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Lesson: Common Problems (and Their Solutions)

If you're having trouble compiling or running your application, this section might be able to help. If nothing in this section helps, please refer to the release documentation for the JDK that you are using.

Can't Locate the Compiler

On UNIX systems, you may see the following error message if your path isn't set properly:

javac: Command not found

The "first steps" section at the beginning of this trail explains how to modify your PATH environment variable to include the directory where the 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 semicolon (;) 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, the following code snippet omits a semicolon (;) 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 issues two error messages because after it processes count++, the compiler's state indicates that it's in the middle of an expression. Without the semicolon, 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.

Can't Find Class

A common error of beginner programmers is to try and run the Java launcher on the .class file that was created by the compiler. For example, if you try to run your program with java HelloWorldApp.class instead of java HelloWorldApp, you'll see this error message:

Can't find class HelloWorldApp.class

The argument is the name of the class that you want to use, not the filename.

The main Method Is Not Defined

The Java Virtual Machine requires that the class you execute with it have a main method at which to begin execution of your application. The main Method discusses the main method in detail. If you are missing this method, you'll see the following error at runtime:

In class classname: void main(String argv[]) is not defined

In the above message, classname is the name of the class that you tried to run.

Changes to My Program Didn't Take Effect

Sometimes when you are in the edit/debug/run cycle, it appears that your changes to an application didn't take effect -- a print statement isn't printing, for example. This is common when running applications on MacOS using Java Runner. If you recompile a .class file, you must quit Java Runner and bring it up again, since Java Runner does not reload classes.

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