Java Maps

Java Map Interface

Library

java.util.Map

java.util.TreeMap

Background

Maps are part of the Collections Framework but technically Maps are not collections
       because they do not implement the Collection interface.

Maps store key / value pairs and all keys must be unique.

All maps implement the Map interface and share a common functionality that allows
       adding a key/value pair or access the value given they key.

Map provides two collection-views to deal with the keys and values separately.

A set of the entries is obtained by calling entrySet(). It returns a Set collection.
       The Set that contains the entries, which are held in an object of type Map.Entry.

The key can be obtained by calling getKey().

The value can be obtained by calling getValue().

The Set of the keys can be obtained by calling keySet().

The Collection of the values can be obtained by calling values().

Methods

# Map is a generic interface
# interface Map<K, V>
public void clear()
public boolean containsKey(Object k)
public boolean containsValue(Object v)
public Set<Map.Entry<K, V>> entrySet()
public V get(Object k)
public boolean isEmpty()
public Set<K> KeySet()
public V put(K k, V v)
public void putAll (Map<? extends K, ? extends V> m)
public v remove(Object k)
public int size()
public Collection<V> values()

Java Map example code

import java.io.*;
import java.util.*;

class MyMaps
{
    public static void main(String args[]) {
        // Create a tree map
        TreeMap<String, Integer> myNums = new TreeMap<String, Integer>();
        myNums.put ("this", 99);
        myNums.put ("one", 1);
        myNums.put ("two", 2);

        System.out.println ("the map contains " + myNums.size() + " items.");

        // Create a set of the entry
        Set<Map.Entry<String, Integer>> set = myNums.entrySet();
        for (Map.Entry<String, Integer> my : set) {
            System.out.println (my.getKey());
            System.out.println (my.getValue());
        }

        // A new map to merge with myNums
        TreeMap<String, Integer> myNums2 = new TreeMap<String, Integer>();
        myNums2.put ("three", 3);
        myNums2.put ("four", 4);
        myNums.putAll (myNums2);

        // To show all entries after merging
        set = myNums.entrySet();
        System.out.println (myNums.size());

        for (Map.Entry<String, Integer> my : set) {
            System.out.println (my.getKey());
            System.out.println (my.getValue());
        }

        // To search a key
        if (myNums.containsKey("six")
             System.out.println (myNums.get("six"));
        // Search for a value
        if (my.Nums.containsValue (1))
            System.out.println ("yes");

        // To remove an entry
        if (myNums.remove("six") != null)
            System.out.println ("six is removed!");
        else
            System.out.println (my.getValue());

        // Show entries after removal
        Set<String> keys = myNums.keySet();
        for (String str : keys)
            System.out.println (str);
        // Display the value set after removal
        Collection<Integer> values = myNums.values();
        for (Integer i : values)
            System.out.println (i);

        // Clear the map
        myNums.clear()
        if (! myNums.isEmpty())
            System.out.println ("The map is not empty! Check again!");        
    }
}