Troubleshooting Compiler Problems

Can't Locate the Compiler
[PENDING: do platform-specific information here for all platforms] On UNIX systems, you may see the following error message if your path isn't set properly.
javac: Command not found 
Use setenv or similar command to modify your path variable so that it includes the directory where the Java compiler lives.
Syntax Errors
If you mistype part of a program, the compiler may issue a syntax error. The message usually displays the type of the error, the line number where the error was detected, the code on that line, and the position of the error within the code. Here's an error caused by omitting a ';' at the end of a statement:
testing.java:14: ';' expected.
    System.out.println("Input has " + count + " chars.")
                                                        ^
1 error
Sometimes the compiler can't guess your intent and prints a confusing error message or multiple error messages if the error cascades over several lines. For example, this code snippet omits a ';' from the bold line.
while (System.in.read() != -1)
    count++
System.out.println("Input has " + count + " chars.");
When processing this code, the compiler issues two error messages:
testing.java:13: Invalid type expression.
        count++
                 ^
testing.java:14: Invalid declaration.
    System.out.println("Input has " + count + " chars.");
                      ^
2 errors
The compiler issued two error messages because after it processed count++, the compiler's state indicates that it's in the middle of an expression. Without the ';' the compiler has no way of knowing that the statement is complete.
If you see any compiler errors, then your program did not successfully compile, and the compiler did not create a .class file. Carefully verify the program, fix any errors that you detect and try again.
Semantic Errors
In addition to verifying that your program is syntactically correct, the compiler checks for other basic correctness. For example, the compiler warns you each time you use a variable that has not been initialized:
testing.java:13: Variable count may not have been initialized.
        count++;
        ^
testing.java:14: Variable count may not have been initialized.
    System.out.println("Input has " + count + " chars.");
                                      ^
2 errors
Again, your program did not successfully compile, and the compiler did not create a .class file. Fix the error and try again.
See Also

The Java Development Environment
Compiler Man Page

Troubleshooting