AbstractMap in Java

The AbstractMap class is a part of the Java Collection Framework. It directly implements the Map interface to provide a structure to it, by doing so it makes the further implementations easier. As the name suggests AbstractMap is an abstract class by definition, therefore it cannot be used to create objects. The concrete classes that inherit from AbstractMap can be used to create objects.

Declaration:

public abstract class AbstractMap<K,V> extends Object, implements Map<K,V>

Here, K is the key type and V is the value type.

Hierarchy of AbstractMap

This class implements Map<K, V> interface and extends Object class.

Constructors of AbstractMap

Basic Operations

1. Adding Elements

To add elements to an AbstractMap we use the put() method. Note that the method is not actually of type AbstractMap but of an extending class, the reference to this object is of type AbstractMap. The entries of the map are of type key, value pair. Here the key is of type Integer and the value id of type String, this is mentioned at the time of instantiation of the object.

The Set view of the mappings:
[1=This, 2=is, 3=an, 4=AbstractMap]

2. Removing Elements

We can remove mappings from an AbstractMap using the remove() method. The syntax of remove is 

mapName.remove(Object key);

This will remove the value mapped to the specified key. We can clear the entire AbstractMap using the clear() method as shown below.

Mappings of the AbstractMap:
{1=This, 2=is, 3=an, 4=AbstractMap}
Mappings of the AbstractMap:
{2=is, 3=an, 4=AbstractMap}

The Set view of the mappings:
{}

3. Replacing entries

We can replace values associated with a key using the replace() method as shown below.

Mappings of the AbstractMap:
{1=This, 2=is, 3=a, 4=AbstractMap}

Mappings of the AbstractMap:
{1=This, 2=is, 3=an, 4=AbstractMap}

4. Traversing

There are different ways to traverse through an AbstractMap, we discuss two ways in the code given below. The first method is to use entrySet() method, which returns a collection view of the map. This view can be iterated through using a for each loop. The method getKey() return the key and getValue() return the associated value. The second method is by using the Iterator interface. We create an iterator over the key set using the keySet() method. This iterator is used to iterate over the map using the next() method, which returns the next entry in the map.

Using the entrySet() method
Key = 1, Value = This
Key = 2, Value = is
Key = 3, Value = a
Key = 4, Value = AbstractMap

Using the Iterator interface
Key = 1, Value =  This
Key = 2, Value =  is
Key = 3, Value =  a
Key = 4, Value =  AbstractMap

Inheriting Classes

1. HashMap

The HashMap class is the basic implementation of the Map interface, which stores the key-value pairs. The HashMap is a basic Map implementation of the Hashtable but HashMap is unsychronised. The order of elements of a HashMap is undefined. The HashMap provides constant time performance for basic operations of insertion, deletion, traversal and manipulation.

Syntax:

HashMap< ? , ? > hmName = new HashMap< ? , ? >();

2. IdentityHashMap

The IdentityHashMap is the Hashtable implementation of a Map, but it uses reference equality instead of object equality. Which means that IdentityHashMap violates the basic rule of maps, therefore it is not a general-purpose map, it is reserved for rare scenarios. In an IdentityHashMap any two objects, o1 and o2, are equal if and only if o1==o2 i.e. the reference of o1 and o2 are the same.

Syntax:

IdentityHashMap< ? , ? > ihmName = new IdentityHashMap< ? , ? >();

3. WeakHashMap

The WeakHashMap is a Hashtable implementation of the Map interface that uses weak keys. When a key in a WeakHashMap is no longer in ordinary use it will be automatically discarded. It means that the existence of a mapping will not prevent the key from being garbage collected. The WeakHashMap is an unsynchronised class.

Syntax:

WeakHashMap< ? , ? > whmName = new WeakHashMap< ? , ? >();

4. TreeMap

The TreeMap class is a Red-Black tree based implementation of the NavigableMap. The TreeMap elements are sorted either by the natural order or by the Comparator provided at the time of construction.

Syntax:

TreeMap< ? , ? > tmName = new TreeMap< ? , ? >();

5. EnumMap

The EnumMap is a specialized implementation of the Map interface. The keys of the EnumMap are of type enum. All the keys must be of the same enum, either defined explicitly or implicitly, specified at the time of construction. The EnumMap is internally represented as an array, it is very compact and efficient.

Syntax:

EnumMap< enumName, ? > emName = new EnumMap< enumName, ? >();

6. ConcurrentHashMap

The ConcurrentHashMap is a Hashtable implementation that supports full concurrency through retrieve and update operations. This class is highly compatible with the Hashtable class and contains all corresponding methods of a Hashtable. Although the operations are thread-safe there is no locking mechanism in place, therefore retrieval can overlap updation. Therefore, retrieval operations reflect the most recently completed update operations.

Syntax:

ConcurrentHashMap< ? , ? > chmName = new ConcurrentHashMap< ? , ? >();

7. ConcurrentSkipListMap

The ConcurrentSkipListMap is a scalable implementation of the ConcurrentNavigableMap interface. The keys in ConcurrentSkipListMap are sorted by natural order or by using a Comparator at the time of construction of the object. The ConcurrentSkipListMap has the expected time cost of log n for insertion, deletion and searching operations. It is a thread-safe class, therefore, all basic operations can be accomplished concurrently.

Syntax:

ConcurrentSkipListMap< ? , ? > cslmName = new ConcurrentSkipListMap< ? , ? >();

Methods of AbstractMap

Methods declared in interface java.util.Map