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/*
Java HashMap example.
This Java HashMap example
describes the basic operations performed on the HashMap.
*/
import java.util.HashMap;
import java.util.Iterator;
public class HashMapExample{
public
static void
main(String args[]){
// constructs
a new empty HashMap with default initial capacity
HashMap hashMap = new HashMap();
/*
To specify initial capacity,
use following constructor.
HashMap HashMap = new
HashMap(100);
To create HashMap from
map use following constructor
HashMap HashMap = new
HashMap(Map myMap);
*/
hashMap.put( "One", new Integer(1) ); // adding value into HashMap
hashMap.put( "Two", new Integer(2) );
hashMap.put( "Three", new Integer(3) );
/*
IMPORTANT : We CAN NOT
add primitives to the HashMap. We have to wrap it into one of
the wrapper before adding.
*/
/*
To copy all key - value
pairs from any Map to HashMap use putAll method.
Signature of putAll
method is,
void putAll(Map m)
*/
//get number of keys present in the HashMap
System.out.println("HashMap contains
" + hashMap.size() + " key value pair.");
/*
To check whether HashMap
is empty or not, use isEmpty() method.
isEmpty() returns true
is HashMap is empty, otherwise false.
/*
Finding particular value
from the HashMap :
HashMap's containsValue
method returns boolean depending upon the presense of the value
in given HashMap
Signature of the containsValue
method is,
boolean containsValue(Object
value)
*/
if( hashMap.containsValue( new
Integer(1) )
){
System.out.println("HashMap
contains 1 as value");
}else{
System.out.println("HashMap
does not contain 1 as value");
}
/*
Finding particular Key
from the HashMap :
HashMap's containsKey
method returns boolean depending upon the presense of the key
in given HashMap
Signature of the method
is,
boolean containsKey(Object
key)
*/
if( hashMap.containsKey("One") ){
System.out.println("HashMap
contains One as key");
}else{
System.out.println("HashMap
does not contain One as value");
}
/*
Use get method of HashMap
to get value mapped to particular key.
Signature of the get
method is,
Object get(Object key)
*/
Integer one = (Integer) hashMap.get("One");
System.out.println("Value mapped
with key \"One\" is " +
one);
/*
IMPORTANT: get method
returns Object, so we need to downcast it.
*/
/*
To get all keys stored
in HashMap use keySet method.
Signature of the keysSet
method is,
Set keySet()
*/
System.out.println("Retriving all
keys from the HashMap");
Iterator iterator = hashMap.keySet().iterator();
while( iterator. hasNext()
){
System.out.println( iterator.next()
);
}
/*
To get all values stored
in HashMap use entrySet() method.
Signature of the entrySet()
method is,
Set entrySet()
*/
System.out.println("Retriving all
values from the HashMap");
iterator = hashMap.entrySet().iterator();
while( iterator. hasNext()
){
System.out.println( iterator.next()
);
}
/*
To remove particular
key - value pair from the HashMap use remove method.
Signature of remove
methid is,
Object remove(Object
key)
This method returns
value that had mapped to the given key, otherwise null if mapping
not found.
*/
System.out.println( hashMap.remove("One")
+ " is removed from
the HashMap.");
}
}
/*
OUTPUT of the above
given Java HashMap Example would be :
HashMap contains 3 key
value pair.
HashMap contains 1 as
value
HashMap contains One
as key
Value mapped with key
"One" is 1
Retriving all keys from
the HashMap
Three
Two
One
Retriving all values
from the HashMap
Three=3
Two=2
One=1
1 is removed from the
HashMap.
*/