How to iterate over a TreeMap?

Question

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How do I iterate over each Entry in a Map?

I want to iterate over a TreeMap, and for all keys which have a particular value, I want them to be added to a new TreeMap. How can I do this?

@Click: "... for all 'key's which have a particular value ...". Do you mean for all keys in a given Set, or for all keys that satisfy a given predicate? — Stephen C, Aug 24 '09 at 00:22

score 272 · Accepted Answer · answered Aug 23 '09 at 16:50

272

Assuming type TreeMap<String,Integer> :

for(Map.Entry<String,Integer> entry : treeMap.entrySet()) {
  String key = entry.getKey();
  Integer value = entry.getValue();

  System.out.println(key + " => " + value);
}

(key and Value types can be any class of course)

answered Aug 23 '09 at 16:50

Zed

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FYI, EntrySet is the preferred way to iterate through any Map since EntrySets are by specification reflective and always represent the state of data in the Map even if the Map underneath would change. – Esko Aug 23 '09 at 16:58
1

Adding to Zed's answer , using entrySet will give the user the power to delete a particular entry while iterating. – SamDJava Sep 15 '14 at 17:49
1

Giving the name `treeMap` to your `TreeMap` threw me off at first. – intcreator Sep 28 '15 at 20:26
11

will this give out the values ordered? – phil294 Sep 26 '16 at 23:51
2

No. TreeMap order by Key value. – André Luiz Reis Mar 28 '18 at 19:21

score 37 · Answer 2 · edited May 01 '14 at 07:01

    //create TreeMap instance
    TreeMap treeMap = new TreeMap();

    //add key value pairs to TreeMap
    treeMap.put("1","One");
    treeMap.put("2","Two");
    treeMap.put("3","Three");

    /*
      get Collection of values contained in TreeMap using
      Collection values()        
    */
    Collection c = treeMap.values();

    //obtain an Iterator for Collection
    Iterator itr = c.iterator();

    //iterate through TreeMap values iterator
    while(itr.hasNext())
      System.out.println(itr.next());

or:

   for (Map.Entry<K,V> entry : treeMap.entrySet()) {
        V value = entry.getValue();
        K key = entry.getKey();
   }

or:

   // Use iterator to display the keys and associated values
   System.out.println("Map Values Before: ");
   Set keys = map.keySet();
   for (Iterator i = keys.iterator(); i.hasNext();) {
     Integer key = (Integer) i.next();
     String value = (String) map.get(key);
     System.out.println(key + " = " + value);
   }

There must be a more performant way that do another treemap lookup here: "(String) map.get(key)". You already know the element at this stage, just curious. — ericcurtin, Mar 27 '20 at 12:24

score 9 · Answer 3 · answered Aug 30 '12 at 04:31

Just to point out the generic way to iterate over any map:

 private <K, V> void iterateOverMap(Map<K, V> map) {
    for (Map.Entry<K, V> entry : map.entrySet()) {
        System.out.println("key ->" + entry.getKey() + ", value->" + entry.getValue());
    }
    }

score 3 · Answer 4 · answered Aug 23 '09 at 17:03

3

Using Google Collections, assuming K is your key type:

Maps.filterKeys(treeMap, new Predicate<K>() {
  @Override
  public boolean apply(K key) {
    return false; //return true here if you need the entry to be in your new map
  }});

You can use filterEntries instead if you need the value as well.

answered Aug 23 '09 at 17:03

Jorn

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This doesn't do any sorting. – zyamys Jan 04 '17 at 04:31

How to iterate over a TreeMap?

4 Answers4

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