title: 可重复key的HashMap tags:
- IdentityHashMap
- hashCode
- identityhashcode
categories: jcf date: 2017-09-27 23:27:18
碰到一些需求需要放入可重复key的HashMap，比如Excel需要报错的行号。
那么如果对象实现过hashCode方法和equals 那么放入到hashMap中会出现可能互相覆盖的情形。
原来你是这样的HashMap 正如这篇文章中的测试所说，互相覆盖。
那么就是IdentityHashMap出场的时候啦~
首先了解一下Object的hashCode方法
/**
* Returns a hash code value for the object. This method is
* supported for the benefit of hash tables such as those provided by
* {@link java.util.HashMap}.
*
* The general contract of {@code hashCode} is:
*
* - Whenever it is invoked on the same object more than once during
* an execution of a Java application, the {@code hashCode} method
* must consistently return the same integer, provided no information
* used in {@code equals} comparisons on the object is modified.
* This integer need not remain consistent from one execution of an
* application to another execution of the same application.
*
- If two objects are equal according to the {@code equals(Object)}
* method, then calling the {@code hashCode} method on each of
* the two objects must produce the same integer result.
*
- It is not required that if two objects are unequal
* according to the {@link java.lang.Object#equals(java.lang.Object)}
* method, then calling the {@code hashCode} method on each of the
* two objects must produce distinct integer results. However, the
* programmer should be aware that producing distinct integer results
* for unequal objects may improve the performance of hash tables.
*
*
* As much as is reasonably practical, the hashCode method defined by
* class {@code Object} does return distinct integers for distinct
* objects. (This is typically implemented by converting the internal
* address of the object into an integer, but this implementation
* technique is not required by the
* JavaTM programming language.)
*
* @return a hash code value for this object.
* @see java.lang.Object#equals(java.lang.Object)
* @see java.lang.System#identityHashCode
*/
public native int hashCode();
复制代码文章中提到了java.lang.System#identityHashCode 那么我们看一下这两个数据有没有差别
这里面包含了IntegerCache的相关知识可以各位详细看看Integer的源码
我们可以看出hashCode计算出来（未复写Object）的值和identityHashCode计算出来的相同（估计就是一个值)
/**
* Returns the same hash code for the given object as
* would be returned by the default method hashCode(),
* whether or not the given object's class overrides
* hashCode().
* The hash code for the null reference is zero.
*
* @param x object for which the hashCode is to be calculated
* @return the hashCode
* @since JDK1.1
*/
public static native int identityHashCode(Object x);
复制代码bingo~就是和hashCode返回一样【而toString中并不是内存地址而是hashCode的hexString】
/**
* Returns a string representation of the object. In general, the
* {@code toString} method returns a string that
* "textually represents" this object. The result should
* be a concise but informative representation that is easy for a
* person to read.
* It is recommended that all subclasses override this method.
*
* The {@code toString} method for class {@code Object}
* returns a string consisting of the name of the class of which the
* object is an instance, the at-sign character `{@code @}', and
* the unsigned hexadecimal representation of the hash code of the
* object. In other words, this method returns a string equal to the
* value of:
*
*
* getClass().getName() + '@' + Integer.toHexString(hashCode())
*
*
* @return a string representation of the object.
*/
public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
复制代码那么来看一下IdentityHashMap对应的实现
/**
* Constructs a new, empty identity hash map with a default expected
* maximum size (21).
*/
public IdentityHashMap() {
init(DEFAULT_CAPACITY);
}
/**
* Constructs a new, empty map with the specified expected maximum size.
* Putting more than the expected number of key-value mappings into
* the map may cause the internal data structure to grow, which may be
* somewhat time-consuming.
*
* @param expectedMaxSize the expected maximum size of the map
* @throws IllegalArgumentException if expectedMaxSize is negative
*/
public IdentityHashMap(int expectedMaxSize) {
if (expectedMaxSize < 0)
throw new IllegalArgumentException("expectedMaxSize is negative: "
+ expectedMaxSize);
init(capacity(expectedMaxSize));
}
/**
* Returns the appropriate capacity for the specified expected maximum
* size. Returns the smallest power of two between MINIMUM_CAPACITY
* and MAXIMUM_CAPACITY, inclusive, that is greater than
* (3 * expectedMaxSize)/2, if such a number exists. Otherwise
* returns MAXIMUM_CAPACITY. If (3 * expectedMaxSize)/2 is negative, it
* is assumed that overflow has occurred, and MAXIMUM_CAPACITY is returned.
*/
private int capacity(int expectedMaxSize) {
// Compute min capacity for expectedMaxSize given a load factor of 2/3
int minCapacity = (3 * expectedMaxSize)/2;
// Compute the appropriate capacity
int result;
if (minCapacity > MAXIMUM_CAPACITY || minCapacity < 0) {
result = MAXIMUM_CAPACITY;
} else {
result = MINIMUM_CAPACITY;
while (result < minCapacity)
result <<= 1;
}
return result;
}
/**
* Initializes object to be an empty map with the specified initial
* capacity, which is assumed to be a power of two between
* MINIMUM_CAPACITY and MAXIMUM_CAPACITY inclusive.
*/
private void init(int initCapacity) {
// assert (initCapacity & -initCapacity) == initCapacity; // power of 2
// assert initCapacity >= MINIMUM_CAPACITY;
// assert initCapacity <= MAXIMUM_CAPACITY;
threshold = (initCapacity * 2)/3;
table = new Object[2 * initCapacity];
}
复制代码在传入容量的时候和HashMap不一样 其优先计算了1.5倍 恩 这样不需要开发者考虑expectedSize了
查看其get方法
private static int nextKeyIndex(int i, int len) {
return (i + 2 < len ? i + 2 : 0);
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key == k)},
* then this method returns {@code v}; otherwise it returns
* {@code null}. (There can be at most one such mapping.)
*
*
A return value of {@code null} does not necessarily
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
*
* @see #put(Object, Object)
*/
public V get(Object key) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k)
return (V) tab[i + 1];
if (item == null)
return null;
i = nextKeyIndex(i, len);
}
}
复制代码很明显其使用了item==k 其实就是地址的比较
nextKeyIndex这个方法很有意思 看来是用来解决Hash冲突的。hashMap通过链表完成了冲突的解决。{
查看hash方法
/**
* Returns index for Object x.
*/
private static int hash(Object x, int length) {
int h = System.identityHashCode(x);
// Multiply by -127, and left-shift to use least bit as part of hash
return ((h << 1) - (h << 8)) & (length - 1);
}
复制代码直接调用了System.identityHashCode 并且和tab的length进行了hash【和hashMap类似】
那么IdentityHashMap通过挪后2个位置【下一个位置用来放value】来占位置【某些情境下性能确实很差，但是通过threshold为2/3稍微稀疏了一些】
继续查看put方法
/**
* Associates the specified value with the specified key in this identity
* hash map. If the map previously contained a mapping for the key, the
* old value is replaced.
*
* @param key the key with which the specified value is to be associated
* @param value the value to be associated with the specified key
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
* @see Object#equals(Object)
* @see #get(Object)
* @see #containsKey(Object)
*/
public V put(K key, V value) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
Object item;
while ( (item = tab[i]) != null) {
if (item == k) {
V oldValue = (V) tab[i + 1];
tab[i + 1] = value;
return oldValue;
}
i = nextKeyIndex(i, len);
}
modCount++;
tab[i] = k;
tab[i + 1] = value;
if (++size >= threshold)
resize(len); // len == 2 * current capacity.
return null;
}
复制代码确实可以看到当hash之后发生碰撞直接找到null可以放入为止。
可以看到查找和插入都比较简单。那么如果移除呢
此时需要注意了。因为当前移除的hash可能是后面如果和当前hash一样的需要前移
/**
* Removes the mapping for this key from this map if present.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with key, or
* null if there was no mapping for key.
* (A null return can also indicate that the map
* previously associated null with key.)
*/
public V remove(Object key) {
Object k = maskNull(key);
Object[] tab = table;
int len = tab.length;
int i = hash(k, len);
while (true) {
Object item = tab[i];
if (item == k) {
modCount++;
size--;
V oldValue = (V) tab[i + 1];
tab[i + 1] = null;
tab[i] = null;
closeDeletion(i);
return oldValue;
}
if (item == null)
return null;
i = nextKeyIndex(i, len);
}
}
复制代码换言之需要当前的数据之后【循环table到头了需要从0从开始】全部数据需要朝前提两个格子知道查找到为null为止
/**
* Rehash all possibly-colliding entries following a
* deletion. This preserves the linear-probe
* collision properties required by get, put, etc.
*
* @param d the index of a newly empty deleted slot
*/
private void closeDeletion(int d) {
// Adapted from Knuth Section 6.4 Algorithm R
Object[] tab = table;
int len = tab.length;
// Look for items to swap into newly vacated slot
// starting at index immediately following deletion,
// and continuing until a null slot is seen, indicating
// the end of a run of possibly-colliding keys.
Object item;
for (int i = nextKeyIndex(d, len); (item = tab[i]) != null;
i = nextKeyIndex(i, len) ) {
// The following test triggers if the item at slot i (which
// hashes to be at slot r) should take the spot vacated by d.
// If so, we swap it in, and then continue with d now at the
// newly vacated i. This process will terminate when we hit
// the null slot at the end of this run.
// The test is messy because we are using a circular table.
int r = hash(item, len);
if ((i < r && (r <= d || d <= i)) || (r <= d && d <= i)) {
tab[d] = item;
tab[d + 1] = tab[i + 1];
tab[i] = null;
tab[i + 1] = null;
d = i;
}
}
}
复制代码其实key也不重复只是判断“equals”的逻辑发生了变化~