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Explain about Map and their types.

Map cares about unique identifier. We can map a unique key to a specific value. It is a key/value pair. We can search a value, based on the key. Like set, Map also uses “equals ( )” method to determine whether two keys are same or different.

Hash Map:

  • Unordered and unsorted map.
  • Hashmap is a good choice when we don’t care about the order.
  • It allows one null key and multiple null values.

Example:

Public class Fruit{
Public static void main(String[ ] args){
HashMap<Sting,String> names =new HashMap<String,String>( );
names.put(“key1”,“cherry”);
names.put (“key2”,“banana”);
names.put (“key3”,“apple”);
names.put (“key4”,“kiwi”);
names.put (“key1”,“cherry”);
System.out.println(names);
}
 }

Output:

{key2 =banana, key1=cherry, key4 =kiwi, key3= apple}

Duplicate keys are not allowed in Map.

Doesn’t maintain any insertion order and is unsorted.

Hash Table:

  • Like vector key, methods of the class are synchronized.
  • Thread safety and therefore slows the performance.
  • Doesn’t allow anything that is null.
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