CODEWITHSUNDEEP
javadictionaryiteratorjava-stream
Dan
Dan

Reputation: 9791

How to remove an element of a HashMap whilst streaming (lambda)

I have the following situation where I need to remove an element from a stream. 

map.entrySet().stream().filter(t -> t.getValue().equals("0")).
            forEach(t -> map.remove(t.getKey()));

in pre Java 8 code one would remove from the iterator - what's the best way to deal with this situation here?

Upvotes: 61

Views: 62246

Answers (4)

Cyberpunk
Cyberpunk

Reputation: 41

1st time replying. Ran across this thread and thought to update if others are searching. Using streams you can return a filtered map<> or whatever you like really. 

  @Test
  public void test() {

    Map<String,String> map1 = new HashMap<>();
    map1.put("dan", "good");
    map1.put("Jess", "Good");
    map1.put("Jaxon", "Bad");
    map1.put("Maggie", "Great");
    map1.put("Allie", "Bad");

    System.out.println("\nFilter on key ...");
    Map<String,String> map2 = map1.entrySet().stream().filter(x -> 
    x.getKey().startsWith("J"))
        .collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));

    map2.entrySet()
      .forEach(s -> System.out.println(s));

    System.out.println("\nFilter on value ...");
    map1.entrySet().stream()
      .filter(x -> !x.getValue().equalsIgnoreCase("bad"))
      .collect(Collectors.toMap(e -> e.getKey(),  e -> e.getValue()))
      .entrySet().stream()
      .forEach(s -> System.out.println(s));
  }

------- output -------

Filter on key ...
Jaxon=Bad
Jess=Good

Filter on value ...
dan=good
Jess=Good
Maggie=Great

Upvotes: 4

abhi169jais
abhi169jais

Reputation: 124

If you want to remove the entire key, then use:

myMap.entrySet().removeIf(map -> map.getValue().containsValue("0"));

Upvotes: 7

Louis Wasserman
Louis Wasserman

Reputation: 198073

map.entrySet().removeIf(entry -> entry.getValue().equals("0"));

You can't do it with streams, but you can do it with the other new methods.

EDIT: even better:

map.values().removeAll(Collections.singleton("0"));

Upvotes: 149

maczikasz
maczikasz

Reputation: 1133

I think it's not possible (or deffinitelly shouldn't be done) due to Streams' desire to have Non-iterference, as described here

If you think about streams as your functional programming constructs leaked into Java, then think about the objects that support them as their Functional counterparts and in functional programming you operate on immutable objects

And for the best way to deal with this is to use filter just like you did

Upvotes: 4

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