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Convert nested for loop into java8 stream

I am trying to convert the below code into Java 8 Stream for nested for loop.
I have tried to take stream for outer loop but not sure how to write condition and assign a variable there.
final Map<String, String> events = new HashMap<>();
for (final Event s : result.getEvents()) {
String eventDetail = "";
for (final Data d : s.getData()) {
if (StringUtils.isNotEmpty(d.getValue()) && StringUtils.isNotEmpty(eventDetail)) {
eventDetail = eventDetail + "-" + d.getValue();
} else {
eventDetail = eventDetail + d.getValue();
}
}
events.put(s.getReferenceID(), eventDetail);
}
Result should be map value.

It looks like your goal is to concatenate the value members of the Data instances of each
Event into a "-" separated String, and map this String to the Event's reference ID.
This can be done with Collectors.joining():
Map<String, String>
events = result.getEvents()
.stream()
.map(s -> new SimpleEntry<>(s.getReferenceID(),s.getData().stream().map(Data::getValue).collect(Collectors.joining("-"))))
.collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));
or, if you wish to eliminate empty values:
Map<String, String>
events = result.getEvents()
.stream()
.map(s -> new SimpleEntry<>(s.getReferenceID(),s.getData().stream().map(Data::getValue).filter(StringUtils::isNotEmpty).collect(Collectors.joining("-"))))
.collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue));

Is this what you want?
Function<Event, String> mapper = event -> event.getData().stream()
.map(Data::getValue)
.filter(StringUtils::isNotEmpty)
.reduce("", (value1, value2) -> value1 + "-" + value2);
final Map<String, String> events = result.getEvents().stream()
.collect(Collectors.toUnmodifiableMap(Event::getReferenceID, mapper));

Usng StringJoiner in complex HashMaps

I have a list of Maps as below:
List<Map<String,Object>> someObjectsList = new ArrayList<Map<String,Object>>();
I am storing the following data in each HashMap
key value
2017-07-21 2017-07-21-07.33.28.429340
2017-07-24 2017-07-24-01.23.33.591340
2017-07-24 2017-07-24-01.23.33.492340
2017-07-21 2017-07-21-07.33.28.429540
I want to iterate through the list of HashMaps and check if the key matches with the first 10 characters of any of the HashMap value, then I want to store those keys and values in the following format. i.e. by using the telemeter 'comma'. The ultimate aim is to group the unique keys of the HashMaps and their relative values (if the key matches with the first 10 characters of any of the HashMap value) in a new HashMap.
key value
2017-07-21 2017-07-21-07.33.28.429340,2017-07-21-07.33.28.429540
2017-07-24 2017-07-24-01.23.33.591340,2017-07-24-01.23.33.492340
I am trying with following java code using StringJoiner, but not getting the results as expected. Any clue on how to frame the logic here?
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.StringJoiner;
public class SampleOne {
public static void main(String[] args) {
// TODO Auto-generated method stub
List<Map<String, Object>> someObjectsList = new ArrayList<Map<String, Object>>();
Map<String, Object> mapOne = new HashMap<String, Object>();
mapOne.put("2017-07-21", "2017-07-21-07.33.28.429340");
Map<String, Object> mapTwo = new HashMap<String, Object>();
mapTwo.put("2017-07-24", "2017-07-24-01.23.33.591340");
Map<String, Object> mapThree = new HashMap<String, Object>();
mapThree.put("2017-07-24", "2017-07-24-01.23.33.492340");
Map<String, Object> mapFour = new HashMap<String, Object>();
mapFour.put("2017-07-21", "2017-07-21-07.33.28.429540");
someObjectsList.add(mapOne);
someObjectsList.add(mapTwo);
someObjectsList.add(mapThree);
someObjectsList.add(mapFour);
for (Map map : someObjectsList) {
StringJoiner sj = new StringJoiner(",");
for (Object key : map.keySet()) {
String value = ((String) map.get(key));
String date = value.substring(0, Math.min(value.length(), 10));
//System.out.println(str);
//System.out.println(value);
if(key.equals(date)) {
sj.add(value);
System.out.println(sj.toString());
}
}
}
}
}
output:
2017-07-21-07.33.28.429340
2017-07-24-01.23.33.591340
2017-07-24-01.23.33.492340
2017-07-21-07.33.28.429540

Make use of the .merge function:
Map<String, Object> finalMap = new HashMap<String, Object>();
for (Map map : someObjectsList) {
for (Object key : map.keySet()) {
String value = ((String) map.get(key));
finalMap.merge((String) key, value, (k, v) -> k + "," + v);
}
}
which outputs:
{2017-07-21=2017-07-21-07.33.28.429340,2017-07-21-07.33.28.429540,
2017-07-24=2017-07-24-01.23.33.591340,2017-07-24-01.23.33.492340}
The same can be achieved by the following one-liner:
someObjectsList.stream()
.flatMap(i -> i.entrySet().stream())
.collect(Collectors.toMap(Entry::getKey, Entry::getValue,
(k, v) -> k + "," + v));

On your code, you are using different StringJoiner on each map. So, it's creating a new instance of it.
You can save your keys on a map. An example code:
(Edit: I did not remove your StringJoiner part.)
public static void main(String[] args) {
// TODO Auto-generated method stub
List<Map<String, Object>> someObjectsList = new ArrayList<Map<String, Object>>();
Map<String, Object> mapOne = new HashMap<String, Object>();
mapOne.put("2017-07-21", "2017-07-21-07.33.28.429340");
Map<String, Object> mapTwo = new HashMap<String, Object>();
mapTwo.put("2017-07-24", "2017-07-24-01.23.33.591340");
Map<String, Object> mapThree = new HashMap<String, Object>();
mapThree.put("2017-07-24", "2017-07-24-01.23.33.492340");
Map<String, Object> mapFour = new HashMap<String, Object>();
mapFour.put("2017-07-21", "2017-07-21-07.33.28.429540");
someObjectsList.add(mapOne);
someObjectsList.add(mapTwo);
someObjectsList.add(mapThree);
someObjectsList.add(mapFour);
Map<String, Object> outputMap = new HashMap<String, Object>();
for (Map map : someObjectsList) {
StringJoiner sj = new StringJoiner(",");
for (Object key : map.keySet()) {
String value = ((String) map.get(key));
String date = value.substring(0, Math.min(value.length(), 10));
//System.out.println(str);
//System.out.println(value);
if(key.equals(date)) {
sj.add(value);
System.out.println(sj.toString());
if(outputMap.containsKey(key)) {
String str = (String) map.get(key);
str = str + "," + value;
outputMap.put((String)key, str);
} else {
outputMap.put((String)key, value);
}
}
}
}
for (String map : outputMap.keySet()) {
System.out.println(map + " " + outputMap.get(map));
}
}

You are looking for the grouping behavior of processing a List. You can use the advantage of java-stream since java-8. In any case, you need a new Map to store the values in order to print them. :
someObjectsList.stream()
.flatMap(i -> i.entrySet().stream()) // flatmapping to entries
.collect(Collectors.groupingBy(Entry::getKey)) // grouping them using the key
In case you want to use for-loops. In this case it is harder since the more entries might appear in each List item:
final Map<String, List<Object>> map = new HashMap<>();
for (Map<String, Object> m: someObjectsList) { // iterate List<Map>
for (Entry<String, Object> entry: m.entrySet()) { // iterate entries of each Map
List<Object> list;
final String key = entry.getKey(); // key of the entry
final Object value = entry.getValue(); // value of the entry
if (map.containsKey(key)) { // if the key exists
list = map.get(key); // ... use it
} else {
list = new ArrayList<>(); // ... or else create a new one
}
list.add(value); // add the new value
map.put(key, list); // and add/update the entry
}
}
Printing out of Map<String, List<Object>> map in both cased will produce the following output:
2017-07-21=[2017-07-21-07.33.28.429340, 2017-07-21-07.33.28.429540],
2017-07-24=[2017-07-24-01.23.33.591340, 2017-07-24-01.23.33.492340]

Any reason you're using Object over String and avoiding safety checks? That said, it's not "the first 10 characters", you want to see if value starts with key full-stop (all your keys are 10 characters). So in that case you can just do if (value.startsWith(key)) { ... }. Don't forget your newlines if the stringjoiner wasn't full. Lastly, you don't need a List, a Map can hold multiple keys at once. An alternative way of doing it:
//LinkedHashMap will preserve our insertion order
Map<String, String> map = new LinkedHashMap<>();
map.put("2017-07-21", "2017-07-21-07.33.28.429340");
map.put("2017-07-24", "2017-07-24-01.23.33.591340");
//note duplicates are overwritten, but no value change here
map.put("2017-07-24", "2017-07-24-01.23.33.492340");
map.put("2017-07-21", "2017-07-21-07.33.28.429540");
// You can also use Java 8 streams for the concatenation
// but I left it simple
List<String> matches = map.entrySet()
.filter(e -> e.getValue().startsWith(e.getKey())
.collect(Collectors.toList());
String concatenated = String.join("\n", matches);
If you wanted to generate that string without streams, it would look like this (again, not using #entrySet for simplicity, but it would be more efficient here):
List<String> matches = new ArrayList<>();
StringJoiner joiner = new StringJoiner("\n");
for (String key : map.keySet()) {
String value = map.get(key);
if (value.startsWith(key)) {
joiner.add(value);
}
}
//joiner#toString will give the expected result

Improper output while filtering a hashmap

I have a hashmap which has key and value in String. The data is in the form of (table1, "table1:ssn1,ssn2,ssn3"). The table name in key is of the source table name and table name in the value is of the destination table name along with the corresponding source systems names separated by a ":".
I am trying to pass source system names in arguments from the command line to filter out the key and value along with the received source system name.
I came up with the following code so far:
public class FilterKeyValues {
public static void main(String[] args) {
String[] valArr;
String ky;
Map<String, String> hmap = new HashMap<String, String>();
Map<String, String> filtered = new HashMap<String, String>();
hmap.put("Table1", "Table1:SSN1,SSN2,SSN3,SSN4,SSN5");
hmap.put("Table2", "Table2:SSN1,SSN4,SSN2,SSN5,SSN8,SSN9,SSN10");
hmap.put("Table3", "Table3:SSN4,SSN1");
hmap.put("Table4", "Table4:SSN5,SSN6,SSN7");
hmap.put("Table5", "Table5:SSN8,SSN1,SSN5,SSN2");
if(args.length > 0) {
for(String ssname: args) {
for (Entry<String, String> entry : hmap.entrySet()) {
if (entry.getValue().contains(ssname)) {
ky = entry.getKey();
valArr = entry.getValue().split(":");
filtered.put(ky, valArr[0]+":"+ssname);
}
}
}
}
for (String iter: filtered.keySet()){
String key = iter.toString();
String value = filtered.get(key).toString();
System.out.println(key + "->" + value);
}
}
}
In the arguments, I am passing: SSN1 SSN2 in the arguments. The output should be
Table1->Table1:SSN1
Table2->Table2:SSN1
Table3->Table3:SSN1
Table5->Table5:SSN1
Table1->Table1:SSN2
Table2->Table2:SSN2
Table5->Table5:SSN2
Instead, I am getting the output of:
Table2->Table2:SSN2
Table3->Table3:SSN1
Table5->Table5:SSN2
Table1->Table1:SSN2
Could anyone let me know what is the mistake I am doing here ?

You are trying to put multiple values into a Map using the same key. For each key, a map can only ever hold one value.
Therefore, only the last value that you add for any given key will be visible in the end.
Generally speaking your code suggests that Strings are not the correct data type for your data and that you should be storing it in a more structured form (such as a Map<String,List<String>>).

As i have observed, key is included in value.. so you can use a simple arraylist fro targeted data.. this may help you
public static void main(String[] args) {
String[] valArr;
String ky;
Map<String, String> hmap = new HashMap<String, String>();
List<String> filtered = new ArrayList<String>();
hmap.put("Table1", "Table1:SSN1,SSN2,SSN3,SSN4,SSN5");
hmap.put("Table2", "Table2:SSN1,SSN4,SSN2,SSN5,SSN8,SSN9,SSN10");
hmap.put("Table3", "Table3:SSN4,SSN1");
hmap.put("Table4", "Table4:SSN5,SSN6,SSN7");
hmap.put("Table5", "Table5:SSN8,SSN1,SSN5,SSN2");
if(args.length > 0) {
for(String ssname: args) {
for (Entry<String, String> entry : hmap.entrySet()) {
if (entry.getValue().contains(ssname)) {
ky = entry.getKey();
valArr = entry.getValue().split(":");
filtered.add(valArr[0]+":"+ssname);
}
}
}
}
for (String iter: filtered){
System.out.println(iter.split(":")[0]+ "->" + iter);
}
}

You need to iterate over the items:
valArr = entry.getValue().substring(entry.getValue().indexOf(":")).split(",");
Here is complete code which produces the output you desire:
public static void main(String[] args) {
String[] valArr;
String ky;
Map<String, String> hmap = new HashMap<String, String>();
Map<String, String> filtered = new HashMap<String, String>();
hmap.put("Table1", "Table1:SSN1,SSN2,SSN3,SSN4,SSN5");
hmap.put("Table2", "Table2:SSN1,SSN4,SSN2,SSN5,SSN8,SSN9,SSN10");
hmap.put("Table3", "Table3:SSN4,SSN1");
hmap.put("Table4", "Table4:SSN5,SSN6,SSN7");
hmap.put("Table5", "Table5:SSN8,SSN1,SSN5,SSN2");
if(args.length > 0) {
for(String ssname: args) {
for (Entry<String, String> entry : hmap.entrySet()) {
if (entry.getValue().contains(ssname)) {
ky = entry.getKey();
valArr = entry.getValue().substring(entry.getValue().indexOf(":")).split(",");
for (String val : valArr) {
if (val.equals(ssname)) {
filtered.put(ky, ky+":"+ssname);
}
}
}
}
}
}
for (String iter: filtered.keySet()){
String key = iter.toString();
String value = filtered.get(key).toString();
System.out.println(key + "->" + value);
}
}

Get value of a param in a string using java

I have string variable String temp="acc=101&name=test"; and now how to get the value of name param from temp string.

temp.split("&")[1].split("=")[1]

public static Map<String, String> getParamMap(String query)
{
String[] params = query.split("&");
Map<String, String> map = new HashMap<String, String>();
for (String param : params)
{
String name = param.split("=")[0];
String value = param.split("=")[1];
map.put(name, value);
}
return map;
}
String temp="acc=101&name=test";
Map<String, String> map = getParamMap(temp);
for(Object object :map.keySet()){
System.out.println("key= "+object +" value= "+map.get(object));
}
System.out.println(map.get("name"));

Here is a non-general way
String str = "name=";
System.out.println(temp.substring(temp.indexOf(str) + str.length()));
It could be implemented in more general way of course:
String temp = "acc=101&name=test";
StringTokenizer st = new StringTokenizer(temp, "&");
String paramName = "name";
String paramValue = "";
while(st.hasMoreElements()) {
String str = st.nextToken();
if (str.contains(paramName)) {
paramValue = str.substring(str.indexOf(paramName) + paramName.length() + 1);
break;
}
}
System.out.println(paramValue);

You can use a method like below
public static String getValue(String queyStr, String paraamName){
String[] queries=queyStr.split("&");
for(String param:queries){
if(param.indexOf(paraamName)!=-1)
return param.split("=")[1];
}
return null;
}
And call the method like
getValue(temp, "name")

public static void main(String[] args)
{
String temp = "acc=101&name=test";
System.out.println(temp.split("&")[1].split("=")[1]);
}

If you are looking for a way to parse GET-Parameters out of an URL:
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("name"), with the URL given in your question this would return "test".

Assuming you have constant format :
String temp="acc=101&name=test";
String result =temp.substring(temp.lastIndexOf("=")+1,temp.length());
//result is test

String temp="acc=101&name=test";
String[] split = temp.split("&");
String[] name = split[1].split("=");
System.out.println(name[1]);

I would put the whole parameter in a HashMap so it is easy to get the values.
HashMap<String, String> valuemap = new HashMap<String, String>();
If you do it like so, you have to split the values at the right place...
String temp="acc=101&name=test";
valuemap.put(temp.split("&")[0].split("=")[0], temp.split("&")[0].split("=")[1]);
valuemap.put(temp.split("&")[1].split("=")[0], temp.split("&")[1].split("=")[1]);
...and put them into your HashMap. Than you have a nice collection of all your values and it is also better if you have more than only that two values. If you want the value back, use:
valuemap.get("acc")
valuemap.get("name")

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("");

In Spring Util, there is a better way..,
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.web.util.UriComponents;
import org.springframework.web.util.UriComponentsBuilder;
MultiValueMap<String, String> params = new LinkedMultiValueMap<String, String>();
params.add("key", key);
params.add("storeId", storeId);
params.add("orderId", orderId);
UriComponents uriComponents = UriComponentsBuilder.fromHttpUrl("http://spsenthil.com/order").queryParams(params).build();
ListenableFuture<ResponseEntity<String>> responseFuture = restTemplate.getForEntity(uriComponents.toUriString(), String.class);

Update June 2016
Felt compelled to add an answer having seen far too many SOF answers with dated or inadequate answers to very common problem - a good library and some solid example usage for both parse and format operations.
Use org.apache.httpcomponents.httpclient library. The library contains this org.apache.http.client.utils.URLEncodedUtils class utility.
For example, it is easy to download this dependency from Maven:
<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpclient</artifactId>
<version>4.5</version>
</dependency>
For my purposes I only needed to parse (read from query string to name-value pairs) and format (read from name-value pairs to query string) query strings. However, there are equivalents for doing the same with a URI (see commented out line below).
// Required imports
import org.apache.http.NameValuePair;
import org.apache.http.client.utils.URLEncodedUtils;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.nio.charset.StandardCharsets;
// code snippet
public static void parseAndFormatExample() throws UnsupportedEncodingException {
final String queryString = "nonce=12345&redirectCallbackUrl=http://www.bbc.co.uk";
System.out.println(queryString);
// => nonce=12345&redirectCallbackUrl=http://www.bbc.co.uk
final List<NameValuePair> params =
URLEncodedUtils.parse(queryString, StandardCharsets.UTF_8);
// List<NameValuePair> params = URLEncodedUtils.parse(new URI(url), "UTF-8");
for (final NameValuePair param : params) {
System.out.println(param.getName() + " : " + param.getValue());
// => nonce : 12345
// => redirectCallbackUrl : http://www.bbc.co.uk
}
final String newQueryStringEncoded =
URLEncodedUtils.format(params, StandardCharsets.UTF_8);
// decode when printing to screen
final String newQueryStringDecoded =
URLDecoder.decode(newQueryStringEncoded, StandardCharsets.UTF_8.toString());
System.out.println(newQueryStringDecoded);
// => nonce=12345&redirectCallbackUrl=http://www.bbc.co.uk
}
This library did exactly what I needed and was able to replace some hacked custom code.

If you actually want to build a complete URI, try URIBuilder from Apache Http Compoments (HttpClient 4).
This does not actually answer the question, but it answered the one I had when I found this question.

I wanted to build on #eclipse's answer using java 8 mapping and reducing.
protected String formatQueryParams(Map<String, String> params) {
return params.entrySet().stream()
.map(p -> p.getKey() + "=" + p.getValue())
.reduce((p1, p2) -> p1 + "&" + p2)
.map(s -> "?" + s)
.orElse("");
}
The extra map operation takes the reduced string and puts a ? in front only if the string exists.

Another 'one class'/no dependency way of doing it, handling single/multiple:
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
public class UrlQueryString {
private static final String DEFAULT_ENCODING = "UTF-8";
public static String buildQueryString(final LinkedHashMap<String, Object> map) {
try {
final Iterator<Map.Entry<String, Object>> it = map.entrySet().iterator();
final StringBuilder sb = new StringBuilder(map.size() * 8);
while (it.hasNext()) {
final Map.Entry<String, Object> entry = it.next();
final String key = entry.getKey();
if (key != null) {
sb.append(URLEncoder.encode(key, DEFAULT_ENCODING));
sb.append('=');
final Object value = entry.getValue();
final String valueAsString = value != null ? URLEncoder.encode(value.toString(), DEFAULT_ENCODING) : "";
sb.append(valueAsString);
if (it.hasNext()) {
sb.append('&');
}
} else {
// Do what you want...for example:
assert false : String.format("Null key in query map: %s", map.entrySet());
}
}
return sb.toString();
} catch (final UnsupportedEncodingException e) {
throw new UnsupportedOperationException(e);
}
}
public static String buildQueryStringMulti(final LinkedHashMap<String, List<Object>> map) {
try {
final StringBuilder sb = new StringBuilder(map.size() * 8);
for (final Iterator<Entry<String, List<Object>>> mapIterator = map.entrySet().iterator(); mapIterator.hasNext();) {
final Entry<String, List<Object>> entry = mapIterator.next();
final String key = entry.getKey();
if (key != null) {
final String keyEncoded = URLEncoder.encode(key, DEFAULT_ENCODING);
final List<Object> values = entry.getValue();
sb.append(keyEncoded);
sb.append('=');
if (values != null) {
for (final Iterator<Object> listIt = values.iterator(); listIt.hasNext();) {
final Object valueObject = listIt.next();
sb.append(valueObject != null ? URLEncoder.encode(valueObject.toString(), DEFAULT_ENCODING) : "");
if (listIt.hasNext()) {
sb.append('&');
sb.append(keyEncoded);
sb.append('=');
}
}
}
if (mapIterator.hasNext()) {
sb.append('&');
}
} else {
// Do what you want...for example:
assert false : String.format("Null key in query map: %s", map.entrySet());
}
}
return sb.toString();
} catch (final UnsupportedEncodingException e) {
throw new UnsupportedOperationException(e);
}
}
public static void main(final String[] args) {
// Examples: could be turned into unit tests ...
{
final LinkedHashMap<String, Object> queryItems = new LinkedHashMap<String, Object>();
queryItems.put("brand", "C&A");
queryItems.put("count", null);
queryItems.put("misc", 42);
final String buildQueryString = buildQueryString(queryItems);
System.out.println(buildQueryString);
}
{
final LinkedHashMap<String, List<Object>> queryItems = new LinkedHashMap<String, List<Object>>();
queryItems.put("usernames", new ArrayList<Object>(Arrays.asList(new String[] { "bob", "john" })));
queryItems.put("nullValue", null);
queryItems.put("misc", new ArrayList<Object>(Arrays.asList(new Integer[] { 1, 2, 3 })));
final String buildQueryString = buildQueryStringMulti(queryItems);
System.out.println(buildQueryString);
}
}
}
You may use either simple (easier to write in most cases) or multiple when required. Note that both can be combined by adding an ampersand...
If you find any problems let me know in the comments.

This is the solution I implemented, using Java 8 and org.apache.http.client.URLEncodedUtils. It maps the entries of the map into a list of BasicNameValuePair and then uses Apache's URLEncodedUtils to turn that into a query string.
List<BasicNameValuePair> nameValuePairs = params.entrySet().stream()
.map(entry -> new BasicNameValuePair(entry.getKey(), entry.getValue()))
.collect(Collectors.toList());
URLEncodedUtils.format(nameValuePairs, Charset.forName("UTF-8"));

There's nothing built into Java to do this. But, hey, Java is a programming language, so. Let's program it!
map
.entrySet()
.stream()
.map(e -> e.getKey() + "=" + e.getValue())
.collect(Collectors.joining("&"))
This gives you param1=12&param2=cat. Now we need to join the URL and this bit together. You'd think you can just do: URL + "?" + theAbove but if the URL already contains a question mark, you have to join it all together with "&" instead. One way to check is to see if there's a question mark in the URL someplace already.
Also, I don't quite know what is in your map. If it's raw stuff, you probably have to safeguard the call to e.getKey() and e.getValue() with URLEncoder.encode or similar.
Yet another way to go is that you take a wider view. Are you trying to append a map's content to a URL, or... Are you trying to make an HTTP (S) request from a Java process with the stuff in the map as (additional) HTTP params? In the latter case, you can look into an HTTP library like OkHttp which has some nice APIs to do this job, then you can forego any need to mess about with that URL in the first place.

Using EntrySet and Streams:
map
.entrySet()
.stream()
.map(e -> e.getKey() + "=" + e.getValue())
.collect(Collectors.joining("&"));

You can use a Stream for this, but instead of appending query parameters myself I'd use a Uri.Builder. For example:
final Map<String, String> map = new HashMap<>();
map.put("param1", "cat");
map.put("param2", "12");
final Uri uri =
map.entrySet().stream().collect(
() -> Uri.parse("relativeUrl").buildUpon(),
(builder, e) -> builder.appendQueryParameter(e.getKey(), e.getValue()),
(b1, b2) -> { throw new UnsupportedOperationException(); }
).build();
//Or, if you consider it more readable...
final Uri.Builder builder = Uri.parse("relativeUrl").buildUpon();
map.entrySet().forEach(e -> builder.appendQueryParameter(e.getKey(), e.getValue())
final Uri uri = builder.build();
//...
assertEquals(Uri.parse("relativeUrl?param1=cat&param2=12"), uri);

Here's a simple kotlin solution:
fun Map<String, String>.toUrlParams(): String =
entries.joinToString("&") {
it.key.toUrlEncoded() + "=" + it.value.toUrlEncoded()
}
fun String.toUrlEncoded(): String = URLEncoder.encode(
this, StandardCharsets.UTF_8
)

To improve a little bit upon #eclipse's answer: In Javaland a request parameter map is usually represented as a Map<String, String[]>, a Map<String, List<String>> or possibly some kind of MultiValueMap<String, String> which is sort of the same thing. In any case: a parameter can usually have multiple values. A Java 8 solution would therefore be something along these lines:
public String getQueryString(HttpServletRequest request, String encoding) {
Map<String, String[]> parameters = request.getParameterMap();
return parameters.entrySet().stream()
.flatMap(entry -> encodeMultiParameter(entry.getKey(), entry.getValue(), encoding))
.reduce((param1, param2) -> param1 + "&" + param2)
.orElse("");
}
private Stream<String> encodeMultiParameter(String key, String[] values, String encoding) {
return Stream.of(values).map(value -> encodeSingleParameter(key, value, encoding));
}
private String encodeSingleParameter(String key, String value, String encoding) {
return urlEncode(key, encoding) + "=" + urlEncode(value, encoding);
}
private String urlEncode(String value, String encoding) {
try {
return URLEncoder.encode(value, encoding);
} catch (UnsupportedEncodingException e) {
throw new IllegalArgumentException("Cannot url encode " + value, e);
}
}

If you need just the query string (not the whole URL) and you are using Spring Framework, you can do this:
import org.springframework.web.util.UriComponentsBuilder;
...
final String queryString = UriComponentsBuilder.newInstance()
.queryParam("many", "7", "14", "21")
.queryParam("single", "XYZ")
.build()
.toUri()
.getQuery();
System.out.println(queryString);
the result is:
many=7&many=14&many=21&single=XYZ

I make these functions than also send just the property name when the value is null.
public static String urlEncode(Map<?, ?> map) {
return map.entrySet().stream().map(
entry -> entry.getValue() == null
? urlEncode(entry.getKey())
: urlEncode(entry.getKey()) + "=" + urlEncode(entry.getValue())
).collect(Collectors.joining("&"));
}
public static String urlEncode(Object obj) {
return URLEncoder.encode(obj.toString(), StandardCharsets.UTF_8);
}

For multivalue map you can do like below (using java 8 stream api's)
Url encoding has been taken cared in this.
MultiValueMap<String, String> params = new LinkedMultiValueMap<>();
String urlQueryString = params.entrySet()
.stream()
.flatMap(stringListEntry -> stringListEntry.getValue()
.stream()
.map(s -> UriUtils.encode(stringListEntry.getKey(), StandardCharsets.UTF_8.toString()) + "=" +
UriUtils.encode(s, StandardCharsets.UTF_8.toString())))
.collect(Collectors.joining("&"));

Personally, I'd go for a solution like this, it's incredibly similar to the solution provided by #rzwitserloot, only subtle differences.
This solution is small, simple & clean, it requires very little in terms of dependencies, all of which are a part of the Java Util package.
Map<String, String> map = new HashMap<>();
map.put("param1", "12");
map.put("param2", "cat");
String output = "someUrl?";
output += map.entrySet()
.stream()
.map(x -> x.getKey() + "=" + x.getValue() + "&")
.collect(Collectors.joining("&"));
System.out.println(output.substring(0, output.length() -1));

Kotlin
mapOf(
"param1" to 12,
"param2" to "cat"
).map { "${it.key}=${it.value}" }
.joinToString("&")

a very lightweight answer it works for me
public static String queryStr(Map<String, String> data) throws UnsupportedEncodingException {
StringBuilder query = new StringBuilder();
for (Entry<String, String> entry : data.entrySet()) {
if (query.length() > 0) {
query.append('&');
}
query.append(entry.getKey()).append('=');
query.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
}
return query.toString();
}