RestContext.response.responseBody = Blob.valueOf(JSON.serialize(locations));

JSON can handle almost any data type, including maps that include serializable types. The exceptions to this rule are items that can't be serialized, like SavePoint, SObjectField and so on.

Desired result is "Moscow 200 usd, Paris 100 USD, Moscow 200 RUR" Try it like this using flatMap.

• stream the entrySet of the main map.
• flatMap the inner map's values via entrySet again.
• the using the key of the outer map and the value of the inner, map them to a string.
• Then join the strings with a collector.

String result = map.entrySet().stream()
        .flatMap(e -> e.getValue().entrySet().stream()
                .map(e2 -> e.getKey() + " " + e2.getValue()))
        .collect(Collectors.joining(", "));
System.out.println(result);

prints

Paris 100 USD, Moscow 200 RUR, Moscow 200 USD

Note: Since maps are unordered, the order of the elements may not be consistent from run to run based on sums, additional Reports, etc.

I don't know if your Map is required as is or a way to get the result, but you could also do it like this.

• use groupingBy as before but use toMap as the collector.

Map<String, Map<String, BigDecimal>> map =
        list.stream()
                .collect(Collectors.groupingBy(
                        Report::getDepartmentName,
                        Collectors.toMap(Report::getCurrency,
                                Report::getAmount, BigDecimal::add)));

• Then you would do it similar to before but convert the BigDecimal to a String here and append the currency type(e2.getKey()) to that.

String result = map1.entrySet().stream().flatMap(e -> e
                .getValue().entrySet().stream()
                .map(e2 -> e.getKey() + " " + e2.getValue().toString()
                        + " " + e2.getKey()))
                .collect(Collectors.joining(", "));

So you can try like this

map<string,string> newMap= new Map<string,string>();
for(String strKey: mapValues.keyset())
{
    newMap.put(strKey, String.valueof(mapValues.get(strKey)));
}

You are really working uphill by doing this all untyped, but you can get the value you want this way:

list<Object> l = (list<Object>) meta.get('addressDetails');
map<String,Object> m = (map<String,Object>) l[0];
Integer i = (Integer) m.get('addressId');

Honestly, if you make use of the Apex type system, you will have an easier time working with the data. If you create a class for the incoming data structure, abbreviated as:

public class Meta{
   public list<addressDetail> addressDetails {get; set;} 

   public class addressDetail{
       public Integer addressId {get; set;}
   }   
}

you will be able to access the data from your JSON strings as so:

Meta MyMeta = (Meta) JSON.deserialize(viewPersonValue,Meta.class);
system.debug(MyMeta.addressDetails[0].addressId);

Do something like,

String jsonInput = '{"New":["Prospect","Open","In Negotiation","Final Negotiation"],"Readoption":["Won"],"Rollover":["Won","Invoiced","Lost"],"Upsell":["In Negotiation","Lost"],"Conversion":[]}';
Map<String,List<String>> m = (Map<String,List<String>>)JSON.deserialize(jsonInput, Map<String,List<String>>.class);

You cant convert the map to a list without an id. Alternatively why to convert it to List. Use the map itself. Below is a code example of how you can use it using your JSON response. For example, I want to insert this JSON response into the contact record, so I can map it using the MAP itself.:

@RestResource(urlMapping='/URIId/*') global class OwneriCRM {

@HttpPut
global static String UpdateOwnerinfo(){
    RestRequest req= RestContext.request;
    RestResponse res= RestContext.response;
    res.addHeader('Content-type','application/JSON');
    Map<String,Object> responseresult=new Map<String,Object>();
    Map<String,Object> results= (Map<String,Object>)JSON.deserializeUntyped(req.requestBody.toString());
     List<contact> insertList = new List<contact>();
    
    for (String key : results.keySet()){
    System.debug(results.get(key));
    contact c= new contact();
    c.ContactField__C = results.get(id);
    c.ContactField1__C = results.get(city);
    c.ContactField2__C = results.get(conference);
    c.ContactField3__C = results.get(division);
    c.ContactField3__C = results.get(full_name);
    c.ContactField3__C = results.get(name);
    insertList.add(c);
    
    }
    if(!insertList.Isempty()){
        update insertList;
    }