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1. Introduction
A HashMap is a widely used data structure that implements the map interface, mapping keys to values. It allows us to store and retrieve objects based on a key. In essence, it's an associative array. Under the hood, a HashMap uses an array and a hashing technique to store the data.
2. Implementation Steps
1. Define an inner Entry class that will represent the individual key-value pairs.
2. Implement a method to compute a hash for the keys.
3. Implement core methods like put, get, remove, and size.
4. Handle collisions using linked lists.
3. Implementation in Kotlin Programming
class HashMap<K, V> {
private data class Entry<K, V>(val key: K, var value: V) {
var next: Entry<K, V>? = null
}
private val capacity = 16
private val table = arrayOfNulls<Entry<K, V>>(capacity)
private var size = 0
// Computes the index in the array
private fun hash(key: K): Int = (key.hashCode() and Int.MAX_VALUE) % capacity
// Inserts or updates the key-value pair
fun put(key: K, value: V): V? {
val index = hash(key)
var existing = table[index]
while (existing != null) {
if (existing.key == key) {
val oldValue = existing.value
existing.value = value
return oldValue
}
existing = existing.next
}
val entry = Entry(key, value)
entry.next = table[index]
table[index] = entry
size++
return null
}
// Fetches the value by key
fun get(key: K): V? {
var current = table[hash(key)]
while (current != null) {
if (current.key == key) {
return current.value
}
current = current.next
}
return null
}
// Removes the key-value pair by key
fun remove(key: K): V? {
val index = hash(key)
var prev: Entry<K, V>? = null
var current = table[index]
while (current != null) {
if (current.key == key) {
if (prev == null) {
table[index] = current.next
} else {
prev.next = current.next
}
size--
return current.value
}
prev = current
current = current.next
}
return null
}
// Returns the size of the map
fun size(): Int = size
}
fun main() {
val hashMap = HashMap<String, Int>()
hashMap.put("One", 1)
hashMap.put("Two", 2)
hashMap.put("Three", 3)
println("Size of map: ${hashMap.size()}")
println("Value of 'Two': ${hashMap.get("Two")}")
hashMap.remove("Two")
println("Size after removal: ${hashMap.size()}")
println("Value of 'Two' after removal: ${hashMap.get("Two")}")
}
Output:
Size of map: 3 Value of 'Two': 2 Size after removal: 2 Value of 'Two' after removal: null
Explanation:
1. Entry class: Represents the individual key-value pair and contains a reference to the next Entry. This reference is useful for handling collisions.
2. hash method: Computes an index in the array for a given key.
3. put method: Inserts a new key-value pair or updates the value if the key already exists.
4. get method: Retrieves the value associated with a given key.
5. remove method: Removes the key-value pair based on the given key.
6. size method: Returns the current number of entries in the hash map.
7. The main function showcases basic operations such as inserting key-value pairs, fetching a value, and removing a key-value pair.
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