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How to iterate over a TreeMap? [duplicate]

People also ask

Are duplicates allowed in TreeMap?

A TreeMap cannot contain duplicate keys. TreeMap cannot contain the null key. However, It can have null values.

How do I iterate through a TreeMap key?

Using keySet(); method and for-each loop. Using keySet(); method and Iterator interface. Using entrySet(); method and for-each loop.

How do you make TreeMap synchronized?

TreeMap can be synchronized using the Collections. synchronizedMap() method. The synchronizedMap() method of the Collections class takes the Map that has to be synchronized as a parameter and returns a thread-safe synchronized Map. The synchronizedMap() method of java.

How do I copy from one TreeMap to another?

TreeMap putAll() Method in Java TreeMap. putAll() is an inbuilt method of TreeMap class that is used for the copy operation. The method copies all of the elements i.e., the mappings, from one map into another.


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)


    //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);
   }

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());
    }
    }

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.