Java集合之LinkedHashSet類詳解

更新時間：2023年09月04日 08:53:09 作者：Neo丶

這篇文章主要介紹了Java集合之LinkedHashSet類詳解,LinkedHashSet 是 Java 中的一個集合類,它是 HashSet 的子類,并實現(xiàn)了 Set 接口,與 HashSet 不同的是,LinkedHashSet 保留了元素插入的順序,并且具有 HashSet 的快速查找特性,需要的朋友可以參考下

LinkedHashSet類

LinkedHashSet是HashSet的子類；

LinkedHashSet底層是一個LinkedHashMap，底層維護了一個數(shù)組+雙向鏈表；

LinkedHashSet根據(jù)元素的hashCode值來決定元素的存儲位置，同時使用鏈表維護元素的次序，這使得元素看起來是以插入順序保存的；

在LinkedHashSet中維護了一個hash表和雙向鏈表(LinkedHshSet有head和tail）；
每一個節(jié)點有pre和next屬性，這樣可以形成雙向鏈表；
在添加一個元素時，先求hash值，再求索引，確定該元素在hashtable的位置，然后將添加的元素加入到雙向鏈表（如果已經(jīng)存儲過相同元素，則不再添加，原則和HashSet相同）

tail.next = newElement; //簡單指定
newElement.pre = tail;
tail = newElement;? //重置tail，為添加下一個元素做準備

遍歷LinkedHashSet能確保插入循序和遍歷順序一致。

LinkedHashSet不允許添加重復(fù)元素。

底層源碼分析

測試代碼

package com.pero.set_;
import java.util.Iterator;
import java.util.LinkedHashSet;
/**
 * LinkedHashSet的底層運行機制
 *
 * @author Pero
 * @version 1.0
 */
@SuppressWarnings({"all"})
public class LinkedHashSetSource {
    public static void main(String[] args) {
        LinkedHashSet linkedHashSet = new LinkedHashSet();
        linkedHashSet.add(new String("AA"));
        linkedHashSet.add(456);
        linkedHashSet.add(456);
        linkedHashSet.add(new Employee("jake", 28));
        linkedHashSet.add(123);
        linkedHashSet.add("pero");
        Iterator iterator = linkedHashSet.iterator();
        while (iterator.hasNext()) {
            Object next = iterator.next();
            System.out.println(next);
        }
        for (Object o : linkedHashSet) {
            System.out.println(o);
        }
        //1.LinkedHashSet 加入順序和取出元素/數(shù)據(jù)的順序一致
        //2.LinkedHashSet 底層維護的是一個LinkedHashMap（是HashMap的子類）
        //3.LinkedHashSet 底層結(jié)構(gòu)（數(shù)組+雙向鏈表）
        //4.第一次添加元素時，直接將 數(shù)組table擴容到16個空間，table類型是HashMap$Node類，
        // 存放的節(jié)點類型不再是Node類型，而是LinkedHashMap$Entry類，并且是HashMap$Node類的子類或者實現(xiàn)類關(guān)系
        // 子類對象可以存放到父類的數(shù)組中（多態(tài)數(shù)組），否則節(jié)點無法存放到table數(shù)組
    }
}

源碼運行流程

1.執(zhí)行構(gòu)造器

public LinkedHashSet() {
        super(16, .75f, true);
    }

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }

    /**
     * Returns a power of two size for the given target capacity.
     */
    static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

2執(zhí)行add()方法，添加元素

public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;
            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        if (newThr == 0) {
            float ft = (float)newCap * loadFactor;
            newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                      (int)ft : Integer.MAX_VALUE);
        }
        threshold = newThr;
        @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
        LinkedHashMap.Entry<K,V> p =
            new LinkedHashMap.Entry<K,V>(hash, key, value, e);
        linkNodeLast(p);
        return p;
    }

static class Entry<K,V> extends HashMap.Node<K,V> {
        //該內(nèi)部類起著作為生成節(jié)點的作用，可以生成雙向鏈表
        //after指向當(dāng)前節(jié)點的下一個節(jié)點
        //before指向當(dāng)前節(jié)點的上一個節(jié)點
        Entry<K,V> before, after;
        Entry(int hash, K key, V value, Node<K,V> next) {
            super(hash, key, value, next);
        }
    }

private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
        LinkedHashMap.Entry<K,V> last = tail;
        tail = p;
        if (last == null)
            head = p;
        else {
            p.before = last;
            last.after = p;
        }
    }

練習(xí)代碼

package com.pero.set_;
import java.util.LinkedHashSet;
import java.util.Objects;
/**
 * Car類(屬性：name ，price)如果name和price都一樣，
 * 則認為是相同元素，不能添加到LinkedHashSet中
 *
 * @author Pero
 * @version 1.0
 */
@SuppressWarnings({"all"})
public class LinkedHashSetExercise {
    public static void main(String[] args) {
        LinkedHashSet linkedHashSet = new LinkedHashSet();
        linkedHashSet.add(new Car("保時捷", 1200000));
        linkedHashSet.add(new Car("奧迪", 600000));
        linkedHashSet.add(new Car("寶馬", 800000));
        linkedHashSet.add(new Car("奔馳", 3000000));
        linkedHashSet.add(new Car("法拉利", 9000000));
        linkedHashSet.add(new Car("保時捷", 1200000));
        for (Object o :linkedHashSet) {
            System.out.println(o);
        }
    }
}
class Car {
    private String name;
    private double price;
    public Car(String name, double price) {
        this.name = name;
        this.price = price;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public double getPrice() {
        return price;
    }
    public void setPrice(double price) {
        this.price = price;
    }
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Car car = (Car) o;
        return Double.compare(car.price, price) == 0 && Objects.equals(name, car.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, price);
    }
    @Override
    public String toString() {
        return "Car{" + name + price + '}';
    }
}

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