HashMap的实现中主要维护一个数组，发生冲突通过链表来解决，链表插入类似于头插法
LinkedHashMap继承自HashMap，在hash的基础上，又维护了一个链表，这个链表是带头结点的双向循环链表，需要注意的链表的元素都是hash里面的元素，链表仅仅是在hash的基础上用指针将hash中的节点连接了起来
LruCache是android的utils包里面的一个类，用来实现缓存防止OOM的一个工具类，用途非常广泛。
关于LRU算法：Least Recently Used最近最少使用算法，在操作系统中，对内存的访问满足局部性原理，于是LRU用在缺页中断发生时的置换算法，将内存中的最近最长未使用的页面置换到磁盘，可以实现的方式可以为维护一个链表，当访问一个页面是，将该页面移动至表头（尾），发生缺页时取链表最后（前）的页面置换，这样存在问题是读取某个页面的复杂度太高，于是可以考虑将其进行hash，这样读取速度会提高，于是用到了LinkedHashMap这种数据结构。
android实现的LruCache类主要使用来进行内存缓存的，维护所用资源的强引用，当内存超过设定的缓存值时，将好久未使用的资源从内存删除。
LruCache的实现中在缓存的值达到最大值时采用的方法是，循环迭代从链表中取eldest的元素进行删除，知道占用的控件小于最大的缓存值。LinkedHashMap中提供的removeEldestEntry函数可以简单实现LRU的功能，但不能很好的满足一些场景，因为里面存放的元素的大小不总是大小一致的，或者说不仅仅是以缓存数据的个数来看的。下面基本上没有太多的文字，所有的解释都详细的列在代码里面
HashMap
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/util/HashMap.java
public class HashMap extends AbstractMap implements Map, Cloneable, Serializable{
// 默认初始容量16
static final int DEFAULT_INITIAL_CAPACITY = 16;
// 最大容量2^30
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默认加载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// hash映射的数组槽
transient Entry[] table;
// 元素个数
transient int size;
// 阈值 = 加载因子 * 容量
int threshold;
// 加载因子
final float loadFactor;
// 修改次数，判断迭代期间容器被修改，不然抛出ConcurrentModificationException
transient int modCount;
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
// 参数调整
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " + loadFactor);
// 找到大于initialCapacity的最小的2次幂
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
// 设置阈值
threshold = (int)(capacity * loadFactor);
// 定义数组，大小为capacity
table = new Entry[capacity];
// 这里是空的实现，实际让其子类覆写该方法
init();
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
// 默认情况下默认的加载因子，默认的容量16
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];
init();
}
// 从已存在的Map创建HashMap
public HashMap(Map m) {
// 容量为Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,DEFAULT_INITIAL_CAPACITY)，默认的加载因子
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
// 遍历m将其元素添加到hashmap中
putAllForCreate(m);
}
void init() {
}
// hash算法
// 可以将1变的松散，可以减少冲突
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
// 根据hash值获得在我们维护的数组的索引
// 即取hash值的小于length的部分，这样才能将其限定在数组大小的范围里面，这样的处理也会带来冲突
static int indexFor(int h, int length) {
return h & (length-1);
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
// 根据键获取值
public V get(Object key) {
if (key == null)
return getForNullKey();
int hash = hash(key.hashCode());
for (Entry e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
return e.value;
}
return null;
}
// 键为null的Entry都放在第0个槽中，相当于null经过hash后为0
private V getForNullKey() {
for (Entry e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
// 返回对应键的Entry，若不存在返回null
final Entry getEntry(Object key) {
// 计算key的hash值
int hash = (key == null) ? 0 : hash(key.hashCode());
// 根据hash值获取其存放的槽，即indexFor函数的作用
// 遍历这个槽上的链表
for (Entry e = table[indexFor(hash, table.length)]; e != null; e = e.next) {
Object k;
// hash值一样且键一样（同一个内存地址或者值相同）即返回。
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
// 添加键值对
public V put(K key, V value) {
// 如果键为null，那么存放在第0个槽上
if (key == null)
return putForNullKey(value);
// 获得键的hash值
int hash = hash(key.hashCode());
// 根据hash值得到保存在我们维护的数组中的那个下标处
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
// 已存在，修改
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
// 将旧的值返回
return oldValue;
}
}
modCount++;
// 不存在则添加
addEntry(hash, key, value, i);
return null;
}
// 添加键位null的键值对
private V putForNullKey(V value) {
// 键位null的放在第0个槽
for (Entry e = table[0]; e != null; e = e.next) {
if (e.key == null) {
// 已存在则替换
V oldValue = e.value;
e.value = value;
// 被子类覆盖
e.recordAccess(this);
return oldValue;
}
}
modCount++;
// 不存在，添加
addEntry(0, null, value, 0);
return null;
}
// 和put类似，用在构造函数、clone
private void putForCreate(K key, V value) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}
createEntry(hash, key, value, i);
}
// 遍历map添加到新建的hashmap中
private void putAllForCreate(Map m) {
for (Map.Entry e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}
// 扩容
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
// 旧的容量已经达到最大了，调整阈值即可
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
// 用新的容量创建新数组
Entry[] newTable = new Entry[newCapacity];
// 并将原数组里面的hash表全部搬移到新的数组槽中
transfer(newTable);
// 将维护的数组引用重新赋值
table = newTable;
// 调整阈值
threshold = (int)(newCapacity * loadFactor);
}
// 将原数组table里面的hash表全部搬移到新的数组槽中填充newTable
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
// 遍历数组的每个槽，每个槽中在一次遍历链表
for (int j = 0; j < src.length; j++) {
Entry e = src[j];
if (e != null) {
src[j] = null;
do {
Entry next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}
//
public void putAll(Map m) {
// map元素个数为0，什么也不用做
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
// 如果待复制的元素个数大于阈值，需要扩容
if (numKeysToBeAdded > threshold) {
// 目标容量为满足当前设置的加载因子情况下的容量
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
// 参数调整
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
// 找到大于targetCapacity的最小2的n次幂
while (newCapacity < targetCapacity)
newCapacity <<= 1;
if (newCapacity > table.length)
// 扩容为新的容量
resize(newCapacity);
}
for (Map.Entry e : m.entrySet())
put(e.getKey(), e.getValue());
}
public V remove(Object key) {
Entry e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
// 移除key所对应的键值对
// 和removeMapping类似，只是在判断相等时有点区别
final Entry removeEntryForKey(Object key) {
int hash = (key == null) ? 0 : hash(key.hashCode());
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;
while (e != null) {
Entry next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
// 依然在删除该键值对时调用，留给LinkedHashMap，因为可能会在访问hashmap时重新整理链表的指向关系
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
// 移除键值对
final Entry removeMapping(Object o) {
// 传递参数不是Entry的子类，什么也不做
if (!(o instanceof Map.Entry))
return null;
Map.Entry entry = (Map.Entry) o;
Object key = entry.getKey();
// 获取要删除的键值对的键的哈希值
int hash = (key == null) ? 0 : hash(key.hashCode());
// 根据hash值得到保存在我们维护的数组中的那个下标处
int i = indexFor(hash, table.length);
Entry prev = table[i];
Entry e = prev;
while (e != null) {
Entry next = e.next;
if (e.hash == hash && e.equals(entry)) {
// hash值相同并且entry内容一样，即找到了
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
// 空的实现，给LinkedHashMap实现，在删除键值对后执行
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
public void clear() {
modCount++;
Entry[] tab = table;
for (int i = 0; i < tab.length; i++)
tab[i] = null;
size = 0;
}
// 判断是否包含值为value的键值对
public boolean containsValue(Object value) {
if (value == null)
return containsNullValue();
Entry[] tab = table;
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
return false;
}
// 判断是否有值为null的键值对
private boolean containsNullValue() {
Entry[] tab = table;
// 依次迭代数组和每个数组槽所对应的链表
for (int i = 0; i < tab.length ; i++)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value == null)
return true;
return false;
}
public Object clone() {
HashMap result = null;
try {
result = (HashMap)super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
result.table = new Entry[table.length];
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);
return result;
}
// hashmap的底层节点结构
static class Entry implements Map.Entry {
final K key;
V value;
Entry next;
final int hash;
Entry(int h, K k, V v, Entry n) {
value = v;
next = n;
key = k;
hash = h;
}
public final K getKey() {
return key;
}
public final V getValue() {
return value;
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}
public final int hashCode() {
return (key==null ? 0 : key.hashCode()) ^
(value==null ? 0 : value.hashCode());
}
public final String toString() {
return getKey() + "=" + getValue();
}
/*******两个空的方法，分别在添加和删除时调用，用以子类实现访问该容器时做一些其他操作*******/
void recordAccess(HashMap m) {
}
void recordRemoval(HashMap m) {
}
}
// 添加一个Entry到bucketIndex槽的位置
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry e = table[bucketIndex];
// 下面这句简单的表述实际上创建了一个Entry节点，下一个节点是e
// 也就是说数组索引所在位置，然后在调整数组索引处为新创建的节点，即链表的头插法
table[bucketIndex] = new Entry<>(hash, key, value, e);
// 元素个数超过了阈值，进行扩容为原来的两倍
if (size++ >= threshold)
resize(2 * table.length);
}
// 逻辑和addEntry一模一样，只是少了扩容的判断，该函数用在构造函数里拷贝另一个map的值
// 此前已经调整了容量，因此不会出现扩容的情况
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
// 迭代器部分
private abstract class HashIterator implements Iterator {
Entry next; // next entry to return
// 迭代器的fast-fail机制，迭代期间不允许修改容器
int expectedModCount; // For fast-fail
int index; // current slot
Entry current; // current entry
HashIterator() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
}
public final boolean hasNext() {
return next != null;
}
final Entry nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry e = next;
if (e == null)
throw new NoSuchElementException();
if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
current = e;
return e;
}
public void remove() {
if (current == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Object k = current.key;
current = null;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
}
}
private final class ValueIterator extends HashIterator {
public V next() {
return nextEntry().value;
}
}
private final class KeyIterator extends HashIterator {
public K next() {
return nextEntry().getKey();
}
}
private final class EntryIterator extends HashIterator> {
public Map.Entry next() {
return nextEntry();
}
}
// Subclass overrides these to alter behavior of views' iterator() method
Iterator newKeyIterator() {
return new KeyIterator();
}
Iterator newValueIterator() {
return new ValueIterator();
}
Iterator> newEntryIterator() {
return new EntryIterator();
}
// Views
// hasp里面的entry所对应的Set
private transient Set> entrySet = null;
// 键对应的Set
public Set keySet() {
Set ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}
private final class KeySet extends AbstractSet {
public Iterator iterator() {
return newKeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return HashMap.this.removeEntryForKey(o) != null;
}
public void clear() {
HashMap.this.clear();
}
}
public Collection values() {
Collection vs = values;
return (vs != null ? vs : (values = new Values()));
}
// 值的集合
private final class Values extends AbstractCollection {
public Iterator iterator() {
return newValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
}
public Set> entrySet() {
return entrySet0();
}
private Set> entrySet0() {
Set> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}
private final class EntrySet extends AbstractSet> {
public Iterator> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry) o;
Entry candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);
}
public boolean remove(Object o) {
return removeMapping(o) != null;
}
public int size() {
return size;
}
public void clear() {
HashMap.this.clear();
}
}
// 序列化部分
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
Iterator> i =
(size > 0) ? entrySet0().iterator() : null;
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
// Write out number of buckets
s.writeInt(table.length);
// Write out size (number of Mappings)
s.writeInt(size);
// Write out keys and values (alternating)
if (i != null) {
while (i.hasNext()) {
Map.Entry e = i.next();
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}
private static final long serialVersionUID = 362498820763181265L;
/**
* Reconstitute the HashMap instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold, loadfactor, and any hidden stuff
s.defaultReadObject();
// Read in number of buckets and allocate the bucket array;
int numBuckets = s.readInt();
table = new Entry[numBuckets];
init(); // Give subclass a chance to do its thing.
// Read in size (number of Mappings)
int size = s.readInt();
// Read the keys and values, and put the mappings in the HashMap
for (int i=0; i LinkedHashMap
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/util/LinkedHashMap.java
public class LinkedHashMap extends HashMap implements Map{
private static final long serialVersionUID = 3801124242820219131L;
// 带头结点的双向循环链表 的头
private transient Entry header;
// 取值代表使用的方式：false链表按照添加顺序组织，true按照使用顺序组织
private final boolean accessOrder;
// 在构造方法中accessOrder均被初始化为false
public LinkedHashMap(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
accessOrder = false;
}
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
public LinkedHashMap() {
super();
accessOrder = false;
}
public LinkedHashMap(Map m) {
super(m);
accessOrder = false;
}
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
// 复写父类的init方法，该方法在父类的构造方法里面调用
void init() {
// 初始化链表头结点header
// 该头结点数字无意义。
header = new Entry<>(-1, null, null, null);
// 双向循环链表
header.before = header.after = header;
}
// hashmap里面的该函数的意义是：将原数组table里面的hash表全部搬移到新的数组槽中填充newTable
// 由于已经将所有元素用链表连起来了所以是用链表来赋值更加快速
//
void transfer(HashMap.Entry[] newTable) {
int newCapacity = newTable.length;
for (Entry e = header.after; e != header; e = e.after) {
int index = indexFor(e.hash, newCapacity);
e.next = newTable[index];
newTable[index] = e;
}
}
// 判断是否含有某个value
// 直接遍历链表会有更好的时间复杂度
public boolean containsValue(Object value) {
// Overridden to take advantage of faster iterator
if (value==null) {
for (Entry e = header.after; e != header; e = e.after)
if (e.value==null)
return true;
} else {
for (Entry e = header.after; e != header; e = e.after)
if (value.equals(e.value))
return true;
}
return false;
}
public V get(Object key) {
Entry e = (Entry)getEntry(key);
if (e == null)
return null;
// 访问即有可能要改变他在链表中的位置
e.recordAccess(this);
return e.value;
}
public void clear() {
super.clear();
header.before = header.after = header;
}
// linkedHashMap的节点
private static class Entry extends HashMap.Entry {
// 比起hashmap的节点多了两个指针，一个指向前一个节点一个指向后一个节点
Entry before, after;
Entry(int hash, K key, V value, HashMap.Entry next) {
super(hash, key, value, next);
}
// 从链表中移除本身节点，仅仅指的是修改指针指向
private void remove() {
before.after = after;
after.before = before;
}
// 从链表中添加本节点至existingEntry的前面
private void addBefore(Entry existingEntry) {
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
// 覆盖父类的方法
void recordAccess(HashMap m) {
LinkedHashMap lm = (LinkedHashMap)m;
// 如果accessOrder为false什么都不做
if (lm.accessOrder) {
lm.modCount++;
// 从链表中移除
remove();
// 将该节点添加到链表header的前面，也就是将其添加到链表末尾（header不变）
addBefore(lm.header);
//前两步其实就是移动该节点到连飙头，因为他刚被访问过
}
}
// 覆盖父类的方法，删除键值对时同时从链表中移除
void recordRemoval(HashMap m) {
remove();
}
}
// 迭代器部分
private abstract class LinkedHashIterator implements Iterator {
Entry nextEntry = header.after;
Entry lastReturned = null;
int expectedModCount = modCount;
public boolean hasNext() {
return nextEntry != header;
}
public void remove() {
if (lastReturned == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
LinkedHashMap.this.remove(lastReturned.key);
lastReturned = null;
expectedModCount = modCount;
}
Entry nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (nextEntry == header)
throw new NoSuchElementException();
Entry e = lastReturned = nextEntry;
nextEntry = e.after;
return e;
}
}
private class KeyIterator extends LinkedHashIterator {
public K next() { return nextEntry().getKey(); }
}
private class ValueIterator extends LinkedHashIterator {
public V next() { return nextEntry().value; }
}
private class EntryIterator extends LinkedHashIterator> {
public Map.Entry next() { return nextEntry(); }
}
// These Overrides alter the behavior of superclass view iterator() methods
Iterator newKeyIterator() { return new KeyIterator(); }
Iterator newValueIterator() { return new ValueIterator(); }
Iterator> newEntryIterator() { return new EntryIterator(); }
// 添加键值对
void addEntry(int hash, K key, V value, int bucketIndex) {
createEntry(hash, key, value, bucketIndex);
// Remove eldest entry if instructed, else grow capacity if appropriate
Entry eldest = header.after;
// 判断最旧的，也就是在链表头部的节点是否需要被删除
if (removeEldestEntry(eldest)) {
removeEntryForKey(eldest.key);
} else {
if (size >= threshold)
resize(2 * table.length);
}
}
// 比起hashmap中的createEntry方法，增加了修改链表
void createEntry(int hash, K key, V value, int bucketIndex) {
HashMap.Entry old = table[bucketIndex];
Entry e = new Entry<>(hash, key, value, old);
table[bucketIndex] = e;
// 添加一个键值对时，总要将其链接到维护的链表结尾
e.addBefore(header);
size++;
}
/******LinkedHashMap暴露的方法，可以用起来实现LRU算法*****/
protected boolean removeEldestEntry(Map.Entry eldest) {
return false;
}
} LruCache
public class LruCache {
// LRC算法底层由LinkedHashMap实现
private final LinkedHashMap map;
// 缓存的数量大小，可以使元素个数、字节数等等任何想要的
private int size;
// 缓存的最大值
private int maxSize;
private int putCount;
private int createCount;
// 由于缓存空间满了被逐出的次数
private int evictionCount;
// 从缓存取命中次数
private int hitCount;
// 为在缓存中找到的次数，即失败次数
private int missCount;
public LruCache(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
this.maxSize = maxSize;
this.map = new LinkedHashMap(0, 0.75f, true);
}
//
public void resize(int maxSize) {
if (maxSize <= 0) {
throw new IllegalArgumentException("maxSize <= 0");
}
synchronized (this) {
this.maxSize = maxSize;
}
trimToSize(maxSize);
}
//
public final V get(K key) {
if (key == null) {
// 不允许出现null的键和HashMap不一样
throw new NullPointerException("key == null");
}
V mapValue;
synchronized (this) {
mapValue = map.get(key);
if (mapValue != null) {
// 每get成功一次hitCount就自加一次，表示命中次数
hitCount++;
// 如果该键对应的值存在，返回之。
return mapValue;
}
missCount++;
}
// 否则，创建该键值对，默认值为null
V createdValue = create(key);
if (createdValue == null) {
return null;
}
synchronized (this) {
createCount++;
// 将创建的value添加到map
mapValue = map.put(key, createdValue);
if (mapValue != null) {
// mapValue部位空，表示本线程在put之前已经被别的线程put了一个值，即产生了冲突
// 此时我们扔掉刚创建的value，而是使用其他地方产生的value
map.put(key, mapValue);
} else {
// 将其放进map中的同时缓存的size增加
size += safeSizeOf(key, createdValue);
}
}
if (mapValue != null) {
entryRemoved(false, key, createdValue, mapValue);
return mapValue;
} else {
// 根据maxSize修改map，因为有可能由于此次的put操作使得容量超过最大值，具体的修改方式在子函数中
trimToSize(maxSize);
return createdValue;
}
}
// 和get基本一样
public final V put(K key, V value) {
if (key == null || value == null) {
throw new NullPointerException("key == null || value == null");
}
V previous;
synchronized (this) {
putCount++;
size += safeSizeOf(key, value);
previous = map.put(key, value);
if (previous != null) {
// 返回值部位null，说明之前该键对应的有值，即使替换，因此占用空间减去之前元素
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
// 移除元素时调用
entryRemoved(false, key, previous, value);
}
trimToSize(maxSize);
return previous;
}
// 根据maxSize增删map
private void trimToSize(int maxSize) {
while (true) {
K key;
V value;
synchronized (this) {
if (size < 0 || (map.isEmpty() && size != 0)) {
throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
}
// 当前占用的空间小于最大空间时跳出
if (size <= maxSize) {
break;
}
// 否则，取出最近最长未使用的元素，也就是链表最前面的一个
// v5.0.1版本的utils包提供的感觉有问题。
/*Map.Entry toEvict = null;
for (Map.Entry entry : map.entrySet()) {
// 循环直到最后一个???
toEvict = entry;
}
if (toEvict == null) {
break;
}
*/
// V4 包里面的实现https://github.com/android/platform_frameworks_support/blob/master/v4/java/android/support/v4/util/LruCache.java
Map.Entry toEvict = map.entrySet().iterator().next();
// 然而google已经提供的LinkedHashMap中就有一个函数获得eldest的元素，于是有些版本（）4.4.2的写法比较好理解
key = toEvict.getKey();
value = toEvict.getValue();
// 移除该元素
map.remove(key);
// 并将占用空间减少
size -= safeSizeOf(key, value);
evictionCount++;
}
entryRemoved(true, key, value, null);
}
}
/**
* Removes the entry for {@code key} if it exists.
*
* @return the previous value mapped by {@code key}.
*/
public final V remove(K key) {
if (key == null) {
throw new NullPointerException("key == null");
}
V previous;
synchronized (this) {
previous = map.remove(key);
if (previous != null) {
size -= safeSizeOf(key, previous);
}
}
if (previous != null) {
entryRemoved(false, key, previous, null);
}
return previous;
}
// true if the entry is being removed to make space, false if the removal was caused by a put or remove.
/****** 可以覆盖进行其他操作 ******/
protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
// 当需要的元素不存在时执行，可以自行覆盖
protected V create(K key) {
return null;
}
// 返回一个值表示占用的空间，这里做了参数检查
private int safeSizeOf(K key, V value) {
int result = sizeOf(key, value);
if (result < 0) {
throw new IllegalStateException("Negative size: " + key + "=" + value);
}
return result;
}
// 返回一个值表示占用的空间
/****** 需要覆盖对不同的元素（键值对）进行不同的处理 ******/
protected int sizeOf(K key, V value) {
return 1;
}
// 逐出所有的元素，参数为-1，只要里面还有元素就会大于-1，于是要全部移除
public final void evictAll() {
trimToSize(-1); // -1 will evict 0-sized elements
}
public synchronized final int size() {
return size;
}
public synchronized final int maxSize() {
return maxSize;
}
public synchronized final int hitCount() {
return hitCount;
}
public synchronized final int missCount() {
return missCount;
}
public synchronized final int createCount() {
return createCount;
}
public synchronized final int putCount() {
return putCount;
}
public synchronized final int evictionCount() {
return evictionCount;
}
public synchronized final Map snapshot() {
return new LinkedHashMap(map);
}
@Override
public synchronized final String toString() {
int accesses = hitCount + missCount;
int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
maxSize, hitCount, missCount, hitPercent);
}
}
android 4.4.2中的LinkedHashMap直接提供了获得最旧元素的方法
/**
* Returns the eldest entry in the map, or {@code null} if the map is empty.
* @hide
*/
public Entry eldest() {
LinkedEntry eldest = header.nxt;
return eldest != header ? eldest : null;
} 上面提到的HashMap和LinkedHashMap在jdk的不同版本变化较大，并且和android包中的实现也有一些差异。
以上。
碎觉！