How to replace null values in map with length of key using java 8 stream

Question

I have a Map<String, Integer>, which has some keys and values. I want to associate all keys with the values as the key's length. I have been able to solve this in pure java and java-8, but somehow I don't think that appending a terminal operation at the end like .collect(Collectors.toList()); which is not required for me in my code.

My code: ( Java ) works fine

 Map<String, Integer> nameLength = new HashMap<>();
   nameLength.put("John", null);
    nameLength.put("Antony", 6);
    nameLength.put("Yassir", 6);
    nameLength.put("Karein", 6);
    nameLength.put("Smith", null);
    nameLength.put("JackeyLent",null);
    for(Entry<String, Integer> length: nameLength.entrySet()){
      if(length.getValue() == null){
        nameLength.put(length.getKey(),length.getKey().length());
      }
    }

Java-8 also works fine but the terminal operation is useless, how I avoid it without using .foreach().

nameLength.entrySet().stream().map(s->{
  if(s.getValue() == null){
    nameLength.put(s.getKey(),s.getKey().length());
  }
  return nameLength;
}).collect(Collectors.toList());
System.out.println(nameLength);

Any other way in which I can do the above logic in Java-8 and above??

Why are you putting these artificial restrictions: no for statements, no forEach? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Apr 23, 2020 at 13:31
This sounds like the XY problem - meta.stackexchange.com/questions/66377/what-is-the-xy-problem . What are you trying to achieve? — Ben R.
– Ben R., Commented Apr 23, 2020 at 13:31
Also please edit your title to reflect your actual issue. Refactoring in Java-8 doesn't tell us anything. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Apr 23, 2020 at 13:34
Whatever this is doing, it is not refactoring. You gain nothing if you randomly put lambdas in your code, this particular case there's no reason to replace a perfectly fine and working foreach loop with arcane constructs of streams and lambdas that barely do what original code did. — M. Prokhorov
– M. Prokhorov, Commented Apr 23, 2020 at 13:52
that's terrible use for map.. mutable operation, that doesn't return anything interesting — Coderino Javarino
– Coderino Javarino, Commented Apr 23, 2020 at 13:59

John Kugelman · Accepted Answer · 2020-04-23 14:08:02Z

4

If you're going to use streams then you should avoid side effects. Functional programming is all about pure operations where the output depends only on the input and functions have no side effects. In other words, create a new map instead of modifying the existing one.

If you do that you might as well just throw away the partially-filled-out map and recompute everything from scratch. Calling String.length() is cheap and it's not really worth the effort to figure out which values are null and which aren't. Recompute all the lengths.

Map<String, Integer> newMap = nameLength.keySet().stream()
    .collect(Collectors.toMap(
        name -> name,
        name -> name.length()
    ));

On the other hand if you just want to patch up your current map streams don't really buy you anything. I'd just modify it in place without involving streams.

for (Map.Entry<String, Integer> entry: nameLength.entrySet()) {
  if (entry.getValue() == null) {
    entry.setValue(entry.getKey().length());
  }
}

Or, as discussed above, you could simplify matters by replacing all of the lengths:

nameLength.replaceAll((name, __) -> name.length());

(__ signifies a variable that isn't used and so doesn't get a meaningful name.)

edited Apr 23, 2020 at 14:08

answered Apr 23, 2020 at 13:48

John Kugelman

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1 Comment

John Kugelman Over a year ago

@M.Prokhorov My answer addresses that. It's easier to recompute everything than to do it piecemeal, so that's what the first and last snippets do. The middle one only replaces null values and has the ugliest code as a result.

Ryuzaki L · Accepted Answer · 2020-04-23 13:49:08Z

3

You almost there, just use the filter to identify the entries with null values and then use Collectors.toMap to collect them into Map with key length as value

Map<String, Integer> nameLengths = nameLength.entrySet()
                 .stream()
                 .filter(entry->entry.getValue()==null)
                 .collect(Collectors.toMap(Map.Entry::getKey, entry->entry.getKey().length()));

Or more simpler way you have that check in Collectors.toMap

  Map<String, Integer> nameLengths = nameLength.entrySet()
                 .stream()
                 .collect(Collectors.toMap(Map.Entry::getKey, entry->entry.getValue() == null ? entry.getKey().length() : entry.getValue()));

edited Apr 23, 2020 at 13:49

answered Apr 23, 2020 at 13:33

Ryuzaki L

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2 Comments

Olivier Grégoire Over a year ago

Of all the answers I feel like you're the closest:

nameLength.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, entry -> entry.getValue() != null ? entry.getValue() : entry.getKey().length()));

.

Ryuzaki L Over a year ago

OMG, thank god for downvotes, any chance of explaining why ?

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