Streams : how map in Streams work

Question

To understand the map function in Streams better I was trying something like this:

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

Where inputString is:

String inputString = "1+3+5";

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

The method map(Function) in the type Stream is not applicable for the arguments (( eachStringLiteral) -> {})

I also need some help in order to understand the syntax in

Function<? super String,? extends R>.

Update

This is whole code illustrating what I am trying 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

Answer 1

Assuming that you provide a Double value to each input String in your map mapOfStringAndNumber, you could use Pattern#splitAsStream(CharSequence input) to split your input String directly as a Stream and use Collectors.joining(CharSequence delimiter) to build your output with + as delimiter, so your final code could 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

Answer 2

I have a couple of suggestions for your code:

1. If you use a {}-block and you want to create a Function (not a Runnable, Consumer or similar) you have to return something.
2. Unless you have a very special case, use Integer.parseInt(String) instead of a Map of Strings to Integers
3. You must not modify output from within a Stream. That could lead to concurrancy issues.
4. Do you really need literal?

My example code for your usecase would look like this:

String inputString="1+3+5";
return

        // split input and create stream
        Stream.of(inputString.split("\\+"))

        // convert each item to Integer
        .mapToInt(Integer::parseInt)

        // sum up everything
        .sum();