Is there a right way to build a URL? [duplicate]

You can use Apache URIBuilder

Sample code: Full Apache Example

URIBuilder builder = new URIBuilder()
    .setScheme("http")
    .setHost("apache.org")
    .setPath("/shindig")
    .addParameter("helloWorld", "foo&bar")
    .setFragment("foo");
builder.toString();

Output: http://apache.org/shindig?helloWorld=foo%26bar#foo

You can also use spring UriComponentsBuilder

UriComponentsBuilder
    .fromUriString(baseUrl)
    .queryParam("name", name)
    .queryParam("surname", surname)
    .build().toUriString();

I have written this up, you can change it where you want extra functionality. It doesn't use any external resources, let me know if I have looked over something!

It's basically a wrapper for the URI class that allows you to more easily add subdirectories and parameters to the URI. You can set default values if you're not interested in some things.

Edit: I have added an option to use a relative URI (per your question).

public class Test {
    public static void main(String[] args) throws URISyntaxException,
            MalformedURLException {
        URLBuilder urlb = new URLBuilder("www.example.com");
        urlb.setConnectionType("http");
        urlb.addSubfolder("somesub");
        urlb.addSubfolder("anothersub");
        urlb.addParameter("param lol", "unknown");
        urlb.addParameter("paramY", "known");
        String url = urlb.getURL();
        System.out.println(url);


        urlb = new URLBuilder();
        urlb.addSubfolder("servlet");
        urlb.addSubfolder("jsp");
        urlb.addSubfolder("somesub");
        urlb.addSubfolder("anothersub");
        urlb.addParameter("param lol", "unknown");
        urlb.addParameter("paramY", "known");
        String relUrl = urlb.getRelativeURL();
        System.out.println(relUrl);
    }
}

class URLBuilder {
    private StringBuilder folders, params;
    private String connType, host;

    void setConnectionType(String conn) {
        connType = conn;
    }

    URLBuilder(){
        folders = new StringBuilder();
        params = new StringBuilder();
    }

    URLBuilder(String host) {
        this();
        this.host = host;
    }

    void addSubfolder(String folder) {
        folders.append("/");
        folders.append(folder);
    }

    void addParameter(String parameter, String value) {
        if(params.toString().length() > 0){params.append("&");}
        params.append(parameter);
        params.append("=");
        params.append(value);
    }

    String getURL() throws URISyntaxException, MalformedURLException {
        URI uri = new URI(connType, host, folders.toString(),
                params.toString(), null);
        return uri.toURL().toString();
    }

    String getRelativeURL() throws URISyntaxException, MalformedURLException{
        URI uri = new URI(null, null, folders.toString(), params.toString(), null);
        return uri.toString();
    }
}

Output:

Absolute

http://www.example.com/somesub/anothersub?param%20lol=unknown&paramY=known

Relative

/servlet/jsp/somesub/anothersub?param%20lol=unknown&paramY=known

I like @Jeroen's suggestion but it didn't quite do all I wanted so, using his idea of gathering the parts together and then using a URI to grow the final result I put together this solution which seems to do what I want:

public class URLBuilder {
  // The scheme - http
  private String scheme = null;
  // The user - user
  private String user = null;
  // The host - example.com
  private String host = null;
  // The port - 8080
  private int port = -1;
  // The paths - /Path/To/Somewhere/index.jsp
  private final ArrayList<String> paths = new ArrayList<String>();
  // The parameters - ?a=b&c=d
  private final ArrayList<Pair<String, String>> queries = new ArrayList<Pair<String, String>>();
  // The fragment - #n
  private String fragment = null;

  public URLBuilder addQuery(String name, String value) {
    queries.add(new Pair(name, value));
    return this;
  }

  public URLBuilder addQuery(String name, long value) {
    addQuery(name, String.valueOf(value));
    return this;
  }

  public URLBuilder addQuery(String name, File file) {
    addQuery(name, file.toURI().getPath());
    return this;
  }

  public URLBuilder addPath(String path) {
    paths.add(path);
    return this;
  }

  @Override
  public String toString() {
    // Build the path.
    StringBuilder path = new StringBuilder();
    for (String p : paths) {
      path.append("/").append(p);
    }
    // Build the query.
    StringBuilder query = new StringBuilder();
    String sep = "";
    for (Pair<String, String> p : queries) {
      query.append(sep).append(p.p).append("=").append(p.q);
      sep = "&";
    }
    String url = null;
    try {
      URI uri = new URI(
              scheme,
              user,
              host,
              port,
              path.length() > 0 ? path.toString() : null,
              query.length() > 0 ? query.toString() : null,
              fragment);
      url = uri.toString();
    } catch (URISyntaxException ex) {
      Logger.getLogger(URLBuilder.class.getName()).log(Level.SEVERE, null, ex);
    }

    return url;
  }

  /**
   * @param host the host to set
   * @return this
   */
  public URLBuilder setHost(String host) {
    this.host = host;
    return this;
  }

  /**
   * @param scheme the scheme to set
   * @return this
   */
  public URLBuilder setScheme(String scheme) {
    this.scheme = scheme;
    return this;
  }

  /**
   * @param user the user to set
   * @return this
   */
  public URLBuilder setUser(String user) {
    this.user = user;
    return this;
  }

  /**
   * @param port the port to set
   * @return this
   */
  public URLBuilder setPort(int port) {
    this.port = port;
    return this;
  }

  /**
   * @param fragment the fragment to set
   * @return this
   */
  public URLBuilder setFragment(String fragment) {
    this.fragment = fragment;
    return this;
  }

  public static void main(String args[]) {
    try {
      URLBuilder url = new URLBuilder();
      System.out.println(url.toString());
      url.setFragment("fragment");
      System.out.println(url.toString());
      url.setHost("host.com");
      System.out.println(url.toString());
      url.addPath("APath");
      System.out.println(url.toString());
      url.addPath("AnotherPath");
      System.out.println(url.toString());
      url.addQuery("query1", "param1");
      System.out.println(url.toString());
      url.addQuery("query 2", "param 2");
      System.out.println(url.toString());
      url.addQuery("file", new File("Hello World.txt"));
      System.out.println(url.toString());
    } catch (Throwable t) {
      t.printStackTrace(System.err);
    }
  }

}