MapUtil.java

package me.falci.util;

import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

import com.google.common.base.Function;
import com.google.common.base.Joiner;
import com.google.common.collect.Lists;

public class MapUtil {
	
	public static String toString(Map<String, String> map, String joiner, Function<Entry<String, String>, String> function){
		List<String> transformed = Lists.transform(Lists.newArrayList(map.entrySet()), function);
		return Joiner.on(joiner).join(Lists.newArrayList(transformed));
	}
	
	public static String serialize(Map<String, String> map){
		return toString(map, "&", new Function<Entry<String, String>, String>() {

			@Override
			public String apply(Entry<String, String> item) {
				return item.getKey() + "=" + item.getValue();
			}
			
		});
	}
}

MapUtilTest.java

package me.falci.util;

import java.util.HashMap;
import java.util.Map;

import junit.framework.Assert;

public class MapUtilTest {

	@Test
	public void test() {
		
		Map<String, String> map = new HashMap<>(3);
		map.put("key1", "value1");
		map.put("key2", "value2");
		map.put("key3", "value3");
		
		String serialized = MapUtil.serialize(map);
		
		Assert.assertTrue(serialized.contains("key1=value1"));
		Assert.assertTrue(serialized.contains("key2=value2"));
		Assert.assertTrue(serialized.contains("key3=value3"));
		Assert.assertEquals(3, serialized.split("&").length);
	}

}