Java实现哈希表的基本功能

Java实现哈希表的基本功能

一、哈希表头插法放入元素

/**

* user：ypc；

* date：2021-05-20;

* time: 11:05;

*/

public class HashBuck {

class Node {

public int key;

int value;

Node next;

Node(int key, int value) {

this.key = key;

this.value = value;

}

}

public int usedSize;

public Node[] array;

HashBuck() {

this.array = new Node[8];

this.usedSize = 0;

}

//JDk1.7及之前是头插法

public void put1(int key, int value) {

int index = key % this.array.length;

Node node = new Node(key, value);

Node cur = array[index];

while (cur != null) {

if (cur.key == key) {

cur.value = value;

return;

}

cur = cur.next;

}

node.next = array[index];

array[index] = node;

this.usedSize++;

if (loadFactor() > 0.75) {

resize1();

}

}

public double loadFactor() {

return this.usedSize / this.array.length * 1.0;

}

}

二、哈希表尾插法放入元素

//JDK1.8是尾插法

public Node findLast(Node head) {

if (head == null) return head;

Node cur = head;

while (cur.next != null) {

cur = cur.next;

}

return cur;

}

public void put2(int key, int value) {

int index = key % this.array.length;

Node node = new Node(key, value);

Node cur = array[index];

while (cur != null) {

if (cur.key == key) {

cur.value = value;

return;

}

cur = cur.next;

}

Node last = findLast(array[index]);

if (last == null) {

array[index] = node;

this.usedSize++;

return;

}

last.next = node;

this.usedSize++;

if (loadFactor() > 0.75) {

resize2();

}

}

三、哈希表头插、尾插扩容

public void resize1() {

Node[] newArray = new Node[this.array.length * 2];

for (int i = 0; i < this.array.length; i++) {

Node cur = array[i];

while (cur != null) {

int index = cur.key % newArray.length;

Node curNext = cur.next;

cur.next = newArray[index];

newArray[index] = cur;

cur = curNext;

}

}

this.array = newArray;

}

//resize尾插

public void resize2() {

Node[] newArray = new Node[this.array.length * 2];

for (int i = 0; i < this.array.length; i++) {

Node cur = array[i];

while (cur != nulXpAZdhlkvl) {

int index = cur.key % newArray.length;

Node curNext = cur.next;

Node last = findLast(newArray[index]);

if (last == null) {

newArray[index] = cur;

break;

}

last.next = cur;

cur = curNext;

}

}

this.array = newArray;

}

public Node findLast(Node head) {

if (head == null) return head;

Node cur = head;

while (cur.next != null) {

cur = cur.next;

}

return cur;

}

四、找到key对应的value

public int get(int key) {

int index = key % this.array.length;

Node cur = this.array[index];

while (cur != null) {

if (cur.key == key) {

return cur.value;

}

cur = cur.next;

}

return -1;

}

五、运行结果

class HashBuckTest {

public static void main(String[] args) {

HashBuck hashBuck = new HashBuck();

//头插

hashBuck.put1(9,451);

hashBuck.put1(17,455);

//尾插

//hashBuck.put2(9,451);

//hashBuck.put2(17,455);

hashBuck.put1(2,45);

hashBuck.put1(3,14);

hashBuck.put1(4,52);

hashBuck.put1(4,89);

System.out.println(hashBuck.get(1));

System.out.println("+=================");

}

}

头插

尾插

扩容

class HashBuckTest {

public static void main(String[] args) {

HashBuck hashBuck = new HashBuck();

// hashBuck.put1(9, 451);

// hashBuck.put1(17, 455);

hashBuck.put1(1, 589);

hashBuck.put1(2, 45);

hashBuck.put1(3, 14);

hashBuck.put1(4, 52);

hashBuck.put1(4, 1);

hashBuck.put1(6, 829);

hashBuck.put1(7, 72);

hashBuck.put1(8, 8279);

hashBuck.put2(9,451);

hashBuck.put2(15,455);

hashBuck.put2(31,451);

System.out.println(hashBuck.get(7));

System.out.println(hashBuck.get(4));

System.out.println(hashBuck.get(15));

System.out.println(hashBuck.get(31));

}

}

get

六、哈希表的泛型实现

public class HashBuck2 {

static class Node {

public K key;

public V val;

public Node next;

public Node(K key, V val) {

this.key = key;

this.val = val;

}

}

public Node[] array;

public int usedSize;

public HashBuck2() {

this.array = (Node[]) new Node[8];

}

public void put(K key, V val) {

int hash = key.hashCode();

int index = hash % array.length;

Node cur = array[index];

while (cur != null) {

if (cur.key.equals(key)) {

cur.val = val;

return;

}

cur = cur.next;

}

Node node = new Node<>(key, val);

node.next = array[index];

array[index] = node;

this.usedSize++;

if (loadFactor() > 0.75) {

resize();

}

}

public V get(K key) {

int hash = key.hashCode();

int index = hash % array.length;

Node cur = array[index];

XpAZdhlkv while (cur != null) {

if (cur.key.equals(key)) {

return cur.val;

}

cur = cur.next;

}

return null;

}

public void resize() {

Node[] newArray = new Node[this.array.length * 2];

for (int i = 0; i < this.array.length; i++) {

Node cur = array[i];

while (cur != null) {

int hash = cur.key.hashCode();

int index = hash % array.length;

Node curNext = cur.next;

cur.next = newArray[index];

newArray[index] = cur;

cur = curNext;

}

}

this.array = newArray;

}

public double loadFactor() {

return this.usedSize / this.array.length * 1.0;

}

}

/**

* user：ypc；

* date：2021-05-20;

* time: 15:25;

*/

class Student{

public int id;

Student(int id){

this.id = id;

}

@Override

public boolean equals(Object o) {

if (this == o) return true;

if (o == null || getClass() != o.getClass()) return false;

Student student = (Student) o;

return id == student.id;

}

@Override

public int hashCode() {

return Objects.hash(id);

}

}

class HashBuck2Test{

public static void main(String[] args) {

HashBuck2 hashBuck2 = new HashBuck2<>();

Student s1 = new Student(10001);

Student s2 = new Student(10001);

Student s3 = new Student(10003);

hashBuck2.put(s1,89);

hashBuck2.put(s1,60);

hashBuck2.put(s2,94);

hashBuck2.put(s3,100);

System.out.println(hashBuck2.get(s1));

System.out.println(hashBuck2.get(s2));

}

}

注意：

要用自定http://义类作为 HashMap 的 key 或者 HashSet 的值，必须覆写 hashCode 和 equals 方 法，而且要做到 equals 相等的对象，hashCode 一定是一致的。

比如Student s1 和 s2 的id一样，得到的却是不同的value，所以要覆写hashCode 和 equals 方 法，如果不覆写，则使用的是Object类的hashCode 和 equals 方 法，比较的是地址。

重写之后

七、为什么JDK1.7及之前使用头插法而JDK1.8使用尾插法

hashmap用数组+链表。数组是固定长度，链表太长就需要扩充数组长度进行rehash减少链表长度。如果两个线程同时触发扩容，在移动节点时会导致一个链表中的2个节点相互引用，从而生成环链表

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