目录一、哈希表头插法放入元素二、哈希表尾插法放入元素三、哈希表头插、尾插扩容四、找到key对应的value五、运行结果六、哈希表的泛型实现七、为什么JDK1.7及之前使用头插法而JDK1.8使用尾插法
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 != null) { 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<K, V> { static class Node<K, V> { public K key; public V val; public Node<K, V> next; public Node(K key, V val) { this.key = key; this.val = val; } } public Node<K, V>[] array; public int usedSize; public HashBuck2() { this.array = (Node<K, V>[]) new Node[8]; } public void put(K key, V val) { int hash = key.hashCode(); int index = hash % array.length; Node<K, V> cur = array[index]; while (cur != null) { if (cur.key.equals(key)) { cur.val = val; return; } cur = cur.next; } Node<K, V> 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<K, V> cur = array[index]; 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<K,V> cur = array[i]; while (cur != null) { int hash = cur.key.hashCode(); int index = hash % array.length; Node <K,V>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<Student,Integer> 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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想象困难做出的反应，不是逃避或绕开它们，
