Map Interface:

A Map cares about unique identifiers. You map a unique key (the ID) to a specific
value, where both the key and the value are, of course, objects.Many languages support data structures that use a key/value or name/value pair. The Map implementations let you do things like search for a value based on the key, ask for a collection of just the values, or ask for a collection of just the keys. Like Sets, Maps rely on the equals() method to determine whether two keys are the same or different.
HashMap The HashMap gives you an unsorted, unordered Map. When you need a Map and you don't care about the order (when you iterate through it), then HashMap is the way to go; the other maps add a little more overhead. Where the keys land in the Map is based on the key's hashcode, so, like HashSet, the more efficient your hashCode() implementation, the better access performance you'll get. HashMap allows one null key and multiple null values in a collection. When using HashSet or LinkedHashSet, the objects you add to them
must override hashCode(). If they don’t override hashCode(), the default Object.
hashCode() method will allow multiple objects that you might consider "meaningfully
equal" to be added to your "no duplicates allowed" set.

Hashtable Like Vector, Hashtable has existed from prehistoric Java times.
For fun, don't forget to note the naming inconsistency: HashMap vs. Hashtable.
Where's the capitalization of t? Oh well, you won't be expected to spell it. Anyway,
just as Vector is a synchronized counterpart to the sleeker, more modern ArrayList,
Hashtable is the synchronized counterpart to HashMap. Remember that you don't
synchronize a class, so when we say that Vector and Hashtable are synchronized, we
just mean that the key methods of the class are synchronized. Another difference,
though, is that while HashMap lets you have null values as well as one null key, a
Hashtable doesn't let you have anything that's null.

LinkedHashMap Like its Set counterpart, LinkedHashSet, the LinkedHash-
Map collection maintains insertion order (or, optionally, access order). Although it
will be somewhat slower than HashMap for adding and removing elements, you can
expect faster iteration with a LinkedHashMap.

TreeMap You can probably guess by now that a TreeMap is a sorted Map.
And you already know that by default, this means "sorted by the natural order of
the elements." Like TreeSet, TreeMap lets you define a custom sort order (via a
Comparable or Comparator) when you construct a TreeMap, that specifies how the
elements should be compared to one another when they're being ordered. As of
Java 6, TreeMap implements NavigableMap.
Map example:
import java.util.*;
class demo
public static void main(String[] args){
Map map=new HashMap();

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