2
2
.
.
6
6
.
.
3
3
H
H
a
a
s
s
h
h
M
M
a
a
p
p
I
I
n
n
f
f
o
o
[
[
C
C
]
]
HashMap
● is Class that contains variable collection of key-value pairs
● can't contain duplicate keys
Both keys and values can be: Object, Primitive Data Type or null value.
There are two versions of HashMap Class
● HashMap is Non-Generic version of the Class (assumes that both key and value are just Objects)
● HashMap<K, V> is Generic version of the Class (you can specify Data Type for key and values)
It is advisable to use Generic version because of Data Type checking & to avoid manual casting when retrieving elements.
When adding elements you can use Primitive Data Types but they will be converted into Non-Primitive Data Types.
When retrieving elements Non-Primitive Data Types are automatically cast to Primitive Data Types if needed and possible.
Use HashMap instead Hashtable since
● it allows null values for both keys and values
● it's iterator is fail-safe throwing Exception if another thread tries to modify collection "structurally" while iterating
● it is not thread-safe making it faster and giving you an option to synchronize it or not
Test.java
import java.util.HashMap;
public class Test {
public static void main (String arg[]) throws Exception {
//PUT ELEMENTS.-------------------------------------------
HashMap hashmap = new HashMap();
hashmap.put("key" , "element" ); //Add key-value pair.
hashmap.put("age" , 33 );
hashmap.put(new Integer(34) , false );
hashmap.put(1 , new Character('A') );
//GET ELEMENTS.-------------------------------------------
String name = (String ) hashmap.get ("key" );
int age = ( (Integer ) hashmap.get ("age" ) ).intValue ();
boolean flag = ( (Boolean ) hashmap.get (new Integer(34) ) ).booleanValue ();
char stuff = ( (Character) hashmap.get (1 ) ).charValue ();
//STATISTICS.---------------------------------------------
boolean containsElement = hashmap.containsValue("element");
boolean containsKey = hashmap.containsKey ("key" );
boolean isEmpty = hashmap.isEmpty ( );
int size = hashmap.size ( );
String removed = (String) hashmap.remove ("key" );
hashmap.clear ( );
//DISPLAY STATISTICS.-------------------------------------
System.out.println ("Contains 'element' = " + containsElement);
System.out.println ("isEmpty = " + isEmpty );
System.out.println ("size = " + size );
System.out.println ("removed element = " + removed );
}
}
