Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Using lambda to format Map into String

I have a map with Integer keys and values. I need to transform it into a String with this specific format: key1 - val1, key2 - val2, key3 - val3. Now, I'm using forEach to format each element, collect them into a List, and then do String.join();

List<String> ships = new ArrayList<>(4); for (Map.Entry<Integer, Integer> entry : damagedMap.entrySet()) {     ships.add(entry.getKey() + " - " + entry.getValue()); } result = String.join(",", ships); 

Is there any shorter way to do it? And it would be good to do it with lambda, because I need some practice using lambdas.

like image 719
TEXHIK Avatar asked May 14 '15 12:05

TEXHIK


People also ask

How do you convert a map to a string?

Use Object#toString() . String string = map. toString();

Can Lambda return string?

A free variable can be a constant or a variable defined in the enclosing scope of the function. The lambda function assigned to full_name takes two arguments and returns a string interpolating the two parameters first and last .

How do you find the value of an object in a string map?

Get Elements From a Java Map Map map = new HashMap(); map. put("key1", "value 1"); String element1 = (String) map. get("key1"); Notice that the get() method returns a Java Object , so we have to cast it to a String (because we know the value is a String).


1 Answers

I think you're looking for something like this:

import java.util.*; import java.util.stream.*; public class Test {      public static void main(String[] args) throws Exception {         Map<Integer, String> map = new HashMap<>();         map.put(1, "foo");         map.put(2, "bar");         map.put(3, "baz");         String result = map.entrySet()             .stream()             .map(entry -> entry.getKey() + " - " + entry.getValue())             .collect(Collectors.joining(", "));         System.out.println(result);     } } 

To go through the bits in turn:

  • entrySet() gets an iterable sequence of entries
  • stream() creates a stream for that iterable
  • map() converts that stream of entries into a stream of strings of the form "key - value"
  • collect(Collectors.joining(", ")) joins all the entries in the stream into a single string, using ", " as the separator. Collectors.joining is a method which returns a Collector which can work on an input sequence of strings, giving a result of a single string.

Note that the order is not guaranteed here, because HashMap isn't ordered. You might want to use TreeMap to get the values in key order.

like image 199
Jon Skeet Avatar answered Sep 26 '22 17:09

Jon Skeet