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In this lesson we look at the Map hierarchy and examine four of the concrete implementations of the Map<K,V> interface. So what type of collection is a Map? Maps have unique identifiers as their keys which are mapped to a value, similar to key/value pairs in other languages. A Map doesn't allow duplicates keys so every entry within a Map must be unique. In practical terms this means that we can never have more than one element within the Map referencing the same object or more than one element within the Map referencing two objects that are considered equal. See the Checking Object Equality section from the OO Concepts - The Object Superclass section of the site for a reminder of what constitutes object equality. We can also only have a maximum of one entry within the set with a value of null.

Another feature of Map collection is how the Map<K,V> interface provides three different collection views, allowing us to view a map collection as a set of keys, a collection of values and as a set of key/value mappings.

Map Hierarchy Diagram Top

The diagram below is a representation of the Map hierarchy and covers the interfaces and classes we will study in this lesson. The diagram has several interfaces missing and also the java.util.WeakHashMap<K,V> and java.util.IdentityHashMap<K,V> concrete implemetations which are not covered on the site, but should help in visualisation:

Map Interfaces & Classes Top

The table below gives a description of each interface and class within the above diagram. Click a link to go to detailed information about a particular class:

Map Types and Ordering Top

The table below extrapolates the commented information about ordering from the diagram above into a more reable tabular format:

HashMap Top

A HashMap is an unordered and unsorted implementation of the Map<K,V> interface that allows null values and a null key. This type of map makes no guarantees over iteration order and in particular no guarantees that the iteration order will remain constant over time.

The hashcode of the object being inserted into the collection is used so the more efficient your override of the hashCode() method of the Object class is, the more efficient the HashMap. This also means that if you want to find out if an object exists within the map, or you want to delete an object from the map, then you need a reference to the object in question.

This type of map is a good choice when you want a collection with no duplicates and fast access and the order doesn't matter when iterating over it.

HashMap<K,V> Method Overview Top

The table below shows the declarations of all the methods for the HashMap<K,V> class implemented from the Map<K,V> interface:

Usage for the the first seven methods is shown in the example below. For more information on the other methods in the HashMap<K,V> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the HashMap<K,V> class which you can find by scrolling down the lower left pane and clicking on HashMap.

HashMap<K,V> Example Top

Lets write a simple class that creates a HashMap and uses some of the methods from the HashMap<K,V> class:

package info.java8;
/*
  Simple class to create a HashMap and use several methods from the class 
*/
import java.util.*; // Import the java.util package

public class HashMapClass {

    public static void main (String[] args) {
        Map<String, String> hm = new HashMap<String, String>();
        hm.put("hello", "goodbye");
        hm.put("goodbye", "hello");
        hm.put("yes", "hello");
        hm.put("no", "goodbye");
        System.out.println("hash map size = " + hm.size());
        System.out.println("hash map contains: " + hm);
        hm.remove("yes");  
        System.out.println(hm.containsKey("yes"));  
        System.out.println("hash map size = " + hm.size());
        System.out.println("hash map contains: " + hm);
        // Collection-views, results differ dependant upon Map implementation
         System.out.println(hm.entrySet());
        System.out.println(hm.keySet());
        System.out.println(hm.values());
        // Use Map static nested class Entry to get a reference and change all values
        for (Map.Entry<String, String> map : hm.entrySet()) {
            map.setValue("aaa");
        }
        System.out.println(hm.entrySet());
    }
}

Save, compile and run the HashMapClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our HashMapClass class and using several of the methods contained within the HashMap<K,V> class.

A thing of note here is how we use the Map.Entry static nested class of Map to get a reference to each map entry and change the values; the only way to obtain a reference to a map entry is from the iterator of this collection-view

LinkedHashMaps Top

A LinkedHashMap is a Hash table and linked list implementation of the Map<K,V> interface with predictable iteration order and differs from a HashSet by maintaintaining a doubly-linked list running through all of its elements. The linked list defines the iteration order of a LinkedHashMap by the order in which elements are inserted into the set.

This type of set is a good choice when you want a collection with no duplicates and fast access and you want to maintain an insertion order when iterating over it.

LinkedHashMap<K,V> Method Overview Top

The table below shows the declarations of all the methods for the LinkedHashMap<K,V> class that are inherited directly from the HashMap<K,V> class:

Usage for the the first seven methods is shown in the example below. For more information on the methods not directly inherited fromthe HashMap<K,V> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the LinkedHashMap<K,V> class which you can find by scrolling down the lower left pane and clicking on LinkedHashMap.

LinkedHashMap<K,V> Example Top

Lets write a simple class that creates a LinkedHashMap and uses some of the inherited methods from the HashMap<K,V> class:

package info.java8;
/*
  Simple class to create a LinkedHashMap and use several methods inherited from the HashMap class 
*/
import java.util.*; // Import the java.util package

public class LinkedHashMapClass {

    public static void main (String[] args) {
        Map<String, String> lhm = new LinkedHashMap<String, String>();
        lhm.put("hello", "goodbye");
        lhm.put("goodbye", "hello");
        lhm.put("yes", "hello");
        lhm.put("no", "goodbye");
        System.out.println("Linked hash map size = " + lhm.size());
        System.out.println("Linked hash map contains: " + lhm);
        lhm.remove("yes");  
        System.out.println(lhm.containsKey("yes"));  
        System.out.println("Linked hash map size = " + lhm.size());
        System.out.println("Linked hash map contains: " + lhm);
        // Collection-views, results differ dependant upon Map implementation
         System.out.println(lhm.entrySet());
        System.out.println(lhm.keySet());
        System.out.println(lhm.values());
        // Use Map static nested class Entry to get a reference and change all values
        for (Map.Entry<String, String> map : lhm.entrySet()) {
            map.setValue("aaa");
        }
        System.out.println(lhm.entrySet());
    }
}

Save, compile and run the LinkedHashMapClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our LinkedHashMapClass class and using several of the methods inherited from the HashMap<K,V> class.

A thing of note here is how we use the Map.Entry static nested class of Map to get a reference to each map entry and change the values; the only way to obtain a reference to a map entry is from the iterator of this collection-view

Hashtables Top

A Hashtable is an unordered and unsorted implementation of the Map<K,V> interface that doesn't allow null values or a null key. This type of map makes no guarantees over iteration order and in particular no guarantees that the iteration order will remain constant over time. Hashtable allows random access to elements and is similar to HashMap apart from being synchronized. In fact Hashtable is one of the two original collections shipped with Java, the other being Vector.

There is no sound reason since the introduction of the HashMap<K,V> class to use the Hashtable<K,V> class. If you need an Hashtable to be synchronized this can be achieved using methods of the Collections class without all the overheads of using Hashtable<K,V> synchronized methods.

Hashtable<K,V> Method Overview Top

The table below shows the declarations of all the methods for the Hashtable<K,V> class implemented from the Map<K,V> interface used in the code example below:

Usage for the the above methods is shown in the example below. For more information on the other methods in the Hashtable<K,V> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the Hashtable<K,V> class which you can find by scrolling down the lower left pane and clicking on Hashtable.

Hashtable<K,V> Example Top

Lets write a simple class that creates a Hashtable and uses some of the methods from the Hashtable<K,V> class:

package info.java8;
/*
  Simple class to create a Hashtable and use several methods from the class 
*/
import java.util.*; // Import the java.util package

public class HashtableClass {

    public static void main (String[] args) {
        Map<String, String> ht = new Hashtable<String, String>();
        ht.put("hello", "goodbye");
        ht.put("goodbye", "hello");
        ht.put("yes", "hello");
        ht.put("no", "goodbye");
        System.out.println("hash table size = " + ht.size());
        System.out.println("hash table contains: " + ht);
        ht.remove("yes");  
        System.out.println(ht.containsKey("yes"));  
        System.out.println("hash table size = " + ht.size());
        System.out.println("hash table contains: " + ht);
        // Collection-views, results differ dependant upon Map implementation
         System.out.println(ht.entrySet());
        System.out.println(ht.keySet());
        System.out.println(ht.values());
        // Use Map static nested class Entry to get a reference and change all values
        for (Map.Entry<String, String> map : ht.entrySet()) {
            map.setValue("aaa");
        }
        System.out.println(ht.entrySet());
    }
}

Save, compile and run the HashtableClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our HashtableClass class and using several of the methods contained within the Hashtable<K,V> class.

A thing of note here is how we use the Map.Entry static nested class of Map to get a reference to each map entry and change the values; the only way to obtain a reference to a map entry is from the iterator of this collection-view

TreeMaps Top

A TreeMap is a sorted implementation of the Map<K,V> interface. This type of map guarantees that the map will be sorted in ascending order, according to the natural order of the elements contained within it, or by a custom comparator/comparable, dependant upon the type of constructor used on creation.

The ordering maintained by a map whether through natural order of the elements contained within it or by a custom comparator/comparable, must be consistent with the equals() method override rules of the Object class if it is to correctly implement the Map<K,V> interface and obey the rules of equality.

This type of map is a good choice when you want a sorted collection with no duplicates with the option of custom sorting.

TreeMap<K,V> Method Overview Top

The table below shows the declarations of all the methods for the TreeMap<K,V> class implemented from the Map<K,V> and SortedMap<K,V> interfaces:

We will go use the first seven methods in in the example below. For more information on the other methods in the TreeMap<K,V> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the TreeMap<K,V> class which you can find by scrolling down the lower left pane and clicking on TreeMap.

TreeMap<K,V> Example Top

Lets write a simple class that creates a TreeMap and uses some of the methods from the TreeMap<K,V> class:

package info.java8;
/*
  Simple class to create a TreeMap and use several methods fron from the class 
*/
import java.util.*; // Import the java.util package

public class TreeMapClass {

    public static void main (String[] args) {
        Map<Object, Object> tm = new TreeMap<Object, Object>();
        tm.put("hello", "goodbye");
        tm.put("goodbye", "hello");
        tm.put("yes", "hello");
        tm.put("no", "goodbye");
        tm.put(2, "goodbye");  // Key autoboxed to Integer
        System.out.println("tree map size = " + tm.size());
        System.out.println("tree map contains: " + tm);
        tm.remove("yes");  
        System.out.println(tm.containsKey("yes"));  
        System.out.println("tree map size = " + tm.size());
        System.out.println("tree map contains: " + tm);
        // Collection-views, results differ dependant upon Map implementation
         System.out.println(tm.entrySet());
        System.out.println(tm.keySet());
        System.out.println(tm.values());
        // Use Map static nested class Entry to get a reference and change all values
        for (Map.Entry<Object, Object> map : tm.entrySet()) {
            map.setValue("aaa");
        }
        System.out.println(tm.entrySet());
    }
}

Save, compile and run the TreeMapClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our TreeSetClass class. We get a ClassCastException and the reason for this is that if a collection is to be sorted, then the objects within it need to be mutually comparable and the String and Integer objects are not. We will talk more about sorting collection, comparables, comparators and natural order in much more detail later in the Sorting Collections lesson. For now remove the put() to the TreeMap for the Integer object and save, compile and run the TreeMapClass class again.

The above screenshot shows the results of creating and running our TreeMapClass class and it now works.

Lesson 6 Complete

In this lesson we looked at Maps.

What's Next?

In the next lessons we look at the Utilities hierarchy and examine the java.util.Arrays and java.util.Collections classes that contains a lot of static utility methods we can use with our collections.

• <<    Queues
• Utilities     >>