Programming Tutorials

HashMap example in Java

By: Charles in Java Tutorials on 2007-09-14  

The HashMap class uses a hash table to implement the Map interface. This allows the execution time of basic operations, such as get() and put(), to remain constant even for large sets.

The following constructors are defined: HashMap()

HashMap(Map m)
HashMap(int capacity)
HashMap(int capacity, float fillRatio)

The first form constructs a default hash map. The second form initializes the hash map by using the elements of m. The third form initializes the capacity of the hash map to capacity. The fourth form initializes both the capacity and fill ratio of the hash map by using its arguments. The meaning of capacity and fill ratio is the same as for HashSet, described earlier.

HashMap implements Map and extends AbstractMap. It does not add any methods of its own. You should note that a hash map does not guarantee the order of its elements. Therefore, the order in which elements are added to a hash map is not necessarily the order in which they are read by an iterator.

The following program illustrates HashMap. It maps names to account balances. Notice how a set-view is obtained and used.

import java.util.*;

class HashMapDemo {
    public static void main(String args[]) {
        // Create a hash map
        HashMap hm = new HashMap();
        // Put elements to the map

        hm.put("John Doe", Double.valueOf(3434.34));
        hm.put("Tom Smith", Double.valueOf(123.22));
        hm.put("Jane Baker", Double.valueOf(1378.00));
        hm.put("Todd Hall", Double.valueOf(99.22));
        hm.put("Ralph Smith", Double.valueOf(-19.08));
        // Get a set of the entries
        Set> set = hm.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 = hm.get("John Doe");
        hm.put("John Doe", balance + 1000);
        System.out.println("John Doe's new balance: " + hm.get("John Doe"));
    }
}

Output from this program is shown here:

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

The program begins by creating a hash map and then adds the mapping of names to balances. Next, the contents of the map are displayed by using a set-view, obtained by calling entrySet(). The keys and values are displayed by calling the getKey() and getValue() methods that are defined by Map.Entry. Pay close attention to how the deposit is made into John Doe's account. The put() method automatically replaces any preexisting value that is associated with the specified key with the new value. Thus, after John Doe's account is updated, the hash map will still contain just one "John Doe" account.