Java TreeMap With Examples

TreeMap in Java is one of the implementation of the Map interface. How it differs from the other implementation HashMap is that unlike HashMap which is unordered, TreeMap is Sorted.

TreeMap is sorted according to the natural ordering of its keys by default. If you want different sort ordering then you will have to provide a Comparator at the TreeMap construction time.

Java TreeMap is a Red-Black tree based implementation of Map. TreeMap class extends AbstractMap and implements NavigableMap, Cloneable and Serializable interfaces.

Features of TreeMap

Some of the features of the TreeMap in Java which are discussed in this post are as follows-

In TreeMap elements are sorted by keys.
In TreeMap values may be duplicate but a key has to be unique.
TreeMap doesn’t permit null keys, null values are permitted though.
TreeMap in Java is not thread safe.
The iterators returned by all of TreeMap’s "collection view methods" are fail-fast. Which means, if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the Iterator throws a ConcurrentModificationException.

Table of contents

Java TreeMap constructors
Java example creating a TreeMap
In TreeMap null keys are not allowed
Changing the sort order in TreeMap
Methods in TreeMap class
TreeMap implementation is not synchronized
Java TreeMap iterator

Java TreeMap constructors

TreeMap()- Constructs a new, empty tree map, using the natural ordering of its keys.
TreeMap(Comparator<? super K> comparator)- Constructs a new, empty tree map, ordered according to the given comparator.
TreeMap(Map<? extends K,? extends V> m)- Constructs a new tree map containing the same mappings as the given map, ordered according to the natural ordering of its keys.
TreeMap(SortedMap<K,? extends V> m)- Constructs a new tree map containing the same mappings and using the same ordering as the specified sorted map.

Java example creating a TreeMap

Let’s see an example where a TreeMap is created and elements inserted in the TreeMap. We’ll display those values to see the sorted ordering.

import java.util.Map;
import java.util.TreeMap;

public class TreeMapDemo {
  public static void main(String[] args) {
    Map<Integer, String> studentMap = new TreeMap<Integer, String>();
    studentMap.put(1001, "Amy");
    studentMap.put(1210, "Root");
    studentMap.put(2340, "Frodo");
    studentMap.put(1345, "Chris");
    studentMap.put(1440, "Chris");
    studentMap.put(1345, "Zidane");
    
    for(Map.Entry<Integer, String> entry : studentMap.entrySet()){
      System.out.println("Key is " + entry.getKey() + " Value is " + entry.getValue());
    }
  }
}

Output

Key is 1001 Value is Amy
Key is 1210 Value is Root
Key is 1345 Value is Zidane
Key is 1440 Value is Chris
Key is 2340 Value is Frodo

From the output you can see that the elements are sorted on keys. Since natural ordering for Integer is ascending that is why keys are sorted in ascending order.

Key 1345 is inserted twice, since key has to be unique that is why value is overwritten for the same key.

In TreeMap null keys are not allowed

We can check that TreeMap doesn’t allow null as key by trying to add null as key in the TreeMap. It should throw NullPointerExpcetion.

public class TreeMapDemo {
  public static void main(String[] args) {
    Map<Integer, String> studentMap = new TreeMap<Integer, String>();
    studentMap.put(1001, "Amy");
    studentMap.put(1210, "Root");
    studentMap.put(2340, "Frodo");
    studentMap.put(1345, "Chris");
    studentMap.put(1440, "Chris");
    studentMap.put(null, "Zidane");

    for(Map.Entry<Integer, String> entry : studentMap.entrySet()){
      System.out.println("Key is " + entry.getKey() + " Value is " + entry.getValue());
    }
  }
}

Output

Exception in thread "main" java.lang.NullPointerException
	at java.base/java.util.TreeMap.put(TreeMap.java:561)
	at com.knpcode.TreeMapDemo.main(TreeMapDemo.java:15)

Changing the sort order in TreeMap

If you want to change the sort order in TreeMap then you will have to provide a Comparator. In the previous example if you want keys to be sorted in descending order then you can do it as follows.

import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;

public class TreeMapDemo {
  public static void main(String[] args) {
    // TreeMap with comparator
    Map<Integer, String> studentMap = new TreeMap<Integer, String>(new KeyComparator());
    studentMap.put(1001, "Amy");
    studentMap.put(1210, "Root");
    studentMap.put(2340, "Frodo");
    studentMap.put(1345, "Chris");
    studentMap.put(1440, "Chris");
    studentMap.put(2134, "Zidane");
    
    for(Map.Entry<Integer, String> entry : studentMap.entrySet()){
      System.out.println("Key is " + entry.getKey() + " Value is " + entry.getValue());
    }
  }
}
// Comparator
class KeyComparator implements Comparator<Integer>{
  @Override
  public int compare(Integer o1, Integer o2) {
    // TODO Auto-generated method stub
    return o2.compareTo(o1);
  }
}

Output

Key is 2340 Value is Frodo
Key is 2134 Value is Zidane
Key is 1440 Value is Chris
Key is 1345 Value is Chris
Key is 1210 Value is Root
Key is 1001 Value is Amy

Methods in TreeMap class

List of some of the methods in TreeMap class in Java.

ceilingEntry(K key)- Returns a key-value mapping associated with the least key greater than or equal to the given key, or null if there is no such key.
ceilingKey(K key)- Returns the least key greater than or equal to the given key, or null if there is no such key.
clear()- Removes all of the mappings from this map.
containsKey(Object key)- Returns true if this map contains a mapping for the specified key.
containsValue(Object value)- Returns true if this map maps one or more keys to the specified value.
floorEntry(K key)- Returns a key-value mapping associated with the greatest key less than or equal to the given key, or null if there is no such key.
floorKey(K key)- Returns the greatest key less than or equal to the given key, or null if there is no such key.
get(Object key)- Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
keySet()- Returns a Set view of the keys contained in this map.
remove(Object key)- Removes the mapping for this key from this TreeMap if present.
size()- Returns the number of key-value mappings in this map.
subMap(K fromKey, K toKey)- Returns a view of the portion of this map whose keys range from fromKey, inclusive, to toKey, exclusive.
tailMap(K fromKey)- Returns a view of the portion of this map whose keys are greater than or equal to fromKey.

TreeMap implementation is not synchronized

TreeMap in Java is not thread safe as it is not synchronized. If multiple threads access a TreeMap concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. You can wrap your TreeMap using the Collections.synchronizedSortedMap method.

SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...));

Java TreeMap iterator

You can’t directly use an iterator with Map. You will have to get the collection view of the Map and then iterate it. The iterators returned by TreeMap's collection view methods are fail-fast. If the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method, the Iterator throws a ConcurrentModificationException.

Iterating TreeMap Java example

public class TreeMapDemo {
  public static void main(String[] args) {
    // TreeMap with comparator
    Map<Integer, String> studentMap = new TreeMap<Integer, String>();
    studentMap.put(1001, "Amy");
    studentMap.put(1210, "Root");
    studentMap.put(2340, "Frodo");
    studentMap.put(1345, "Chris");
    studentMap.put(1440, "Chris");
    studentMap.put(2134, "Zidane");
        
    // iterating map
    Iterator<Map.Entry<Integer, String>> itr = studentMap.entrySet().iterator();
    while(itr.hasNext()) {
      Map.Entry<Integer, String> entry = itr.next();
      System.out.println("Key is " + entry.getKey() + " Value is " + entry.getValue());
    }
  }
}

Output

Key is 1001 Value is Amy
Key is 1210 Value is Root
Key is 1345 Value is Chris
Key is 1440 Value is Chris
Key is 2134 Value is Zidane
Key is 2340 Value is Frodo

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September 5, 2022