cannot run a simple java code

问题

I have downloaded a java developers kit for my 64bit windows 7, wrote down my code in the notepad, though the code is compiling from the command prompt and creating a .class file, but its refusing to run showing the error code:

java.lang.NoClassDefFoundError: first Caused by: java.lang.ClassNotFoundException: first   
  at java.net.URLClassLoader$1.run(Unknown Source)   
  at java.security.AccessController.doPrivileged(Native Method)   
  at java.net.URLClassLoader.findClass(Unknown Source)   
  at java.lang.ClassLoader.loadClass(Unknown Source)   
  at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)   
  at java.lang.ClassLoader.loadClass(Unknown Source) 
Could not find the main class: first.  Program will exit. Exception in thread "main"

I have made sure more than once that the file name and the class name are exactly same(i have kept them smallcase 'a' just to be sure). But still no avail, could you please suggest a few solutions please.. I'm new to java i'm basically a C/C++ programmer.

回答1:

A java program has this basic structure:

ClassName.java

public class ClassName
{
    public static void main(String[] args)
    {

    }
}

try using this outline to generate your code.

compile and run with:

javac ClassName.java
java ClassName

回答2:

I used to get this error when I ran a Class file.

Try doing: java NameOfClass

You don't need the .java extension when running, but you need it for compiling. That was always the problem I used to have.

回答3:

Did you set your classpath?

http://download.oracle.com/javase/1.3/docs/tooldocs/win32/classpath.html

java -classpath <path> <classname>

回答4:

You must provide with the code.

Anyway, from the Java Docs, class ClassNotFoundException:

Thrown when an application tries to load in a class through its string name using:

The forName method in class Class.

The findSystemClass method in class ClassLoader.

The loadClass method in class ClassLoader.

but no definition for the class with the specified name could be found.

Must read link : Tip: Causes of java.lang.ClassNotFoundException

回答5:

It is quite possible after compiling your code you would be writing java filename.java. Because this sought of exception occurs then.

After compiling your program using javac filename.java use the command to start your interpreter java filename.

As you enter this command java interpreter automatically starts interpreting filename.class.

commands :

javac filename.java   // to start compiling
java filename         // to start interpreting

来源：https://stackoverflow.com/questions/6549825/cannot-run-a-simple-java-code

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java

compilation