TreeMap

In this chapter you will learn:

What is Java TreeMap and how to use TreeMap
A demo for TreeMap

TreeMap class

The TreeMap class extends AbstractMap and implements the NavigableMap interface.

It creates maps stored in a tree structure.

A tree map guarantees that its elements will be sorted in an order. TreeMap is a generic class that has this declaration:

class TreeMap<K, V>

K specifies the type of keys
V specifies the type of values.

The following TreeMap constructors are defined:

TreeMap( ) constructs an empty tree map.
TreeMap(Comparator<? super K> comp) constructs an empty tree-based map using the Comparator comp.
TreeMap(Map<? extends K, ? extends V> m) initializes a tree map with the entries from m.
TreeMap(SortedMap<K, ? extends V> sm) initializes a tree map with the entries from SortedMap.

TreeMap has no methods beyond those specified by the NavigableMap interface and the AbstractMap class.

Put TreeMap to work

The following code sorts a map's entries according to the natural ordering of their String-based keys

import java.util.Map;
import java.util.TreeMap;
// j a  v  a2 s .co  m
public class Main {
  public static void main(String[] args) {
    Map<String, Integer> msi = new TreeMap<String, Integer>();
    String[] fruits = { "apples", "pears", "grapes", "bananas", "kiwis" };
    int[] quantities = { 10, 15, 8, 17, 30 };
    for (int i = 0; i < fruits.length; i++){
      msi.put(fruits[i], quantities[i]);
    }      
    for (Map.Entry<String, Integer> entry : msi.entrySet()){
      System.out.println(entry.getKey() + ": " + entry.getValue());
    }      
  }
}