HashMap、HashTable和TreeMap详解

叁歲伎倆 2022-05-22 09:40 323阅读 0赞

# 概述 #

HashMap、HashTable和TreeMap 是Map接口下三个重要的集合类。

树默认的是红黑树算法

![70][]

# HashMap #

默认的初始化大小是16

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

最大容量是2的30次方

static final int MAXIMUM_CAPACITY = 1 << 30;

默认的扩充因子是0.75。比如在大小为16时，如果存储的数据达到了16\*0.75=12时，就会扩充容量

static final float DEFAULT_LOAD_FACTOR = 0.75f;

有一个node内部类，判断node相等，必须是key的hashcode和value的hashcode同时相等。

static class Node<K,V> implements Map.Entry<K,V> {
            final int hash;
            final K key;
            V value;
            Node<K,V> next;
    
            Node(int hash, K key, V value, Node<K,V> next) {
                this.hash = hash;
                this.key = key;
                this.value = value;
                this.next = next;
            }
    
            public final K getKey()        { return key; }
            public final V getValue()      { return value; }
            public final String toString() { return key + "=" + value; }
    
            public final int hashCode() {
                return Objects.hashCode(key) ^ Objects.hashCode(value);
            }
    
            public final V setValue(V newValue) {
                V oldValue = value;
                value = newValue;
                return oldValue;
            }
    
            public final boolean equals(Object o) {
                if (o == this)
                    return true;
                if (o instanceof Map.Entry) {
                    Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                    if (Objects.equals(key, e.getKey()) &&
                        Objects.equals(value, e.getValue()))
                        return true;
                }
                return false;
            }
        }

# HashTable #

默认大小是11

public Hashtable() {
            this(11, 0.75f);
        }

默认的容量因子是0.75

public Hashtable(int initialCapacity) {
            this(initialCapacity, 0.75f);
        }

它里面的每一个操作方法都有synchronized修饰，用以保证同步

public synchronized boolean contains(Object value) {
            if (value == null) {
                throw new NullPointerException();
            }
    
            Entry<?,?> tab[] = table;
            for (int i = tab.length ; i-- > 0 ;) {
                for (Entry<?,?> e = tab[i] ; e != null ; e = e.next) {
                    if (e.value.equals(value)) {
                        return true;
                    }
                }
            }
            return false;
        }

添加一项时

public synchronized V put(K key, V value) {
            // Make sure the value is not null
            if (value == null) {
                throw new NullPointerException();
            }
    
            // Makes sure the key is not already in the hashtable.
            Entry<?,?> tab[] = table;
            int hash = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;
            @SuppressWarnings("unchecked")
            Entry<K,V> entry = (Entry<K,V>)tab[index];
            for(; entry != null ; entry = entry.next) {
                if ((entry.hash == hash) && entry.key.equals(key)) {
                    V old = entry.value;
                    entry.value = value;
                    return old;
                }
            }
    
            addEntry(hash, key, value, index);
            return null;
        }

其中addEntrty。

private void addEntry(int hash, K key, V value, int index) {
            modCount++;
    
            Entry<?,?> tab[] = table;
            if (count >= threshold) {
                // Rehash the table if the threshold is exceeded
                rehash();
    
                tab = table;
                hash = key.hashCode();
                index = (hash & 0x7FFFFFFF) % tab.length;
            }
    
            // Creates the new entry.
            @SuppressWarnings("unchecked")
            Entry<K,V> e = (Entry<K,V>) tab[index];
            tab[index] = new Entry<>(hash, key, value, e);
            count++;
        }

删除

public synchronized V remove(Object key) {
            Entry<?,?> tab[] = table;
            int hash = key.hashCode();
            int index = (hash & 0x7FFFFFFF) % tab.length;
            @SuppressWarnings("unchecked")
            Entry<K,V> e = (Entry<K,V>)tab[index];
            for(Entry<K,V> prev = null ; e != null ; prev = e, e = e.next) {
                if ((e.hash == hash) && e.key.equals(key)) {
                    modCount++;
                    if (prev != null) {
                        prev.next = e.next;
                    } else {
                        tab[index] = e.next;
                    }
                    count--;
                    V oldValue = e.value;
                    e.value = null;
                    return oldValue;
                }
            }
            return null;
        }

toString（）方法将键值对都打印出来

public synchronized String toString() {
            int max = size() - 1;
            if (max == -1)
                return "{}";
    
            StringBuilder sb = new StringBuilder();
            Iterator<Map.Entry<K,V>> it = entrySet().iterator();
    
            sb.append('{');
            for (int i = 0; ; i++) {
                Map.Entry<K,V> e = it.next();
                K key = e.getKey();
                V value = e.getValue();
                sb.append(key   == this ? "(this Map)" : key.toString());
                sb.append('=');
                sb.append(value == this ? "(this Map)" : value.toString());
    
                if (i == max)
                    return sb.append('}').toString();
                sb.append(", ");
            }
        }

属性都有volatile关键字修饰

private transient volatile Set<K> keySet;
        private transient volatile Set<Map.Entry<K,V>> entrySet;
        private transient volatile Collection<V> values;

# TreeMap #

添加时

public V put(K key, V value) {
            Entry<K,V> t = root;
            if (t == null) {
                compare(key, key); // type (and possibly null) check
    
                root = new Entry<>(key, value, null);
                size = 1;
                modCount++;
                return null;
            }
            int cmp;
            Entry<K,V> parent;
            // split comparator and comparable paths
            Comparator<? super K> cpr = comparator;
            if (cpr != null) {
                do {
                    parent = t;
                    cmp = cpr.compare(key, t.key);
                    if (cmp < 0)
                        t = t.left;
                    else if (cmp > 0)
                        t = t.right;
                    else
                        return t.setValue(value);
                } while (t != null);
            }
            else {
                if (key == null)
                    throw new NullPointerException();
                @SuppressWarnings("unchecked")
                    Comparable<? super K> k = (Comparable<? super K>) key;
                do {
                    parent = t;
                    cmp = k.compareTo(t.key);
                    if (cmp < 0)
                        t = t.left;
                    else if (cmp > 0)
                        t = t.right;
                    else
                        return t.setValue(value);
                } while (t != null);
            }
            Entry<K,V> e = new Entry<>(key, value, parent);
            if (cmp < 0)
                parent.left = e;
            else
                parent.right = e;
            fixAfterInsertion(e);
            size++;
            modCount++;
            return null;
        }

里面的红黑树机制：

// Red-black mechanics
        private static final boolean RED   = false;
        private static final boolean BLACK = true;

K key;
            V value;
            Entry<K,V> left;
            Entry<K,V> right;
            Entry<K,V> parent;
            boolean color = BLACK;

取出第一个

final Entry<K,V> getFirstEntry() {
            Entry<K,V> p = root;
            if (p != null)
                while (p.left != null)
                    p = p.left;
            return p;
        }

取出最后一个

final Entry<K,V> getLastEntry() {
            Entry<K,V> p = root;
            if (p != null)
                while (p.right != null)
                    p = p.right;
            return p;
        }

# 示例 #

TreeMap相对于HashMap,key是有序排列

public static void main(String[] args) {
    		Map<String,String> map = new TreeMap<>();
    		map.put("a", "aaa");
    		map.put("b", "bbb");
    		map.put("c", "ccc");
    		map.put("2", "222");
    		map.put("5", "555");
    		map.put("e", "eee");
    		map.put("d", "ddd");
    		
    		Set<String> sets = map.keySet();
    		for(String s : sets){
    			System.out.println(s+" : " +map.get(s));
    		}
    		
    		System.out.println("=================================");
    		
    		Map<String,String> map2 = new HashMap<>();
    		map2.put("c", "ccc");
    		map2.put("e", "eee");
    		map2.put("d", "ddd");
    		map2.put("2", "222");
    		map2.put("5", "555");
    		map2.put("a", "aaa");
    		map2.put("b", "bbb");
    		
    		Set<String> sets2 = map2.keySet();
    		for(String s : sets2){
    			System.out.println(s+" : " +map2.get(s));
    		}
    	}

运行结果是：

2 : 222
    5 : 555
    a : aaa
    b : bbb
    c : ccc
    d : ddd
    e : eee
    =================================
    a : aaa
    2 : 222
    b : bbb
    c : ccc
    d : ddd
    e : eee
    5 : 555

[70]: /images/20220522/da4499f3efa74e699f12bd6f4a229545.png