[OT] Errore nel setup JDK installando JCreator

[minidiable]

Ho provato ad eseguire questo programma:

public class Ex3 {
    private static int[] arr=new int[2];

    public static void main(String[] args) {
    	try{
    		arr[2]=4; //Provo a mettere il valore 4 nel 3°elemento dell'array
    	}catch(ArrayIndexOutOfBoundsException e){
    		System.out.println("Ho rilevato l'eccezione");
    		e.printStackTrace();
    	}
    	}
    }

Ma mi da il seguente errore, ma perchè?
--------------------Configuration: <Default>--------------------
java.lang.NoClassDefFoundError: Ex3
Caused by: java.lang.ClassNotFoundException: Ex3
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)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"
Process completed.