Couldn`t join two files with one key via Cascading

Lets see what we have. First file [Interface Class]:

list arrayList
list linkedList

Second file[Class1 amount]:

arrayList 120
linkedList 4

I would like to join this two files by key[Class] and get count per each Interface:

list arraylist 120
list linkedlist 4

Code:

public class Main
{
public static void main( String[] args )
{
String docPath = args[ 0 ];
String wcPath = args[ 1 ];
String doc2Path = args[ 2 ];

Properties properties = new Properties();
AppProps.setApplicationJarClass( properties, Main.class );
AppProps.setApplicationName( properties, "Part 1" );
AppProps.addApplicationTag( properties, "lets:do:it" );
AppProps.addApplicationTag( properties, "technology:Cascading" );
FlowConnector flowConnector = new Hadoop2MR1FlowConnector( properties );

// create source and sink taps
Tap wcTap = new Hfs(new TextDelimited(true, ","), wcPath);


    Fields classInterfaceFiles = new Fields("interface", "class");
    Tap classInterfaceTap = new Hfs(new TextDelimited(classInterfaceFiles, true, ","), docPath);

    Fields classAmountFields = new Fields("class1", "amount");
    Tap classAmountFileTap = new Hfs(new TextDelimited(classAmountFields, true, ","), doc2Path);

    Tap outTap = new MultiSinkTap(); // just saying, create your own tap
    Pipe classInterfaceFilePipe = new Pipe("classInterfaceFilePipe");

    Pipe classIAmountFilePipe = new Pipe("classIAmountFilePipe");

    Fields groupFields = new Fields("class");
    Fields groupFields1 = new Fields("class1"); // fields used as joining keys
    Pipe outPipe = new CoGroup(classInterfaceFilePipe, groupFields, classIAmountFilePipe, groupFields1, new InnerJoin());


 // build flow definition
    FlowDef flowDef = FlowDef.flowDef().setName("myFlow")
            .addSource(classInterfaceFilePipe, classInterfaceTap)
            .addSource(classIAmountFilePipe, classAmountFileTap)
            .addTailSink(outPipe, wcTap);
         //   .addTailSink( outPipe, wcTap );


// write a DOT file and run the flow
Flow wcFlow = flowConnector.connect( flowDef );
wcFlow.writeDOT( "dot/wc.dot" );
wcFlow.complete();
}

}

[this is one step of bigger task]

This is happening because you have same field in two pipes which are joined together i.e. "class". Probably you can rename those to "class_interface" and "class_amount". You will also have to make change in the groupFields you used in CoGroup pipe.