1、map接口定义了键值对操作的基本常用方法。
2、size：key-value的数量
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of key-value mappings in this map
*/
int size();3、isEmpty：key是否为空
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
boolean isEmpty();3、containsKey：是否包含某个key
containsValue：是否包含某个Value
/**
* Returns <tt>true</tt> if this map contains a mapping for the specified
* key. More formally, returns <tt>true</tt> if and only if
* this map contains a mapping for a key <tt>k</tt> such that
* <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
* at most one such mapping.)
*
* @param key key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
*/
boolean containsKey(Object key);
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value. More formally, returns <tt>true</tt> if and only if
* this map contains at least one mapping to a value <tt>v</tt> such that
* <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
* will probably require time linear in the map size for most
* implementations of the <tt>Map</tt> interface.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
* @throws ClassCastException if the value is of an inappropriate type for
* this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified value is null and this
* map does not permit null values
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
*/
boolean containsValue(Object value);
4、get：获取值
put：设置值
remove：删除key
putAll：添加某个键值对的所有值
clear：清空键值对的所有值
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>If this map permits null values, then a return value of
* {@code null} does not <i>necessarily</i> 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.
*
* @param key the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or
* {@code null} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
*/
V get(Object key);
// Modification Operations
/**
* Associates the specified value with the specified key in this map
* (optional operation). If the map previously contained a mapping for
* the key, the old value is replaced by the specified value. (A map
* <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
* if {@link #containsKey(Object) m.containsKey(k)} would return
* <tt>true</tt>.)
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>,
* if the implementation supports <tt>null</tt> values.)
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if the specified key or value is null
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
*/
V put(K key, V value);
/**
* Removes the mapping for a key from this map if it is present
* (optional operation). More formally, if this map contains a mapping
* from key <tt>k</tt> to value <tt>v</tt> such that
* <code>(key==null ? k==null : key.equals(k))</code>, that mapping
* is removed. (The map can contain at most one such mapping.)
*
* <p>Returns the value to which this map previously associated the key,
* or <tt>null</tt> if the map contained no mapping for the key.
*
* <p>If this map permits null values, then a return value of
* <tt>null</tt> does not <i>necessarily</i> indicate that the map
* contained no mapping for the key; it's also possible that the map
* explicitly mapped the key to <tt>null</tt>.
*
* <p>The map will not contain a mapping for the specified key once the
* call returns.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
* is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this
* map does not permit null keys
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
*/
V remove(Object key);
// Bulk Operations
/**
* Copies all of the mappings from the specified map to this map
* (optional operation). The effect of this call is equivalent to that
* of calling {@link #put(Object,Object) put(k, v)} on this map once
* for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
* specified map. The behavior of this operation is undefined if the
* specified map is modified while the operation is in progress.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the <tt>putAll</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of a key or value in the
* specified map prevents it from being stored in this map
* @throws NullPointerException if the specified map is null, or if
* this map does not permit null keys or values, and the
* specified map contains null keys or values
* @throws IllegalArgumentException if some property of a key or value in
* the specified map prevents it from being stored in this map
*/
void putAll(Map<? extends K, ? extends V> m);
/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> operation
* is not supported by this map
*/
void clear();5、查看方法：
keySet：获取所有的键集合
values：获取键值对中所有的值集合
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @return a set view of the keys contained in this map
*/
Set<K> keySet();
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the values contained in this map
*/
Collection<V> values();6、entrySet：获取键值对的集合。map的实际值是保存在Map.Entry中
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation, or through the
* <tt>setValue</tt> operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations. It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a set view of the mappings contained in this map
*/
Set<Map.Entry<K, V>> entrySet();7、Entry：定义了key，value的get/set方法，用于保存map的键值对信息。
/**
* A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
* a collection-view of the map, whose elements are of this class. The
* <i>only</i> way to obtain a reference to a map entry is from the
* iterator of this collection-view. These <tt>Map.Entry</tt> objects are
* valid <i>only</i> for the duration of the iteration; more formally,
* the behavior of a map entry is undefined if the backing map has been
* modified after the entry was returned by the iterator, except through
* the <tt>setValue</tt> operation on the map entry.
*
* @see Map#entrySet()
* @since 1.2
*/
interface Entry<K,V> {
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
K getKey();
/**
* Returns the value corresponding to this entry. If the mapping
* has been removed from the backing map (by the iterator's
* <tt>remove</tt> operation), the results of this call are undefined.
*
* @return the value corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V getValue();
/**
* Replaces the value corresponding to this entry with the specified
* value (optional operation). (Writes through to the map.) The
* behavior of this call is undefined if the mapping has already been
* removed from the map (by the iterator's <tt>remove</tt> operation).
*
* @param value new value to be stored in this entry
* @return old value corresponding to the entry
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by the backing map
* @throws ClassCastException if the class of the specified value
* prevents it from being stored in the backing map
* @throws NullPointerException if the backing map does not permit
* null values, and the specified value is null
* @throws IllegalArgumentException if some property of this value
* prevents it from being stored in the backing map
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V setValue(V value);
/**
* Compares the specified object with this entry for equality.
* Returns <tt>true</tt> if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
* if<pre>
* (e1.getKey()==null ?
* e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
* (e1.getValue()==null ?
* e2.getValue()==null : e1.getValue().equals(e2.getValue()))
* </pre>
* This ensures that the <tt>equals</tt> method works properly across
* different implementations of the <tt>Map.Entry</tt> interface.
*
* @param o object to be compared for equality with this map entry
* @return <tt>true</tt> if the specified object is equal to this map
* entry
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry <tt>e</tt> is defined to be: <pre>
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* This ensures that <tt>e1.equals(e2)</tt> implies that
* <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
* <tt>e1</tt> and <tt>e2</tt>, as required by the general
* contract of <tt>Object.hashCode</tt>.
*
* @return the hash code value for this map entry
* @see Object#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on key.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with a null key.
*
* @param <K> the {@link Comparable} type of then map keys
* @param <V> the type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on key.
* @see Comparable
* @since 1.8
*/
public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getKey().compareTo(c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} in natural order on value.
*
* <p>The returned comparator is serializable and throws {@link
* NullPointerException} when comparing an entry with null values.
*
* @param <K> the type of the map keys
* @param <V> the {@link Comparable} type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order on value.
* @see Comparable
* @since 1.8
*/
public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> c1.getValue().compareTo(c2.getValue());
}
/**
* Returns a comparator that compares {@link Map.Entry} by key using the given
* {@link Comparator}.
*
* <p>The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param <K> the type of the map keys
* @param <V> the type of the map values
* @param cmp the key {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the key.
* @since 1.8
*/
public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
Objects.requireNonNull(cmp);
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} by value using the given
* {@link Comparator}.
*
* <p>The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param <K> the type of the map keys
* @param <V> the type of the map values
* @param cmp the value {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the value.
* @since 1.8
*/
public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
Objects.requireNonNull(cmp);
return (Comparator<Map.Entry<K, V>> & Serializable)
(c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
}
}7、getOrDefault：底层提供了值为空时，获取默认值的方法
default V getOrDefault(Object key, V defaultValue) {
V v;
return (((v = get(key)) != null) || containsKey(key))
? v
: defaultValue;
}8、putIfAbsent：如果原有值不存在，则将新值设置进去
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}) associates it with the given value and returns
* {@code null}, else returns the current value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code
* map}:
*
* <pre> {@code
* V v = map.get(key);
* if (v == null)
* v = map.put(key, value);
*
* return v;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default V putIfAbsent(K key, V value) {
V v = get(key);
if (v == null) {
v = put(key, value);
}
return v;
}9、remove(Object key, Object value)：key-value同时相等时进行删除。如果key不存在，则不操作，并返回false。
只有实际删除了，才返回true
/**
* Removes the entry for the specified key only if it is currently
* mapped to the specified value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.remove(key);
* return true;
* } else
* return false;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param value value expected to be associated with the specified key
* @return {@code true} if the value was removed
* @throws UnsupportedOperationException if the {@code remove} operation
* is not supported by this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @since 1.8
*/
default boolean remove(Object key, Object value) {
Object curValue = get(key);
if (!Objects.equals(curValue, value) ||
(curValue == null && !containsKey(key))) {
return false;
}
remove(key);
return true;
}10、replace(K key, V oldValue, V newValue)：校验key-oldValue匹配后，且key存在之后；才设置新的newValue
/**
* Replaces the entry for the specified key only if currently
* mapped to the specified value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.put(key, newValue);
* return true;
* } else
* return false;
* }</pre>
*
* The default implementation does not throw NullPointerException
* for maps that do not support null values if oldValue is null unless
* newValue is also null.
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param oldValue value expected to be associated with the specified key
* @param newValue value to be associated with the specified key
* @return {@code true} if the value was replaced
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of a specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if a specified key or newValue is null,
* and this map does not permit null keys or values
* @throws NullPointerException if oldValue is null and this map does not
* permit null values
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws IllegalArgumentException if some property of a specified key
* or value prevents it from being stored in this map
* @since 1.8
*/
default boolean replace(K key, V oldValue, V newValue) {
Object curValue = get(key);
if (!Objects.equals(curValue, oldValue) ||
(curValue == null && !containsKey(key))) {
return false;
}
put(key, newValue);
return true;
}11、replace(K key, V value)：值存在或者键存在，则替换新的value
/**
* Replaces the entry for the specified key only if it is
* currently mapped to some value.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key)) {
* return map.put(key, value);
* } else
* return null;
* }</pre>
*
* <p>The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param key key with which the specified value is associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* @since 1.8
*/
default V replace(K key, V value) {
V curValue;
if (((curValue = get(key)) != null) || containsKey(key)) {
curValue = put(key, value);
}
return curValue;
}
