What is HashMap?

What is HashMap?

MAP(I)::   

• It is not the child interface of collection
• If we want to represent a group of objects as key-value pairs then we should go for Map
• Both keys and values are objects only
• Duplicate key is not allowed but values can be duplicated
• Each key-value pair is called Entry. Hence map is considered as a collection of entry objects

Entry(I)::

• A map is a group of key-value pairs and each key-value pair is called an Entry. Hence map is considered as a collection of entry objects.
• Without existing map object there is no chance of existing entry object. Hence entry interface is defined inside map interface.

interface map{
    interface Entry{
        Object getKey();
        Object getValue()
        Object SetValue(Object newobj)
    }
}

Map Interface Methods ::

• Object put(Object key,Object value) :: To add 1 key-value pair to the map 

Note:: If the key is already is present then old value will be replaced with new value and returns old value.

• void putAll(Map m)
• Object get (Object key)
• Object remove(Object key)
• boolean containsKey(Object key)
• boolean containsValue(Object value)
• boolean isEmpty()
• int size()
• void clear();
• Below 3 methods  are collection views of map:
• Set keySet()
• Collection values()
• Set entrySet() 

##sample demo program ###

import java.util.*;
public class HashMapDemo {
    public static void main(String[] args) {
    HashMap<String, String> hmap = new HashMap();
    hmap.put("key1", "value1");
    hmap.put("key2", "value2");
    hmap.put("key3", "value3");
    hmap.put("key4", "value4");
    System.out.println("keySet()::=" + hmap.keySet() + "\n" + hmap.keySet().getClass());
    System.out.println("values()::=" + hmap.values() + "\n" + hmap.values().getClass());
    System.out.println("entrySet() ::=" + hmap.entrySet() + "\n" + hmap.entrySet().getClass());

    Set<String> set = hmap.keySet();
    System.out.println("keys=" + set);
    System.out.println("Iterating keys");
    // iterate keys
    for (String d : set) {
        System.out.println(d);
    }
    System.out.println("--------------");

    Collection<String> values = hmap.values();
    System.out.println("Values=" + values);
    System.out.println("Iterating Values");
    System.out.println("Iterating values() using Iterator interface");
    Iterator it = values.iterator();
    while (it.hasNext()) {
        System.out.println(it.next());
    }
    System.out.println("Iterating values() using for-each loop");
    for (String va : hmap.values()) {
        System.out.println(va);
    }
    System.out.println("Iterating entries of a hashmap");
    for (Map.Entry<String, String> s : hmap.entrySet()) {
        System.out.println(s.getKey() + "==" + s.getValue());
    }
    // Get specific value based on its key
    System.out.println("Getting specific value of a key using get() method." + "\n" + "value of key3=" + hmap.get("key3"));
    // Get all hashmap values based on its key using get() method
    System.out.println("Get all hashmap values based on its key using get() method");
    for (Map.Entry<String, String> s : hmap.entrySet()) {
        System.out.println(s.getKey() + "==" + hmap.get(s.getKey()));
    }
  }
}

Output:

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