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Enhance docs, fix & add tests in `GenericHashMapUsingArrayL… (#5973)

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This commit is contained in:
Hardik Pawar
2024-10-26 22:15:16 +05:30
committed by GitHub
parent f5bc2c807d
commit bc6ea1ec87
2 changed files with 128 additions and 10 deletions
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95
src/main/java/com/thealgorithms/datastructures/hashmap/hashing/GenericHashMapUsingArrayList.java
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@ -3,12 +3,34 @@ package com.thealgorithms.datastructures.hashmap.hashing;
import java.util.ArrayList;
import java.util.LinkedList;
/**
* A generic implementation of a hash map using an array list of linked lists for collision resolution.
* This class allows storage of key-value pairs with average-case constant time complexity for insertion,
* deletion, and retrieval operations.
*
* <p>
* The hash map uses separate chaining to handle collisions. Each bucket in the hash map is represented
* by a linked list that holds nodes containing key-value pairs. When multiple keys hash to the same index,
* they are stored in the same linked list.
* </p>
*
* <p>
* The hash map automatically resizes itself when the load factor exceeds 0.5. The load factor is defined
* as the ratio of the number of entries to the number of buckets. When resizing occurs, all existing entries
* are rehashed and inserted into the new buckets.
* </p>
*
* @param <K> the type of keys maintained by this hash map
* @param <V> the type of mapped values
*/
public class GenericHashMapUsingArrayList<K, V> {
ArrayList<LinkedList<Node>> buckets;
private float lf = 0.5f;
private int size;
private ArrayList<LinkedList<Node>> buckets; // Array list of buckets (linked lists)
private int size; // Number of key-value pairs in the hash map
/**
* Constructs a new empty hash map with an initial capacity of 10 buckets.
*/
public GenericHashMapUsingArrayList() {
buckets = new ArrayList<>();
for (int i = 0; i < 10; i++) {
@ -17,6 +39,13 @@ public class GenericHashMapUsingArrayList<K, V> {
size = 0;
}
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old value is replaced.
*
* @param key the key with which the specified value is to be associated
* @param value the value to be associated with the specified key
*/
public void put(K key, V value) {
int hash = Math.abs(key.hashCode() % buckets.size());
LinkedList<Node> nodes = buckets.get(hash);
@ -31,25 +60,36 @@ public class GenericHashMapUsingArrayList<K, V> {
nodes.add(new Node(key, value));
size++;
if ((float) size / buckets.size() > lf) {
// Load factor threshold for resizing
float loadFactorThreshold = 0.5f;
if ((float) size / buckets.size() > loadFactorThreshold) {
reHash();
}
}
/**
* Resizes the hash map by doubling the number of buckets and rehashing existing entries.
*/
private void reHash() {
ArrayList<LinkedList<Node>> old = buckets;
ArrayList<LinkedList<Node>> oldBuckets = buckets;
buckets = new ArrayList<>();
size = 0;
for (int i = 0; i < old.size() * 2; i++) {
for (int i = 0; i < oldBuckets.size() * 2; i++) {
buckets.add(new LinkedList<>());
}
for (LinkedList<Node> nodes : buckets) {
for (LinkedList<Node> nodes : oldBuckets) {
for (Node node : nodes) {
put(node.key, node.val);
}
}
}
/**
* Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
*
* @param key the key whose associated value is to be returned
* @return the value associated with the specified key, or null if no mapping exists
*/
public V get(K key) {
int hash = Math.abs(key.hashCode() % buckets.size());
LinkedList<Node> nodes = buckets.get(hash);
@ -61,6 +101,11 @@ public class GenericHashMapUsingArrayList<K, V> {
return null;
}
/**
* Removes the mapping for the specified key from this map if present.
*
* @param key the key whose mapping is to be removed from the map
*/
public void remove(K key) {
int hash = Math.abs(key.hashCode() % buckets.size());
LinkedList<Node> nodes = buckets.get(hash);
@ -72,18 +117,36 @@ public class GenericHashMapUsingArrayList<K, V> {
break;
}
}
nodes.remove(target);
size--;
if (target != null) {
nodes.remove(target);
size--;
}
}
/**
* Returns true if this map contains a mapping for the specified key.
*
* @param key the key whose presence in this map is to be tested
* @return true if this map contains a mapping for the specified key
*/
public boolean containsKey(K key) {
return get(key) != null;
}
/**
* Returns the number of key-value pairs in this map.
*
* @return the number of key-value pairs
*/
public int size() {
return this.size;
}
/**
* Returns a string representation of the map, containing all key-value pairs.
*
* @return a string representation of the map
*/
@Override
public String toString() {
StringBuilder builder = new StringBuilder();
@ -96,15 +159,27 @@ public class GenericHashMapUsingArrayList<K, V> {
builder.append(", ");
}
}
// Remove trailing comma and space if there are any elements
if (builder.length() > 1) {
builder.setLength(builder.length() - 2);
}
builder.append("}");
return builder.toString();
}
/**
* A private inner class representing a key-value pair (node) in the hash map.
*/
private class Node {
K key;
V val;
/**
* Constructs a new Node with the specified key and value.
*
* @param key the key of the key-value pair
* @param val the value of the key-value pair
*/
Node(K key, V val) {
this.key = key;
this.val = val;

43
src/test/java/com/thealgorithms/datastructures/hashmap/hashing/GenericHashMapUsingArrayListTest.java
View File

@ -50,4 +50,47 @@ class GenericHashMapUsingArrayListTest {
assertEquals("Washington DC", map.get(101));
assertTrue(map.containsKey(46));
}
@Test
void testRemoveNonExistentKey() {
GenericHashMapUsingArrayList<String, String> map = new GenericHashMapUsingArrayList<>();
map.put("USA", "Washington DC");
map.remove("Nepal"); // Attempting to remove a non-existent key
assertEquals(1, map.size()); // Size should remain the same
}
@Test
void testRehashing() {
GenericHashMapUsingArrayList<String, String> map = new GenericHashMapUsingArrayList<>();
for (int i = 0; i < 20; i++) {
map.put("Key" + i, "Value" + i);
}
assertEquals(20, map.size()); // Ensure all items were added
assertEquals("Value5", map.get("Key5")); // Check retrieval after rehash
}
@Test
void testUpdateValueForExistingKey() {
GenericHashMapUsingArrayList<String, String> map = new GenericHashMapUsingArrayList<>();
map.put("USA", "Washington DC");
map.put("USA", "New Washington DC"); // Updating value for existing key
assertEquals("New Washington DC", map.get("USA"));
}
@Test
void testToStringMethod() {
GenericHashMapUsingArrayList<String, String> map = new GenericHashMapUsingArrayList<>();
map.put("USA", "Washington DC");
map.put("Nepal", "Kathmandu");
String expected = "{USA : Washington DC, Nepal : Kathmandu}";
assertEquals(expected, map.toString());
}
@Test
void testContainsKey() {
GenericHashMapUsingArrayList<String, String> map = new GenericHashMapUsingArrayList<>();
map.put("USA", "Washington DC");
assertTrue(map.containsKey("USA"));
assertFalse(map.containsKey("Nepal"));
}
}