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
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