Difference between HashMap and LinkedHashMap

[Robert Darling]

Hi,

While studying Java Collections , i cannot understand the main difference between HashMap and LinkedHashMap ? From what i understand The below given codes, HashMap does not maintain insertion order, but LinkedHashMap does ? Is this the only difference.

HashMap<String, String> a=new HashMap<String, String>();
		a.put("a", "apple");
		a.put("b", "banana");
		a.put("c","coliflower");
		
		
		Set<String> b= a.keySet();
		
		for(String c: b)
			System.out.println(c);

Output b,a,c

LinkedHashMap<String, String> a=new LinkedHashMap<String, String>();
a.put("a", "apple");
a.put("b", "banana");
a.put("c","coliflower");

		
		Set<String> b= a.keySet();
		
		for(String c: b)
			System.out.println(c);

Output : a,b,c

Secondly how do you iterate over the values in a HashMap and HashTable ? Are we supposed to reuse the key returned by the keySet to get the value of the key ? Secondly a Set does not allow duplicates, so what about duplicate keys ?

[Ankit Garg]

A map doesn't allow duplicate keys.
Yes you need to use the keySet's key to iterate over a map.
You can find the details of both HashMap and LinkedHashMap in the javadocs to find their differences, you already pointed out the main difference between the two...

[Robert Darling]

Hi .. Thanks for the reply. You said that a MAP does not allow duplicate keys, but i understand a Hashtable does ? Is this correct. Both implement the MAP interface ?

[Paul Clapham]

Sajjad Dar wrote:You said that a MAP does not allow duplicate keys, but i understand a Hashtable does ? Is this correct. Both implement the MAP interface ?

If it is correct, then the documentation would mention it. The documentation would also mention what interfaces it implements. So how about if you have a look there? That's what the docs are for, after all. Let us know what you find out.