java.lang.NoClassDefFoundError: org/spongycastle/util/encoders/Hex in Java when dealing with Bitcoin addresses
You need to resolve the dependency org.spongycastle.util.encoders.Hex, more than likely you need to add this dependency to your pom.xml file if you are using Maven. If you are not using maven, you will have download the JAR file from somewhere and I add it to your build path in Eclipse. 

From a quick Google search this appears this is  the dependency that you want.