Error: Could not find or load main class concurrency.MainTest

I installed eclipse kepler and JDK 1.7.45 on lubuntu 13.04

Here is java version:

nazar@nazar-desktop:~$ java -version
java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)
nazar@nazar-desktop:~$ echo $JAVA_HOME
/opt/java/jdk1.7.0_45

I configured at eclipse at:

Window => preferences => java => installed jre => add my jdk

I wrote easy test class:

package concurrency;

public class MainTest {

    public static void main(String[] args) {
        System.out.println("Hello world ");
    }    
}

But when I run I see error message:

Error: Could not find or load main class concurrency.MainTest

It even doesn't compile this class into src/bin.

I tried to do it by myself as

javac MainTest.java => java MainTest

It compiles but at run time throws:

Exception in thread "main" java.lang.NoClassDefFoundError: MainTest (wrong name: concurrency/MainTest)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:482)

I couldn't figure out what it can be. Coz installed jdk is compatible with OS system.

UPDATE:

This project was from another machine there was installed java.1.7_40. Here I imported this project and keep doing it.

Here is content of eclipse.ini:

-startup
plugins/org.eclipse.equinox.launcher_1.3.0.v20130327-1440.jar
--launcher.library
plugins/org.eclipse.equinox.launcher.gtk.linux.x86_64_1.1.200.v20130807-1835
-product
org.eclipse.epp.package.jee.product
--launcher.defaultAction
openFile
-showsplash
org.eclipse.platform
--launcher.XXMaxPermSize
256m
--launcher.defaultAction
openFile
--launcher.appendVmargs
-vm
/opt/java/jdk1.7.0_45/bin/java
-vmargs
-Dosgi.requiredJavaVersion=1.6
-XX:MaxPermSize=256m
-Xms40m
-Xmx1024m
-Dorg.eclipse.swt.browser.IEVersion=10001

Any suggestions?

• How to solve this trouble?