Map, thesarfo

Key points about the Map interface

Map is part of the Java Collections Framework, but it doesn’t inherit the Collection interface.
A Map cannot contain duplicate keys — each key maps to at most one value. It models the mathematical function abstraction.
Each key-value pair of the map is stored as a Map.Entry object — an inner interface of Map.
The Java platform has three general-purpose Map implementations: HashMap, TreeMap, and LinkedHashMap.
Order of elements is implementation-dependent: HashMap doesn’t maintain any order, LinkedHashMap maintains insertion order, and TreeMap orders elements according to the supplied Comparator.
Map provides three methods that let its contents be viewed as a set of keys (keySet()), a collection of values (values()), or a set of key-value mappings (entrySet()).

HashMap implementation of a Map

public class CreateHashMapExample {
    public static void main(String[] args) {
        // Creating a HashMap
        Map<String, Integer> numberMapping = new HashMap<>();

        // Adding key-value pairs to a HashMap
        numberMapping.put("One", 1);
        numberMapping.put("Two", 2);
        numberMapping.put("Three", 3);

        // Add a new key-value pair only if the key does not exist, or is mapped to null
        numberMapping.putIfAbsent("Four", 4);

        // Access element by its key
        numberMapping.get("One");

        // Remove element by its key
        numberMapping.remove("Two");

        // Getting the size of the map
        int size = numberMapping.size();

        System.out.println(numberMapping);
    }
}

Looping through a HashMap

There are different ways to loop through a HashMap:

Using entrySet() and a for-each loop
Using keySet() and a for-each loop
Using values() and a for-each loop
Using an Iterator
Using Java 8’s forEach method

public class Main {
    public static void main(String[] args) {
        // Creating a HashMap
        HashMap<String, String> map = new HashMap<>();
        map.put("Apple", "Red");
        map.put("Orange", "Orange");
        map.put("Banana", "Yellow");

        // Using entrySet and a for-each loop
        System.out.println("Using entrySet and a for-each loop:");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
        }
        System.out.println();

        // Using keySet and a for-each loop
        System.out.println("Using keySet and a for-each loop:");
        for (String key : map.keySet()) {
            System.out.println("Key = " + key + ", Value = " + map.get(key));
        }
        System.out.println();

        // Using values and a for-each loop
        System.out.println("Using values and a for-each loop:");
        for (String value : map.values()) {
            System.out.println("Value = " + value);
        }
        System.out.println();

        // Using an Iterator
        System.out.println("Using an Iterator:");
        Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();
        while (iterator.hasNext()) {
            Map.Entry<String, String> entry = iterator.next();
            System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
        }
        System.out.println();

        // Using Java 8's forEach method
        System.out.println("Using Java 8's forEach method:");
        map.forEach((key, value) -> System.out.println("Key = " + key + ", Value = " + value));
    }
}

Output of the loops

Using entrySet and a for-each loop:
Key = Apple, Value = Red
Key = Orange, Value = Orange
Key = Banana, Value = Yellow

Using keySet and a for-each loop:
Key = Apple, Value = Red
Key = Orange, Value = Orange
Key = Banana, Value = Yellow

Using values and a for-each loop:
Value = Red
Value = Orange
Value = Yellow

Using an Iterator:
Key = Apple, Value = Red
Key = Orange, Value = Orange
Key = Banana, Value = Yellow

Using Java 8's forEach method:
Key = Apple, Value = Red
Key = Orange, Value = Orange
Key = Banana, Value = Yellow

Methods available on the Map interface:

Map interface methods