Java - The TreeMap Class

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The TreeMap class implements the Map interface by using a tree. A TreeMap provides an efficient means of storing key/value pairs in sorted order, and allows rapid retrieval.

You should note that, unlike a hash map, a tree map guarantees that its elements will be sorted in ascending key order.

The TreeMap class supports four constructors. The first form constructs an empty tree map that will be sorted by using the natural order of its keys:

TreeMap( )

The second form constructs an empty tree-based map that will be sorted by using the Comparator comp:

TreeMap(Comparator comp)

The third form initializes a tree map with the entries from m, which will be sorted by using the natural order of the keys:

TreeMap(Map m)

The fourth form initializes a tree map with the entries from sm, which will be sorted in the same order as sm:

TreeMap(SortedMap sm)

Apart from the methods inherited from its parent classes, TreeMap defines following methods:

SNMethods with Description
1void clear()
Removes all mappings from this TreeMap.
2Object clone()
Returns a shallow copy of this TreeMap instance.
3Comparator comparator()
Returns the comparator used to order this map, or null if this map uses its keys' natural order.
4boolean containsKey(Object key)
Returns true if this map contains a mapping for the specified key.
5boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value.
6Set entrySet()
Returns a set view of the mappings contained in this map.
7Object firstKey()
Returns the first (lowest) key currently in this sorted map.
8Object get(Object key)
Returns the value to which this map maps the specified key.
9SortedMap headMap(Object toKey)
Returns a view of the portion of this map whose keys are strictly less than toKey.
10Set keySet()
Returns a Set view of the keys contained in this map.
11Object lastKey()
Returns the last (highest) key currently in this sorted map.
12Object put(Object key, Object value)
Associates the specified value with the specified key in this map.
13void putAll(Map map)
Copies all of the mappings from the specified map to this map.
14Object remove(Object key)
Removes the mapping for this key from this TreeMap if present.
15int size()
Returns the number of key-value mappings in this map.
16SortedMap subMap(Object fromKey, Object toKey)
Returns a view of the portion of this map whose keys range from fromKey, inclusive, to toKey, exclusive.
17SortedMap tailMap(Object fromKey)
Returns a view of the portion of this map whose keys are greater than or equal to fromKey.
18Collection values()
Returns a collection view of the values contained in this map.

Example:

The following program illustrates several of the methods supported by this collection:

import java.util.*;

public class TreeMapDemo {

   public static void main(String args[]) {
      // Create a hash map
      TreeMap tm = new TreeMap();
      // Put elements to the map
      tm.put("Zara", new Double(3434.34));
      tm.put("Mahnaz", new Double(123.22));
      tm.put("Ayan", new Double(1378.00));
      tm.put("Daisy", new Double(99.22));
      tm.put("Qadir", new Double(-19.08));
      
	  // Get a set of the entries
      Set set = tm.entrySet();
      // Get an iterator
      Iterator i = set.iterator();
      // Display elements
      while(i.hasNext()) {
         Map.Entry me = (Map.Entry)i.next();
         System.out.print(me.getKey() + ": ");
         System.out.println(me.getValue());
      }
      System.out.println();
      // Deposit 1000 into Zara's account
      double balance = ((Double)tm.get("Zara")).doubleValue();
      tm.put("Zara", new Double(balance + 1000));
      System.out.println("Zara's new balance: " +
      tm.get("Zara"));
   }
}

This would produce the following result:

Ayan: 1378.0
Daisy 99.22
Mahnaz: 123.22
Qadir: -19.08
Zara: 3434.34
Zara's current balance: 4434.34



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