尽管这是一个非常老的问题，但我认为我应该为初学者提供一个易于理解的答案。
HashMap 简单解决方案的自定义实现：
class HashMapCustom<K, V> {
private Entry<K, V>[] table; //Array of Entry.
private int capacity = 4; //Initial capacity of HashMap
static class Entry<K, V> {
K key;
V value;
Entry<K, V> next;
public Entry(K key, V value, Entry<K, V> next) {
this.key = key;
this.value = value;
this.next = next;
}
}
@SuppressWarnings("unchecked")
public HashMapCustom() {
table = new Entry[capacity];
}
/**
* Method allows you put key-value pair in HashMapCustom.
* If the map already contains a mapping for the key, the old value is replaced.
* Note: method does not allows you to put null key though it allows null values.
* Implementation allows you to put custom objects as a key as well.
* Key Features: implementation provides you with following features:-
* >provide complete functionality how to override equals method.
* >provide complete functionality how to override hashCode method.
*
* @param newKey
* @param data
*/
public void put(K newKey, V data) {
if (newKey == null)
return; //does not allow to store null.
//calculate hash of key.
int hash = hash(newKey);
//create new entry.
Entry<K, V> newEntry = new Entry<K, V>(newKey, data, null);
//if table location does not contain any entry, store entry there.
if (table[hash] == null) {
table[hash] = newEntry;
} else {
Entry<K, V> previous = null;
Entry<K, V> current = table[hash];
while (current != null) { //we have reached last entry of bucket.
if (current.key.equals(newKey)) {
if (previous == null) { //node has to be insert on first of bucket.
newEntry.next = current.next;
table[hash] = newEntry;
return;
} else {
newEntry.next = current.next;
previous.next = newEntry;
return;
}
}
previous = current;
current = current.next;
}
previous.next = newEntry;
}
}
/**
* Method returns value corresponding to key.
*
* @param key
*/
public V get(K key) {
int hash = hash(key);
if (table[hash] == null) {
return null;
} else {
Entry<K, V> temp = table[hash];
while (temp != null) {
if (temp.key.equals(key))
return temp.value;
temp = temp.next; //return value corresponding to key.
}
return null; //returns null if key is not found.
}
}
/**
* Method removes key-value pair from HashMapCustom.
*
* @param key
*/
public boolean remove(K deleteKey) {
int hash = hash(deleteKey);
if (table[hash] == null) {
return false;
} else {
Entry<K, V> previous = null;
Entry<K, V> current = table[hash];
while (current != null) { //we have reached last entry node of bucket.
if (current.key.equals(deleteKey)) {
if (previous == null) { //delete first entry node.
table[hash] = table[hash].next;
return true;
} else {
previous.next = current.next;
return true;
}
}
previous = current;
current = current.next;
}
return false;
}
}
/**
* Method displays all key-value pairs present in HashMapCustom.,
* insertion order is not guaranteed, for maintaining insertion order
* refer LinkedHashMapCustom.
*
* @param key
*/
public void display() {
for (int i = 0; i < capacity; i++) {
if (table[i] != null) {
Entry<K, V> entry = table[i];
while (entry != null) {
System.out.print("{" + entry.key + "=" + entry.value + "}" + " ");
entry = entry.next;
}
}
}
}
/**
* Method implements hashing functionality, which helps in finding the appropriate
* bucket location to store our data.
* This is very important method, as performance of HashMapCustom is very much
* dependent on this method's implementation.
*
* @param key
*/
private int hash(K key) {
return Math.abs(key.hashCode()) % capacity;
}
}