Programming Tutorials

TreeMap example in Java

By: Daniel Malcolm in Java Tutorials on 2007-09-14  

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 following TreeMap constructors are defined:

TreeMap()
TreeMap(Comparator comp)
TreeMap(Map m)
TreeMap(SortedMap sm)

The first form constructs an empty tree map that will be sorted by using the natural order of its keys. The second form constructs an empty tree-based map that will be sorted by using the Comparator comp. (Comparators are discussed later in this chapter.) The third form initializes a tree map with the entries from m, which will be sorted by using the natural order of the keys. The fourth form initializes a tree map with the entries from sm, which will be sorted in the same order as sm.

TreeMap implements SortedMap and extends AbstractMap. It does not define any additional methods of its own. The following program reworks the preceding example so that it uses TreeMap:

import java.util.*;

class TreeMapDemo {
    public static void main(String args[]) {
        // Create a tree map
        TreeMap tm = new TreeMap<>();
        // Put elements to the map
        tm.put("John Doe", 3434.34);
        tm.put("Tom Smith", 123.22);
        tm.put("Jane Baker", 1378.00);
        tm.put("Todd Hall", 99.22);
        tm.put("Ralph Smith", -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 = i.next();
            System.out.print(me.getKey() + ": ");
            System.out.println(me.getValue());
        }
        System.out.println();
        // Deposit 1000 into John Doe's account
        double balance = tm.get("John Doe");
        tm.put("John Doe", balance + 1000);
        System.out.println("John Doe's new balance: " +
                tm.get("John Doe"));
    }
}

The following is the output from this program:

Jane Baker: 1378.0
John Doe: 3434.34
Ralph Smith: -19.08
Todd Hall: 99.22
Tom Smith: 123.22
John Doe's current balance: 4434.34

Notice that TreeMap sorts the keys. However, in this case, they are sorted by first name instead of last name. You can alter this behavior by specifying a comparator when the map is created. The next section describes how.