How can I solve java lang NoClassDefFoundError

Question

I've tried both the examples in Oracle's Java Tutorials. They both compile fine, but at run time, both come up with this error:

Exception in thread "main" java.lang.NoClassDefFoundError: graphics/shapes/Square
    at Main.main(Main.java:7)
Caused by: java.lang.ClassNotFoundException: graphics.shapes.Square
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 1 more

I think I might have the Main.java file in the wrong folder.

Here is the directory hierarchy:

graphics
├ Main.java
├ shapes
|   ├ Square.java
|   ├ Triangle.java
├ linepoint
|   ├ Line.java
|   ├ Point.java
├ spaceobjects
|   ├ Cube.java
|   ├ RectPrism.java

And here is Main.java:

import graphics.shapes.*;
import graphics.linepoint.*
import graphics.spaceobjects.*;

public class Main {
    public static void main(String args[]) {
        Square s = new Square(2, 3, 15);
        Line l = new Line(1, 5, 2, 3);
        Cube c = new Cube(13, 32, 22);
    }
}

Why am I doing this wrong?

UPDATE

The Main class was added to the graphics package (I added package graphics; to it), the classpath was set to the "_test" folder (which contains graphics), it was compiled, and Java Graphics was used to run it.

It functioned in main (from the command line).

Extremely tardy UPDATE #2

Since I wasn't using Eclipse and only had Notepad++ and the JDK, the aforementioned update fixed my issue. Though they share similar concepts, it appears that many of these responses are for Eclipse and IntelliJ IDEA.