Java – streams: how maps in streams work

In order to better understand the map function in streams, I tried this:

String inputString="1+3+5";
Stream.of(inputString.split("\\+")).map(
    eachStringLiteral -> {
        output += mapOfStringAndNumber.get(eachStringLiteral) + literal;
    }
);

Inputstring is:

String inputString = "1+3+5";

However, the compiler complains and I don't know why:

I need some help to understand grammar

Function & lt;? Super string,? Extension r >

to update

This is the complete code, which explains the goal I want to achieve:

HashMap<String,Double> mapOfStringAndNumber=new HashMap<String,Double>();

        mapOfStringAndNumber.put("1",270.5);
        mapOfStringAndNumber.put("2",377.5);
        mapOfStringAndNumber.put("3",377.5);
        String inputString="1+3+5";
        String literal="+";
       String output;
      java.util.stream.Stream.of(inputString.split("+")).map(eachStringLiteral->
      output+=mapOfStringAndNumber.get(eachStringLiteral)+literal

Solution

Assuming that you provide a double value for each input string in map mapofstringandnumber, you can directly split the input string into streams using pattern #splitasstream (charsequence input) and use collectors Joining (charsequence delimiter) builds the output with the as delimiter, so your final code may be:

Map<String,Double> mapOfStringAndNumber = new HashMap<>();
mapOfStringAndNumber.put("1",270.5);
mapOfStringAndNumber.put("3",377.5);
mapOfStringAndNumber.put("5",377.5);
String inputString = "1+3+5";
String output = Pattern.compile("\\+")
    .splitAsStream(inputString)
    .map(mapOfStringAndNumber::get)
    .map(d -> Double.toString(d))
    .collect(Collectors.joining("+"));

System.out.println(output);

Output:

270.5+377.5+377.5