Map & HashMap in Java

Store key-value pairs using HashMap, LinkedHashMap (insertion order), and TreeMap (sorted order), and learn common map operations and iteration patterns.

Map Basics

1. Creating an Empty Map

Use `HashMap` for a general-purpose key-value store with no ordering guarantee.

2. Setting a Key-Value Pair

`put(key, value)` adds or replaces an entry. If the key already exists, the old value is overwritten.

3. Checking if a Key Exists

`containsKey()` returns true if the key is present. `containsValue()` checks for a value.

4. Accessing Value on a Specific Key

`get(key)` returns the value, or `null` if not found. `getOrDefault(key, fallback)` returns a fallback instead of null.

5. Updating Value on a Specific Key

`put()` overwrites the existing value. `replace()` only updates if the key already exists.

6. Deleting a Key-Value Pair

`remove(key)` deletes the entry and returns the removed value, or `null` if not found.

7. Clearing the Map

`clear()` removes all entries. The map object still exists and can be reused.

8. Getting Map Size

`size()` returns the number of key-value pairs. `isEmpty()` returns true if the map has no entries.

Map Keys & Values

1. Getting Keys List

`keySet()` returns a `Set` of all keys. Convert to a `List` for indexed access.

2. Getting Values List

`values()` returns a `Collection` of all values. Duplicate values are included.

Traversing a Map

1. Traversing using for-each (entrySet)

`entrySet()` gives a set of `Map.Entry<K,V>` objects. Use `.getKey()` and `.getValue()` on each entry.

2. Traversing using forEach (lambda)

`forEach(BiConsumer)` is the cleanest way to iterate when you just need key and value.

3. Traversing Keys Only

Loop over `keySet()` when you only need keys, or use the key to look up the value.

Merging Maps

1. Merging Two Maps

`putAll()` copies all entries from one map into another. Duplicate keys are overwritten by the second map.

2. Merging with Conflict Strategy

`merge(key, value, remappingFunction)` lets you decide what to do when a key already exists — e.g. sum the values.

Map Variants

1. LinkedHashMap (Insertion Order)

`LinkedHashMap` preserves the order in which entries were inserted — unlike `HashMap` which has no guaranteed order.

2. TreeMap (Sorted Order)

`TreeMap` keeps entries sorted by key in natural (alphabetical / numerical) order.

3. Immutable Map (Map.of)

`Map.of()` creates a read-only map. Any attempt to add, update, or remove entries throws `UnsupportedOperationException`.