Source to accompany blog post, http://octodecillion.com/blog/detect-dupe-key-with-commons-config/

DuplicatePropDetectWithCommonsConfig.java

package com.octodecillion.util;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Properties;

import org.apache.commons.configuration.ConfigurationException;
import org.apache.commons.configuration.PropertiesConfiguration;
import org.junit.Assert;
import org.junit.Test;
import org.junit.experimental.runners.Enclosed;
import org.junit.runner.RunWith;

/**
 * Detect duplicate properties when loading Properties from stream.
 * <p>
 * 
 * @author jbetancourt
 * @since
 */
@RunWith(Enclosed.class)
public class DuplicatePropDetectWithCommonsConfig {

    /**
     * Detect duplicate keys in Properties resource.
     * <p>
     * <a href="">Commons Config</a> is used.
     * {@link Properties} class.
     * 
     * @see Properties#load(Reader)
     * 
     * @param reader The reader is NOT closed.
     * @param delimiterParsingDisabled 
     * @return Map of keys that are duplicated with a list value of each value of duplicate. 
     * @throws ConfigurationException 
     */
    @SuppressWarnings("unchecked")
    public Map<String, List<String>> loadWithConfig(Reader reader, boolean delimiterParsingDisabled) throws ConfigurationException {
        PropertiesConfiguration config = new PropertiesConfiguration();
        final Map<String, List<String>> results = new HashMap<>();

        config.setDelimiterParsingDisabled(delimiterParsingDisabled);
        config.load(reader);

        Iterator<String> keys = config.getKeys();
        while (keys.hasNext()) {
            String theKey = keys.next();
            List<?> valueObject = config.getList(theKey); 
            if(valueObject.size() > 1){
                results.put(theKey, (List<String>) valueObject);
            }
        }

        return results;
    }
    
    /**
     * @author jbetancourt
     *
     */
    public static class DuplicatePropDetectWithCommonsConfigTest {
        private static final String DATA1_PROPERTIES = "/Data1.properties";

        /**
         * @throws ConfigurationException
         */
        @Test
        public void testLoadFileWithConfig() throws ConfigurationException {
            System.out.println("*********************** running test ");
            InputStream is = this.getClass().getResourceAsStream(DATA1_PROPERTIES);
            Assert.assertNotNull(is);

            DuplicatePropDetectWithCommonsConfig detector = new DuplicatePropDetectWithCommonsConfig();
            Map<String, List<String>> map = detector.loadWithConfig(new InputStreamReader(is), true);
            Assert.assertEquals(1, map.size());

            Assert.assertTrue(map.containsKey("three"));

        }
    }

}