Hello.java:3: cannot find symbol
symbol : method pruntln(java.lang.String)
Continued on next page
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