How to convert map to url query string?

Do you know of any utility class/library, that can convert Map into URL-friendly query string?

Example:

I have a map:

"param1"=12,
"param2"="cat"

I want to get:

param1=12&param2=cat

final output

relativeUrl+param1=12&param2=cat

The most robust one I saw off-the-shelf is the URLEncodedUtils class from Apache Http Compoments (HttpClient 4.0).

The method URLEncodedUtils.format() is what you need.

It doesn't use map so you can have duplicate parameter names, like,

  a=1&a=2&b=3

Not that I recommend this kind of use of parameter names.

Here's something that I quickly wrote; I'm sure it can be improved upon.

import java.util.*;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;

public class MapQuery {
    static String urlEncodeUTF8(String s) {
        try {
            return URLEncoder.encode(s, "UTF-8");
        } catch (UnsupportedEncodingException e) {
            throw new UnsupportedOperationException(e);
        }
    }
    static String urlEncodeUTF8(Map<?,?> map) {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<?,?> entry : map.entrySet()) {
            if (sb.length() > 0) {
                sb.append("&");
            }
            sb.append(String.format("%s=%s",
                urlEncodeUTF8(entry.getKey().toString()),
                urlEncodeUTF8(entry.getValue().toString())
            ));
        }
        return sb.toString();       
    }
    public static void main(String[] args) {
        Map<String,Object> map = new HashMap<String,Object>();
        map.put("p1", 12);
        map.put("p2", "cat");
        map.put("p3", "a & b");         
        System.out.println(urlEncodeUTF8(map));
        // prints "p3=a+%26+b&p2=cat&p1=12"
    }
}

I found a smooth solution using java 8 and polygenelubricants' solution.

parameters.entrySet().stream()
    .map(p -> urlEncodeUTF8(p.getKey()) + "=" + urlEncodeUTF8(p.getValue()))
    .reduce((p1, p2) -> p1 + "&" + p2)
    .orElse("");