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Need to replace spaces inside string with percentual symbol Java

I need to replace the spaces inside a string with the % symbol but I'm having some issues, what I tried is:
imageUrl = imageUrl.replace(' ', "%20");
But It gives me an error in the replace function.
Then:
imageUrl = imageUrl.replace(' ', "%%20");
But It still gives me an error in the replace function.
The I tried with the unicode symbol:
imageUrl = imageUrl.replace(' ', (char) U+0025 + "20");
But it still gives error.
Is there an easy way to do it?

String.replace(String, String) is the method you want.
replace
imageUrl.replace(' ', "%");
with
imageUrl.replace(" ", "%");
System.out.println("This is working".replace(" ", "%"));
I suggest you to use a URL Encoder for Encoding Strings in java.
String searchQuery = "list of banks in the world";
String url = "http://mypage.com/pages?q=" + URLEncoder.encode(searchQuery, "UTF-8");

I've ran into issues like this in the past with certain frameworks. I don't have enough of your code to know for sure, but what might be happening is whatever http framework you are using, in my case it was spring, is encoding the URL again. I spent a few days trying to solve a similar problem where I thought that string replace and the URI.builder() was broken. What ended up being the problem was that my http framework had taken my encoded url, and encoded it again. that means that any place it saw a "%20", it would see the '%' charictor and switch it out for '%' http code, "%25", resulting in. "%2520". The request would then fail because %2520 didn't translate into the space my server was expecting. While the issue apeared to be one of my encoding not working, it was really an issue of encoding too many times. I have an example from some working code in one of my projects below
//the Url of the server
String fullUrl = "http://myapiserver.com/path/";
//The parameter to append. contains a space that will need to be encoded
String param 1 = "parameter 1"
//Use Uri.Builder to append parameter
Uri.Builder uriBuilder = Uri.parse(fullUrl).buildUpon();
uriBuilder.appendQueryParameter("parameter1",param1);
/* Below is where it is important to understand how your
http framework handles unencoded url. In my case, which is Spring
framework, the urls are encoded when performing requests.
The result is that a url that is already encoded will be
encoded twice. For instance, if you're url is
"http://myapiserver.com/path?parameter1=param 1"
and it needs to be read by the server as
"http://myapiserver.com/path?parameter1=param%201"
it makes sense to encode the url using URI.builder().append, or any valid
solutions listed in other posts. However, If the framework is already
encoding your url, then it is likely to run into the issue where you
accidently encode the url twice: Once when you are preparing the URL to be
sent, and once again when you are sending the message through the framework.
this results in sending a url that looks like
"http://myapiserver.com/path?parameter1=param%25201"
where the '%' in "%20" was replaced with "%25", http's representation of '%'
when what you wanted was
"http://myapiserver.com/path?parameter1=param%201"
this can be a difficult bug to squash because you can copy the url in the
debugger prior to it being sent and paste it into a tool like fiddler and
have the fiddler request work but the program request fail.
since my http framework was already encoding the urls, I had to unencode the
urls after appending the parameters so they would only be encoded once.
I'm not saying it's the most gracefull solution, but the code works.
*/
String finalUrl = uriBuilder.build().toString().replace("%2F","/")
.replace("%3A", ":").replace("%20", " ");
//Call the server and ask for the menu. the Menu is saved to a string
//rest.GET() uses spring framework. The url is encoded again as
part of the framework.
menuStringFromIoms = rest.GET(finalUrl);
There is likely a more graceful way to keep a url from encoding twice. I hope this example helps point you on the right direction or eliminate a possability. Good luck.

Try this:
imageUrl = imageUrl.replaceAll(" ", "%20");

Replace spaces is not enought, try this
url = java.net.URLEncoder.encode(url, "UTF-8");

java servlet: request parameter contains plus

The request parameter is like decrypt?param=5FHjiSJ6NOTmi7/+2tnnkQ==.
In the servlet, when I try to print the parameter by String param = request.getParameter("param"); I get 5FHjiSJ6NOTmi7/ 2tnnkQ==. It turns the character + into a space. How can I keep the orginal paramter or how can I properly handle the character +.
Besides, what else characters should I handle?

You have two choices
URL encode the parameter
If you have control over the generation of the URL you should choose this. If not...
Manually retrieve the parameter
If you can't change how the URL is generated (above) then you can manually retrieve the raw URL. Certain methods decode parameters for you. getParameter is one of them. On the other hand, getQueryString does not decode the String. If you have only a few parameters it shouldn't be difficult to parse the value yourself.
request.getQueryString();
//?param=5FHjiSJ6NOTmi7/+2tnnkQ==

If you want to use the '+' character in a URL you need to encode it when it is generated. For '+' the correct encoding is %2b

Use URLEncoder,URLDecoder's static methods for encoding and decoding URLs.
For example : -
Encode the URL param using
URLEncoder.encode(url,"UTF-8")
Back in the server side , decode this parameter using
URLDecoder.decode(url,"UTF-8")
decode method returns a String type of the decoded URL.

Allthough the question is some years old, I'd like to write down how I fixed the problem in my case: the download link to a file is created in a GWT page where
com.google.gwt.http.client.URL.encode(finalurl)
is used to encode the URL.
The problem was that the "+" sign a customer of us had in the filename wasn't encoded/escaped. So I had to remove the URL.encode(finalurl) and encode each parameter in the url with
URL.encodePathSegment(fileName)
I know my question is bound to GWT but it seems, URLEncoder.encode(string, encoding) should be applied to the parameter only aswell.

URL encoding + sign

I have an application with + sign in its name (eg. DB+JSP.jws).
I get an error when trying to create connection as java encodes url + with spaces and hence cannot add the connection to DB JSP/../META-INF/connection.xml (File not found exception).
Any way to circumvent this only by using URLEncoder.encode() and URLDecoder.decode() methods?

You need to encode the URL correctly since '+' is a reserved character in a URL and can only be used in the correct context otherwise needs to be encoded with %2B.
Your URL string would encoded as "DB%2BJSP.jws".
So if you defined the following:
String url = URLEncoder.encode("DB+JSP.jws");
System.out.println(url);
The output would be the same:
DB%2BJSP.jws
You can prepend "http://localhost/" to the encoded URL as you need to.

Encode URL query parameters

How can I encode URL query parameter values? I need to replace spaces with %20, accents, non-ASCII characters etc.
I tried to use URLEncoder but it also encodes / character and if I give a string encoded with URLEncoder to the URL constructor I get a MalformedURLException (no protocol).

URLEncoder has a very misleading name. It is according to the Javadocs used encode form parameters using MIME type application/x-www-form-urlencoded.
With this said it can be used to encode e.g., query parameters. For instance if a parameter looks like &/?# its encoded equivalent can be used as:
String url = "http://host.com/?key=" + URLEncoder.encode("&/?#");
Unless you have those special needs the URL javadocs suggests using new URI(..).toURL which performs URI encoding according to RFC2396.
The recommended way to manage the encoding and decoding of URLs is to use URI
The following sample
new URI("http", "host.com", "/path/", "key=| ?/#ä", "fragment").toURL();
produces the result http://host.com/path/?key=%7C%20?/%23ä#fragment. Note how characters such as ?&/ are not encoded.
For further information, see the posts HTTP URL Address Encoding in Java or how to encode URL to avoid special characters in java.
EDIT
Since your input is a string URL, using one of the parameterized constructor of URI will not help you. Neither can you use new URI(strUrl) directly since it doesn't quote URL parameters.
So at this stage we must use a trick to get what you want:
public URL parseUrl(String s) throws Exception {
URL u = new URL(s);
return new URI(
u.getProtocol(),
u.getAuthority(),
u.getPath(),
u.getQuery(),
u.getRef()).
toURL();
}
Before you can use this routine you have to sanitize your string to ensure it represents an absolute URL. I see two approaches to this:
Guessing. Prepend http:// to the string unless it's already present.
Construct the URI from a context using new URL(URL context, String spec)

So what you're saying is that you want to encode part of your URL but not the whole thing. Sounds to me like you'll have to break it up into parts, pass the ones that you want encoded through the encoder, and re-assemble it to get your whole URL.

response.sendredirect with url with foreign chars - how to encode?

I have a jsf app that has international users so form inputs can have non-western strings like kanjii and chinese - if I hit my url with ..?q=東日本大 the output on the page is correct and I see the q input in my form gets populated fine. But if I enter that same string into my form and submit, my app does a redirect back to itself after constructing the url with the populated parameters in the url (seems redundant but this is due to 3rd party integration) but the redirect is not encoding the string properly. I have
url = new String(url.getBytes("ISO-8859-1"), "UTF-8");
response.sendRedirect(url);
But url redirect ends up being q=???? I've played around with various encoding strings (switched around ISO and UTF-8 and just got a bunch of gibberish in the url) in the String constructor but none seem to work to where I get q=東日本大 Any ideas as to what I need to do to get the q=東日本大 populated in the redirect properly? Thanks.

How are you making your url? URIs can't directly have non-ASCII characters in; they have to be turned into bytes (using a particular encoding) and then %-encoded.
URLEncoder.encode should be given an encoding argument, to ensure this is the right encoding. Otherwise you get the default encoding, which is probably wrong and always to be avoided.
String q= "\u6771\u65e5\u672c\u5927"; // 東日本大
String url= "http://example.com/query?q="+URLEncoder.encode(q, "utf-8");
// http://example.com/query?q=%E6%9D%B1%E6%97%A5%E6%9C%AC%E5%A4%A7
response.sendRedirect(url);
This URI will display as the IRI ‘http://example.com/query?q=東日本大’ in the browser address bar.
Make sure you're serving your pages as UTF-8 (using Content-Type header/meta) and interpreting query string input as UTF-8 (server-specific; see this faq for Tomcat.)

Try
response.setContentType("text/html; charset=UTF-16");
response.setCharacterEncoding("utf-16");