cross validation

Hello,
in Java I need to split a list and generate a TEST and TRAINING set list; I did this below that doesn't work because of an exception during of the 'removeAll'; is it a good way to do it? how can I do that RemoveALL? thanks

Code:

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File dir = new File("..\\..\\dir");
String[] files = dir.list();
 int block = files.length*9/10;
	    for (int numbOfTest=0; numbOfTest < 10; numbOfTest++ ) {
	    	List<String> list = Arrays.asList(files);
		    List<String> listTEST = list.subList( numbOfTest*block, block);		    		    		    
		    Collection<String> c = listTEST;
                 //now list will be the training list
                 list.removeAll( list ); //error

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