Common Error Messages on Microsoft Windows Systems

'javac' is not recognized as an internal or external command, operable program or batch file

If you receive this error, Windows cannot find the compiler (javac).

Here's one way to tell Windows where to find javac. Suppose you installed the JDK in C:\jdk6. At the prompt you would type the following command and press Enter:

C:\jdk6\bin\javac HelloWorldApp.java

If you choose this option, you'll have to precede your javac and java commands with C:\jdk6\bin\ each time you compile or run a program. To avoid this extra typing, consult the section Update the PATH variable in the JDK 6 installation instructions.

Common Error Messages on UNIX Systems

javac: Command not found

If you receive this error, UNIX cannot find the compiler, javac.

Here's one way to tell UNIX where to find javac. Suppose you installed the JDK in /usr/local/jdk6. At the prompt you would type the following command and press Return:

/usr/local/jdk6/javac HelloWorldApp.java

Note: If you choose this option, each time you compile or run a program, you'll have to precede your javac and java commands with /usr/local/jdk6/. To avoid this extra typing, you could add this information to your PATH variable. The steps for doing so will vary depending on which shell you are currently running.

Syntax Errors (All Platforms)

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.

Error Messages on Microsoft Windows Systems

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class.

One of the places java tries to find your .class file is your current directory. So if your .class file is in C:\java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Enter:

cd c:\java

The prompt should change to C:\java>. If you enter dir at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH variable. To see if this is necessary, try clobbering the classpath with the following command.

set CLASSPATH=

Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable. To set this variable, consult the Update the PATH variable section in the JDK 6 installation instructions. The CLASSPATH variable is set in the same manner.

Error Messages on UNIX Systems

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class.

One of the places java tries to find your bytecode file is your current directory. So, for example, if your bytecode file is in /home/jdoe/java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Return:

cd /home/jdoe/java

If you enter pwd at the prompt, you should see /home/jdoe/java. If you enter ls at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH environment variable. To see if this is necessary, try clobbering the classpath with the following command.

unset CLASSPATH

Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable in the same manner as the PATH variable above.

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:

Exception in thread "main" java.lang.NoClassDefFoundError: 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. A Closer Look at the "Hello World!" Application discusses the main method in detail. If you are missing this method, you'll see the following error at runtime:

Exception in thread "main" java.lang.NoSuchMethodError: main

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