Parse a URI String into Name-Value Collection

If you are looking for a way to achieve it without using an external library, the following code will help you.

public static Map<String, String> splitQuery(URL url) throws UnsupportedEncodingException {
    Map<String, String> query_pairs = new LinkedHashMap<String, String>();
    String query = url.getQuery();
    String[] pairs = query.split("&");
    for (String pair : pairs) {
        int idx = pair.indexOf("=");
        query_pairs.put(URLDecoder.decode(pair.substring(0, idx), "UTF-8"), URLDecoder.decode(pair.substring(idx + 1), "UTF-8"));
    }
    return query_pairs;
}

You can access the returned Map using <map>.get("client_id"), with the URL given in your question this would return "SS".

UPDATE URL-Decoding added

UPDATE As this answer is still quite popular, I made an improved version of the method above, which handles multiple parameters with the same key and parameters with no value as well.

public static Map<String, List<String>> splitQuery(URL url) throws UnsupportedEncodingException {
  final Map<String, List<String>> query_pairs = new LinkedHashMap<String, List<String>>();
  final String[] pairs = url.getQuery().split("&");
  for (String pair : pairs) {
    final int idx = pair.indexOf("=");
    final String key = idx > 0 ? URLDecoder.decode(pair.substring(0, idx), "UTF-8") : pair;
    if (!query_pairs.containsKey(key)) {
      query_pairs.put(key, new LinkedList<String>());
    }
    final String value = idx > 0 && pair.length() > idx + 1 ? URLDecoder.decode(pair.substring(idx + 1), "UTF-8") : null;
    query_pairs.get(key).add(value);
  }
  return query_pairs;
}

UPDATE Java8 version

public Map<String, List<String>> splitQuery(URL url) {
    if (Strings.isNullOrEmpty(url.getQuery())) {
        return Collections.emptyMap();
    }
    return Arrays.stream(url.getQuery().split("&"))
            .map(this::splitQueryParameter)
            .collect(Collectors.groupingBy(SimpleImmutableEntry::getKey, LinkedHashMap::new, mapping(Map.Entry::getValue, toList())));
}

public SimpleImmutableEntry<String, String> splitQueryParameter(String it) {
    final int idx = it.indexOf("=");
    final String key = idx > 0 ? it.substring(0, idx) : it;
    final String value = idx > 0 && it.length() > idx + 1 ? it.substring(idx + 1) : null;
    return new SimpleImmutableEntry<>(
        URLDecoder.decode(key, "UTF-8"),
        URLDecoder.decode(value, "UTF-8")
    );
}

Running the above method with the URL

https://stackoverflow.com?param1=value1&param2=&param3=value3&param3

returns this Map:

{param1=["value1"], param2=[null], param3=["value3", null]}

org.apache.http.client.utils.URLEncodedUtils

is a well known library that can do it for you

import org.apache.hc.client5.http.utils.URLEncodedUtils

String url = "http://www.example.com/something.html?one=1&two=2&three=3&three=3a";

List<NameValuePair> params = URLEncodedUtils.parse(new URI(url), Charset.forName("UTF-8"));

for (NameValuePair param : params) {
  System.out.println(param.getName() + " : " + param.getValue());
}

Outputs

one : 1
two : 2
three : 3
three : 3a

If you are using Spring Framework:

public static void main(String[] args) {
    String uri = "http://my.test.com/test?param1=ab&param2=cd&param2=ef";
    MultiValueMap<String, String> parameters =
            UriComponentsBuilder.fromUriString(uri).build().getQueryParams();
    List<String> param1 = parameters.get("param1");
    List<String> param2 = parameters.get("param2");
    System.out.println("param1: " + param1.get(0));
    System.out.println("param2: " + param2.get(0) + "," + param2.get(1));
}

You will get:

param1: ab
param2: cd,ef

use google Guava and do it in 2 lines:

import java.util.Map;
import com.google.common.base.Splitter;

public class Parser {
    public static void main(String... args) {
        String uri = "https://google.com.ua/oauth/authorize?client_id=SS&response_type=code&scope=N_FULL&access_type=offline&redirect_uri=http://localhost/Callback";
        String query = uri.split("\\?")[1];
        final Map<String, String> map = Splitter.on('&').trimResults().withKeyValueSeparator('=').split(query);
        System.out.println(map);
    }
}

which gives you

{client_id=SS, response_type=code, scope=N_FULL, access_type=offline, redirect_uri=http://localhost/Callback}