AbstractMap 源代码
源代码
/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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*/
package java.util;
import java.util.Map.Entry;
/**
* This class provides a skeletal implementation of the Map
* interface, to minimize the effort required to implement this interface.
*
* To implement an unmodifiable map, the programmer needs only to extend this
* class and provide an implementation for the entrySet method, which
* returns a set-view of the map's mappings. Typically, the returned set
* will, in turn, be implemented atop AbstractSet. This set should
* not support the add or remove methods, and its iterator
* should not support the remove method.
*
*
To implement a modifiable map, the programmer must additionally override
* this class's put method (which otherwise throws an
* UnsupportedOperationException), and the iterator returned by
* entrySet().iterator() must additionally implement its
* remove method.
*
*
The programmer should generally provide a void (no argument) and map
* constructor, as per the recommendation in the Map interface
* specification.
*
*
The documentation for each non-abstract method in this class describes its
* implementation in detail. Each of these methods may be overridden if the
* map being implemented admits a more efficient implementation.
*
*
This class is a member of the
*
* Java Collections Framework.
*
* @param the type of keys maintained by this map
* @param the type of mapped values
*
* @author Josh Bloch
* @author Neal Gafter
* @see Map
* @see Collection
* @since 1.2
*/
/*
此类提供 Map 接口的骨干实现，以最大限度地减少实现此接口所需的工作。
要实现不可修改的映射，编程人员只需扩展此类并提供 entrySet 方法的实现即可，
该方法将返回映射的映射关系 set 视图。通常，返回的 set 将依次在 AbstractSet 上实现。
此 set 不支持 add 或 remove 方法，其迭代器也不支持 remove 方法。
要实现可修改的映射，编程人员必须另外重写此类的 put 方法（否则将抛出
UnsupportedOperationException），entrySet().iterator() 返回的迭代器也必须另外实现其 remove 方法。
按照 Map 接口规范中的建议，编程人员通常应该提供一个 void（无参数）构造方法和 map 构造方法。
此类中每个非抽象方法的文档详细描述了其实现。如果要实现的映射允许更有效的实现，则可以重写所有这些方法。
*/
public abstract class AbstractMap implements Map {
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractMap() {
}
// Query Operations
/**
* {@inheritDoc}
*
* @implSpec
* This implementation returns entrySet().size().
*/
//返回容量
public int size() {
//entrySet()方法是继承该类时需要实现的方法
return entrySet().size();
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation returns size() == 0.
*/
//是否为空
public boolean isEmpty() {
return size() == 0;
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation iterates over entrySet() searching
* for an entry with the specified value. If such an entry is found,
* true is returned. If the iteration terminates without
* finding such an entry, false is returned. Note that this
* implementation requires linear time in the size of the map.
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
//是否包含值value
public boolean containsValue(Object value) {
//获取键值对的迭代器
Iterator> i = entrySet().iterator();
if (value==null) {
//如果value为null,则通过 == 对比
while (i.hasNext()) {
Entry e = i.next();
if (e.getValue()==null)
return true;
}
} else {
//value不为null,通过equals对比
while (i.hasNext()) {
Entry e = i.next();
if (value.equals(e.getValue()))
return true;
}
}
return false;
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation iterates over entrySet() searching
* for an entry with the specified key. If such an entry is found,
* true is returned. If the iteration terminates without
* finding such an entry, false is returned. Note that this
* implementation requires linear time in the size of the map; many
* implementations will override this method.
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
//判断键是否包含key
public boolean containsKey(Object key) {
Iterator> i = entrySet().iterator();
if (key==null) {
//通过 == 对比
while (i.hasNext()) {
Entry e = i.next();
if (e.getKey()==null)
return true;
}
} else {
//通过 equals 对比
while (i.hasNext()) {
Entry e = i.next();
if (key.equals(e.getKey()))
return true;
}
}
return false;
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation iterates over entrySet() searching
* for an entry with the specified key. If such an entry is found,
* the entry's value is returned. If the iteration terminates without
* finding such an entry, null is returned. Note that this
* implementation requires linear time in the size of the map; many
* implementations will override this method.
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
//通过key找到对应的value
public V get(Object key) {
Iterator> i = entrySet().iterator();
if (key==null) {
while (i.hasNext()) {
Entry e = i.next();
if (e.getKey()==null)
return e.getValue();
}
} else {
while (i.hasNext()) {
Entry e = i.next();
if (key.equals(e.getKey()))
return e.getValue();
}
}
return null;
}
// Modification Operations
/**
* {@inheritDoc}
*
* @implSpec
* This implementation always throws an
* UnsupportedOperationException.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
//添加，此类不具体实现
public V put(K key, V value) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation iterates over entrySet() searching for an
* entry with the specified key. If such an entry is found, its value is
* obtained with its getValue operation, the entry is removed
* from the collection (and the backing map) with the iterator's
* remove operation, and the saved value is returned. If the
* iteration terminates without finding such an entry, null is
* returned. Note that this implementation requires linear time in the
* size of the map; many implementations will override this method.
*
* Note that this implementation throws an
* UnsupportedOperationException if the entrySet
* iterator does not support the remove method and this map
* contains a mapping for the specified key.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
//移除掉键为key的键值对
public V remove(Object key) {
//获取迭代器
Iterator> i = entrySet().iterator();
//用来记录键为key的键值对
Entry correctEntry = null;
if (key==null) {
while (correctEntry==null && i.hasNext()) {
Entry e = i.next();
if (e.getKey()==null)
correctEntry = e;
}
} else {
while (correctEntry==null && i.hasNext()) {
Entry e = i.next();
if (key.equals(e.getKey()))
correctEntry = e;
}
}
//记录被移除的键值对的值
V oldValue = null;
if (correctEntry !=null) {
//有该key的键值对
oldValue = correctEntry.getValue();
//移除掉
i.remove();
}
return oldValue;
}
// Bulk Operations
/**
* {@inheritDoc}
*
* @implSpec
* This implementation iterates over the specified map's
* entrySet() collection, and calls this map's put
* operation once for each entry returned by the iteration.
*
* Note that this implementation throws an
* UnsupportedOperationException if this map does not support
* the put operation and the specified map is nonempty.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
//将所有m中的键值对添加到当前Map中
public void putAll(Map m) {
for (Map.Entry e : m.entrySet())
//循环添加
put(e.getKey(), e.getValue());
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation calls entrySet().clear().
*
*
Note that this implementation throws an
* UnsupportedOperationException if the entrySet
* does not support the clear operation.
*
* @throws UnsupportedOperationException {@inheritDoc}
*/
//清除掉
public void clear() {
entrySet().clear();
}
// Views
/**
* Each of these fields are initialized to contain an instance of the
* appropriate view the first time this view is requested. The views are
* stateless, so there's no reason to create more than one of each.
*/
transient volatile Set keySet = null;
transient volatile Collection values = null;
/**
* {@inheritDoc}
*
* @implSpec
* This implementation returns a set that subclasses {@link AbstractSet}.
* The subclass's iterator method returns a "wrapper object" over this
* map's entrySet() iterator. The size method
* delegates to this map's size method and the
* contains method delegates to this map's
* containsKey method.
*
* The set is created the first time this method is called,
* and returned in response to all subsequent calls. No synchronization
* is performed, so there is a slight chance that multiple calls to this
* method will not all return the same set.
*/
//返回所有key组成的set
public Set keySet() {
if (keySet == null) {
keySet = new AbstractSet() {
//实现iterator方法
public Iterator iterator() {
return new Iterator() {
//创建Iterator
private Iterator> i = entrySet().iterator();
public boolean hasNext() {
return i.hasNext();
}
public K next() {
return i.next().getKey();
}
public void remove() {
i.remove();
}
};
}
//以下的方法调用本身的方法
public int size() {
return AbstractMap.this.size();
}
public boolean isEmpty() {
return AbstractMap.this.isEmpty();
}
public void clear() {
AbstractMap.this.clear();
}
public boolean contains(Object k) {
return AbstractMap.this.containsKey(k);
}
};
}
return keySet;
}
/**
* {@inheritDoc}
*
* @implSpec
* This implementation returns a collection that subclasses {@link
* AbstractCollection}. The subclass's iterator method returns a
* "wrapper object" over this map's entrySet() iterator.
* The size method delegates to this map's size
* method and the contains method delegates to this map's
* containsValue method.
*
* The collection is created the first time this method is called, and
* returned in response to all subsequent calls. No synchronization is
* performed, so there is a slight chance that multiple calls to this
* method will not all return the same collection.
*/
//获取所有value
public Collection values() {
if (values == null) {
values = new AbstractCollection() {
public Iterator iterator() {
return new Iterator() {
private Iterator> i = entrySet().iterator();
public boolean hasNext() {
return i.hasNext();
}
public V next() {
return i.next().getValue();
}
public void remove() {
i.remove();
}
};
}
public int size() {
return AbstractMap.this.size();
}
public boolean isEmpty() {
return AbstractMap.this.isEmpty();
}
public void clear() {
AbstractMap.this.clear();
}
public boolean contains(Object v) {
return AbstractMap.this.containsValue(v);
}
};
}
return values;
}
public abstract Set> entrySet();
// Comparison and hashing
/**
* Compares the specified object with this map for equality. Returns
* true if the given object is also a map and the two maps
* represent the same mappings. More formally, two maps m1 and
* m2 represent the same mappings if
* m1.entrySet().equals(m2.entrySet()). This ensures that the
* equals method works properly across different implementations
* of the Map interface.
*
* @implSpec
* This implementation first checks if the specified object is this map;
* if so it returns true. Then, it checks if the specified
* object is a map whose size is identical to the size of this map; if
* not, it returns false. If so, it iterates over this map's
* entrySet collection, and checks that the specified map
* contains each mapping that this map contains. If the specified map
* fails to contain such a mapping, false is returned. If the
* iteration completes, true is returned.
*
* @param o object to be compared for equality with this map
* @return true if the specified object is equal to this map
*/
//判断对象是否相等
public boolean equals(Object o) {
// == 引用相同则相等
if (o == this)
return true;
//不是Map类型的对象则不相等
if (!(o instanceof Map))
return false;
Map m = (Map) o;
//容量不同则不相等
if (m.size() != size())
return false;
try {
Iterator> i = entrySet().iterator();
while (i.hasNext()) {
Entry e = i.next();
K key = e.getKey();
V value = e.getValue();
//一个一个元素遍历，如果有不相同，则返回false
if (value == null) {
if (!(m.get(key)==null && m.containsKey(key)))
return false;
} else {
if (!value.equals(m.get(key)))
return false;
}
}
} catch (ClassCastException unused) {
return false;
} catch (NullPointerException unused) {
return false;
}
return true;
}
/**
* Returns the hash code value for this map. The hash code of a map is
* defined to be the sum of the hash codes of each entry in the map's
* entrySet() view. This ensures that m1.equals(m2)
* implies that m1.hashCode()==m2.hashCode() for any two maps
* m1 and m2, as required by the general contract of
* {@link Object#hashCode}.
*
* @implSpec
* This implementation iterates over entrySet(), calling
* {@link Map.Entry#hashCode hashCode()} on each element (entry) in the
* set, and adding up the results.
*
* @return the hash code value for this map
* @see Map.Entry#hashCode()
* @see Object#equals(Object)
* @see Set#equals(Object)
*/
//计算hashCode
public int hashCode() {
int h = 0;
Iterator> i = entrySet().iterator();
//将每个键值对的hashCode相加
while (i.hasNext())
h += i.next().hashCode();
return h;
}
/**
* Returns a string representation of this map. The string representation
* consists of a list of key-value mappings in the order returned by the
* map's entrySet view's iterator, enclosed in braces
* ("{}"). Adjacent mappings are separated by the characters
* ", " (comma and space). Each key-value mapping is rendered as
* the key followed by an equals sign ("=") followed by the
* associated value. Keys and values are converted to strings as by
* {@link String#valueOf(Object)}.
*
* @return a string representation of this map
*/
//重写toString方法
public String toString() {
Iterator> i = entrySet().iterator();
if (! i.hasNext())
return "{}";
StringBuilder sb = new StringBuilder();
sb.append('{');
for (;;) {
Entry e = i.next();
K key = e.getKey();
V value = e.getValue();
sb.append(key == this ? "(this Map)" : key);
sb.append('=');
sb.append(value == this ? "(this Map)" : value);
if (! i.hasNext())
return sb.append('}').toString();
sb.append(',').append(' ');
}
}
/**
* Returns a shallow copy of this AbstractMap instance: the keys
* and values themselves are not cloned.
*
* @return a shallow copy of this map
*/
//克隆
protected Object clone() throws CloneNotSupportedException {
AbstractMap result = (AbstractMap)super.clone();
result.keySet = null;
result.values = null;
return result;
}
/**
* Utility method for SimpleEntry and SimpleImmutableEntry.
* Test for equality, checking for nulls.
*
* NB: Do not replace with Object.equals until JDK-8015417 is resolved.
*/
//用于SimpleEntry和SimpleImmutableEntry的工具方法，判断2个对象是否相同
private static boolean eq(Object o1, Object o2) {
return o1 == null ? o2 == null : o1.equals(o2);
}
// Implementation Note: SimpleEntry and SimpleImmutableEntry
// are distinct unrelated classes, even though they share
// some code. Since you can't add or subtract final-ness
// of a field in a subclass, they can't share representations,
// and the amount of duplicated code is too small to warrant
// exposing a common abstract class.
/**
* An Entry maintaining a key and a value. The value may be
* changed using the setValue method. This class
* facilitates the process of building custom map
* implementations. For example, it may be convenient to return
* arrays of SimpleEntry instances in method
* Map.entrySet().toArray.
*
* @since 1.6
*/
/*
维护键和值的 Entry。可以使用 setValue 方法更改值。此类简化了构建自定义映射实现的过程。
例如，可以使用 Map.entrySet().toArray 方法方便地返回 SimpleEntry 实例数组
*/
public static class SimpleEntry
implements Entry, java.io.Serializable
{
private static final long serialVersionUID = -8499721149061103585L;
// key 为不可变
private final K key;
private V value;
/**
* Creates an entry representing a mapping from the specified
* key to the specified value.
*
* @param key the key represented by this entry
* @param value the value represented by this entry
*/
public SimpleEntry(K key, V value) {
this.key = key;
this.value = value;
}
/**
* Creates an entry representing the same mapping as the
* specified entry.
*
* @param entry the entry to copy
*/
public SimpleEntry(Entry entry) {
this.key = entry.getKey();
this.value = entry.getValue();
}
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
*/
//获取key
public K getKey() {
return key;
}
/**
* Returns the value corresponding to this entry.
*
* @return the value corresponding to this entry
*/
//获取value
public V getValue() {
return value;
}
/**
* Replaces the value corresponding to this entry with the specified
* value.
*
* @param value new value to be stored in this entry
* @return the old value corresponding to the entry
*/
//设置value
public V setValue(V value) {
V oldValue = this.value;
this.value = value;
return oldValue;
}
/**
* Compares the specified object with this entry for equality.
* Returns {@code true} if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries {@code e1} and {@code e2} represent the same mapping
* if
* (e1.getKey()==null ?
* e2.getKey()==null :
* e1.getKey().equals(e2.getKey()))
* &&
* (e1.getValue()==null ?
* e2.getValue()==null :
* e1.getValue().equals(e2.getValue()))
* This ensures that the {@code equals} method works properly across
* different implementations of the {@code Map.Entry} interface.
*
* @param o object to be compared for equality with this map entry
* @return {@code true} if the specified object is equal to this map
* entry
* @see #hashCode
*/
//判断是否相等
public boolean equals(Object o) {
//如果不是Map.Entry
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
//键跟值都相等才相等
return eq(key, e.getKey()) && eq(value, e.getValue());
}
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry {@code e} is defined to be:
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* This ensures that {@code e1.equals(e2)} implies that
* {@code e1.hashCode()==e2.hashCode()} for any two Entries
* {@code e1} and {@code e2}, as required by the general
* contract of {@link Object#hashCode}.
*
* @return the hash code value for this map entry
* @see #equals
*/
public int hashCode() {
return (key == null ? 0 : key.hashCode()) ^
(value == null ? 0 : value.hashCode());
}
/**
* Returns a String representation of this map entry. This
* implementation returns the string representation of this
* entry's key followed by the equals character ("=")
* followed by the string representation of this entry's value.
*
* @return a String representation of this map entry
*/
public String toString() {
return key + "=" + value;
}
}
/**
* An Entry maintaining an immutable key and value. This class
* does not support method setValue. This class may be
* convenient in methods that return thread-safe snapshots of
* key-value mappings.
*
* @since 1.6
*/
/*
维护不可变的键和值的 Entry。此类不支持 setValue 方法。在返回线程安全的键-值映射关系快照的方法中，此类也许很方便
*/
public static class SimpleImmutableEntry
implements Entry, java.io.Serializable
{
private static final long serialVersionUID = 7138329143949025153L;
//key value 都是不可变的
private final K key;
private final V value;
/**
* Creates an entry representing a mapping from the specified
* key to the specified value.
*
* @param key the key represented by this entry
* @param value the value represented by this entry
*/
public SimpleImmutableEntry(K key, V value) {
this.key = key;
this.value = value;
}
/**
* Creates an entry representing the same mapping as the
* specified entry.
*
* @param entry the entry to copy
*/
public SimpleImmutableEntry(Entry entry) {
this.key = entry.getKey();
this.value = entry.getValue();
}
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
*/
public K getKey() {
return key;
}
/**
* Returns the value corresponding to this entry.
*
* @return the value corresponding to this entry
*/
public V getValue() {
return value;
}
/**
* Replaces the value corresponding to this entry with the specified
* value (optional operation). This implementation simply throws
* UnsupportedOperationException, as this class implements
* an immutable map entry.
*
* @param value new value to be stored in this entry
* @return (Does not return)
* @throws UnsupportedOperationException always
*/
public V setValue(V value) {
throw new UnsupportedOperationException();
}
/**
* Compares the specified object with this entry for equality.
* Returns {@code true} if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries {@code e1} and {@code e2} represent the same mapping
* if
* (e1.getKey()==null ?
* e2.getKey()==null :
* e1.getKey().equals(e2.getKey()))
* &&
* (e1.getValue()==null ?
* e2.getValue()==null :
* e1.getValue().equals(e2.getValue()))
* This ensures that the {@code equals} method works properly across
* different implementations of the {@code Map.Entry} interface.
*
* @param o object to be compared for equality with this map entry
* @return {@code true} if the specified object is equal to this map
* entry
* @see #hashCode
*/
public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
return eq(key, e.getKey()) && eq(value, e.getValue());
}
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry {@code e} is defined to be:
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* This ensures that {@code e1.equals(e2)} implies that
* {@code e1.hashCode()==e2.hashCode()} for any two Entries
* {@code e1} and {@code e2}, as required by the general
* contract of {@link Object#hashCode}.
*
* @return the hash code value for this map entry
* @see #equals
*/
public int hashCode() {
return (key == null ? 0 : key.hashCode()) ^
(value == null ? 0 : value.hashCode());
}
/**
* Returns a String representation of this map entry. This
* implementation returns the string representation of this
* entry's key followed by the equals character ("=")
* followed by the string representation of this entry's value.
*
* @return a String representation of this map entry
*/
public String toString() {
return key + "=" + value;
}
}
}