CS1 for Java (Keyboard Input)

For my Computer Science class, we have to download and use the Keyboard Class. So I downloaded the cs1.jar file and Keyboard.java (from which I compiled the Keyboard.class file) and put all three things in the folder which I save my Java files to. I then opened DrJava and tried to compile the program the teachers told us to use to test it:
// File: InputTest.java
import cs1.Keyboard;

public class InputTest {
public static void main(String[] args) {
int i;
System.out.print("Type a number, followed by the Enter key: ");
i = Keyboard.readInt();
System.out.println("The number is: " + i);
}
}

It didn't work. I then opened the Keyboard.java file in DrJava, so both were open at the same time. Then, I was able to compile InputTest.java without a problem. But when I tried to run it, it gave me this error message: NoClassDefFoundError: cs1/Keyboard
at InputTest.main(InputTest.java:10)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.ja va:25)
at java.lang.reflect.Method.invoke(Method.java:585)


I'm tearing my hair out trying to get this to work. Please help!