15.5. Map Interface and Its Implementations

A Map stores key-value pairs. Keys must be unique.

A Map is a collection designed to hold pairs of keys and values. Each key in the Map must be unique, meaning no two entries can have the same key. This uniqueness allows you to quickly access a specific value using its corresponding key. For example, if you wanted to look up a person's contact number using their name, the name would be the key and the number would be the value stored in the Map.

Think of a Map like a dictionary. The word you look up is the key, and the definition of that word is the value. Just like each word in a dictionary is unique, each key in a Map must also be unique.

• HashMap o Unordered, allows null key and values. • LinkedHashMap o Maintains insertion order. • TreeMap o Sorted by keys. • Hashtable o Legacy synchronized implementation.

Java provides several implementations of the Map interface, each with its own characteristics:

1. HashMap: This is the most commonly used implementation. It stores entries in an unordered fashion, which allows for very fast retrieval of data. It permits null keys and values, making it flexible in terms of the types of data you can store.
2. LinkedHashMap: This implementation keeps track of the order in which entries are added, so when you iterate through it, the output is in the order items were inserted.
3. TreeMap: This type sorts the keys in their natural order or according to a specified comparator. It provides a way to store key-value pairs while ensuring the keys are sorted.
4. Hashtable: An older implementation that is synchronized, which makes it thread-safe but can be slower than the others. It doesn't allow null keys or values.

Consider a library system:

• A HashMap is like an index of books where the books can be retrieved quickly by their unique identifier, irrespective of the order they were added.
• A LinkedHashMap is like a bookshelf where books are arranged based on the order you placed them.
• A TreeMap is similar to a categorized system where books are sorted by genre or title.
• Lastly, a Hashtable would be an archived version of the catalog that could be old and may have limitations on how it stores entries.

• put(K key, V value) • get(Object key) • remove(Object key) • keySet(), values(), entrySet()

Maps come with a set of common methods that allow you to manipulate the data:

1. put(K key, V value): This method adds a key-value pair to the map. If the key already exists, it updates the value associated with that key.
2. get(Object key): This retrieves the value associated with a given key. If the key doesn't exist, it returns null.
3. remove(Object key): This method removes the entry for the specified key from the map, if it exists.
4. keySet(): This returns a Set of all the keys in the map.
5. values(): This returns a Collection of all the values in the map.
6. entrySet(): This provides a Set view of the key-value pairs.

Think of a Map like a storage box containing index cards:

• The put method is like adding a new index card with a name (key) and a phone number (value).
• The get method is like looking up a name on an index card to find the corresponding phone number.
• The remove method is like taking a card out of the box when it's no longer needed.
• The keySet method lets you view all the names on the cards, while values shows all the phone numbers, and entrySet gives you a view of the whole box containing pairs of names and numbers.

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