从命令行运行Java程序

Question

I am trying to run a java program from command line. I tried following the steps mentioned here . But when I try to run javac Hello.java , it's throwing error that such a program is not there. I tried giving java Hello.java and got the error:

Exception in thread "main" java.lang.NoClassDefFoundError: Hello/java
Caused by: java.lang.ClassNotFoundException: Hello.java
        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: Hello.java.  Program will exit.

What is the problem here. How can I do it?

EDIT: I have many classes in my code file, Hello.java. Will that cause any problem?

Answer 1

First you should compile the java code with

 javac Hello.java

Then run it

 java Hello

In both cases, make sure your classpath is set correctly...

Answer 2

To run the program you need to do:

java Hello

which is java followed by the class name without extension .

Answer 3

First off, java requires at most one public class per file. No

public class this {
}

public class that {
}

You can have

class this {
   class that {
   }
}

if you need.

EDIT or in file this.java:

public class this {
}

class that {
}

javac won't be in the jre folder. Have you installed the jdk? it doesn't come by default on many computers. it's often in "C:\\Program Files\\Java\\jdk1.6.0_05\\bin\\javac.exe" or a similiar path.

Answer 4

As stated by the others answer, first, you have to run your application using java Hello and not java Hello.java

Second, you have to check that your CLASSPATH is correctly set. It seems that your variable is not set or does not integrate the current directory, ie .

So run :

javac -classpath . Hello.java
java -classpath Hello

or

set CLASSPATH=.
javac Hello.java
java Hello

Of course, defining the CLASSPATH as a user / system variable in your Windows system is a better solution!

Answer 5

In case Hello.java is contained in a package, you will have to create an appropriate directory structure. Ie in case Hello.java is contained in the package com.stackoverflow , you must create the folders com/stackoverflow and put Hello.java in this folder. From the root folder you must then launch

java com.stackoverflow.Hello