Using a HashMap
A Map cares about unique identifiers. You map a unique key to a specific value. Both the key and the value are objects. A HashMap is an unsorted, unordered Map. When you need a Map and the order of the values is the least of your worldly concerns, the HashMap is here to serve you.
What can you do with a Hashmap?
- Search for a value based on the key
- Retrieve a collection of just keys or just values
- Add a value to be located by a key
- Delete a value by its corresponding key
- A HashMap can contain one
nullkey and any number ofnullvalues - If you put a value in a HashMap where a key exists, the old value is replaced
How do you declare a HashMap?
The following will create a HashMap called MyMap with the keys as String and the values as String
import java.util.HashMap;
HashMap<String, String> stateCapitals = new HashMap<String, String>();
How do you add values to the HashMap?
MyMap.put("Colorado","Denver");
How do you retrieve the value of a key?
MyMap.get("Colorado");
Here's a complete example
import java.util.HashMap;
public class HashmapApp
{
public static void main( String[] args )
{
HashMap<String,String> map = new HashMap<String,String>();
map.put( "cat", "Meow" );
map.put( "pig", "Oink" );
map.put( "dog", "Woof" );
map.put( "bat", "Squeak" );
System.out.println( "map = " + map );
System.out.println();
System.out.println("A cat says... " + map.get("cat"));
System.out.println("A pig says... " + map.get("pig"));
System.out.println("A dog says... " + map.get("dog"));
System.out.println("A bat says... " + map.get("bat"));
System.out.println("A unicorn says...." + map.get("unicorn"));
//check if the map contains a goldfish
if (map.containsKey("goldfish")){
System.out.println("A goldfish says... " + map.get("goldfish"));
}else{
System.out.println("I don't have a goldfish!");
}
}
}
The output of the above example
A cat says... Meow A pig says... Oink A dog says... Woof A bat says... Squeak A unicorn says....null