从命令行运行java程序时，在线程“main”java.lang.NoClassDefFoundError'中出现异常

Question

What am I doing wrong here:

class Helo { 
   // main: generate some simple output 
   public static void main (String[] args) { 
      System.out.println ("Hello, world."); // print one line 
      System.out.println ("How are you?"); // print another 
   } 
} 

When I go into terminal I do:

cd ~
javac Atempt2.java (//that's the file name) 
java Atempt2 

and then it gives me this error message:

Exception in thread "main" java.lang.NoClassDefFoundError: Atempt2

So all in all this is what I do and what happens:

david-allenders-macbook-pro:~ davidallender$ cd ~
david-allenders-macbook-pro:~ davidallender$ javac Atempt2.java
david-allenders-macbook-pro:~ davidallender$ java Atempt2
Exception in thread "main" java.lang.NoClassDefFoundError: Atempt2
david-allenders-macbook-pro:~ davidallender$ 

I am very new at this so please explain things in a very simple manner.

Thanks.

Answer 1

自从我完成任何java工作以来已经有一段时间了，但我很确定你的类名需要与你的文件名相匹配。

Answer 2

javac uses the class name to generate the output not the filename. So it will generate a Helo.class classfile.
java will take a class name and call the main function in the corresponding class file, here Hello.class.

The ClassNotFoundError is thrown because javac never generated an Atemp2 classfile as there is no Atemp2 class in your source file.

Answer 3

Rename your Atempt2.java to Hello.java to get going, then:

javac Helo.java
java Helo

See here for more discussion and the reasoning .

Answer 4

change:

class Helo

to

class Atempt2

in your source file.

A .java file that declares a class must have the file name match the declared class name.

Answer 5

The filename must match the name of the public class defined in the file. In this case, you would either have to name the file "Helo.java" or renamed the class to Atempt2 .

Answer 6

This is the very basic to start with java programming.Any program you write the name of the file must match with the public class of the program. Here in your program public class of the file is Helo so your file name must be Helo.java.Here the compiler is able to compile but JVM will search for Helo.class file to run. As there is no Helo.class file you are getting runtime Exception Exception in thread "main" java.lang.NoClassDefFoundError: Atempt2

Answer 7

to complement josefx 's answer.

The argument to the compiler ( javac ) is the name of the file or files to compile (as you did).

On the other side, the virtual machine ( java ) gets the name of the class whose main method is to be executed.

One option would be

javac Atempt2.java    // the file name
java Helo             // the class name

Normally it is a good idea to have the file named the same way as the class. For public class this is a must (checked by compiler).