Any Link Between ArrayList and HashMap

[Rajendra Prakash]

Hi Friends, Can you give few interview questions which relates ArrayList and Hashmap. I mean how to link ArrayList and HashMap.

[Jesper de Jong]

I don't know what you mean by "how to link ArrayList and HashMap". Do you mean you want to know what the similarities and differences between these two are?

ArrayList and HashMap are both standard Java collection classes, but they have a different structure and a different purpose. An ArrayList is a an ordered sequence (a list) of objects backed by an array, and a HashMap is an associative array - a data structure that maps keys to values.

See Sun's Collections tutorial and The Collections Framework.

[Rajendra Prakash]

ArrayList vs HashMap : I have stored objects in ArrayList , can i retrieve same objects in HashMap. is it possible?

[Campbell Ritchie]

If you put the same objects into the HashMap, then you can retrieve them.

You appear to have misunderstood the two. It is a bit like asking whether you can use a boat or a bicycle. The answer is yes, but where boats work bicycles don't, and vice versa

[Rob Spoor]

You can put your ArrayList elements in a Map as follows:

Map<KeyType,ElementType> map = new HashMap<KeyType,ElementType>(list.size());
for (ElementType element: list)
{
    KeyType key = element.getKey();
    map.put(key, element);
}

The KeyType is usually something immutable like String, Integer or Long. The name of any object is often used. For instance:

Map<String,Employee> map = new HashMap<String,Employee>(list.size());
for (Employee employee: list)
{
    String name = employee.getName();
    map.put(name, employee);
}

Vice versa you have two options:
1) store map.entrySet() in the List
2) ignore the keys and store map.values() in the List.