Java Eclipse issuing a ClassNotFoundException while using AutoCompleteDecorator.decorate method from swingx-all-1.6.4.jar

Question

I downloaded swingx-all-1.6.4.jar from here http://www.java2s.com/Code/Jar/s/Downloadswingxall164jar.htm, I added it to my Java Build Path in Eclipse. then I wrote java code using AutoCompleteDecorator such that:

...

import org.jdesktop.swingx.autocomplete.AutoCompleteDecorator;

...

    AutoCompleteDecorator.decorate(myComboBox);

I do not have any issues by Eclipse about the code, it seems all written correctly, but when I run the application I get the following message

Exception in thread "main" java.lang.NoClassDefFoundError: org/jdesktop/swingx/autocomplete/AutoCompleteDecorator at DD_StartNewActivity.(DD_StartNewActivity.java:636) at DD_StartNewActivity.main(DD_StartNewActivity.java:529) Caused by: java.lang.ClassNotFoundException: org.jdesktop.swingx.autocomplete.AutoCompleteDecorator at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641) at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188) at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520) ... 2 more

What's going wrong, please? Thank you so much!