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/*
* Copyright (c) 1997, 2026, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package java.util;
import jdk.internal.javac.PreviewFeature;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Function;
import java.io.Serializable;
/**
* An object that maps keys to values. A map cannot contain duplicate keys;
* each key can map to at most one value.
*
* <p>This interface takes the place of the {@code Dictionary} class, which
* was a totally abstract class rather than an interface.
*
* <p>The {@code Map} interface provides three <i>collection views</i>, which
* allow a map's contents to be viewed as a set of keys, collection of values,
* or set of key-value mappings. The <i>order</i> of a map is defined as
* the order in which the iterators on the map's collection views return their
* elements. Some map implementations, like the {@code TreeMap} class, make
* specific guarantees as to their encounter order; others, like the
* {@code HashMap} class, do not. Maps with a defined
* <a href="SequencedCollection.html#encounter">encounter order</a>
* are generally subtypes of the {@link SequencedMap} interface.
*
* <p>Note: great care must be exercised if mutable objects are used as map
* keys. The behavior of a map is not specified if the value of an object is
* changed in a manner that affects {@code equals} comparisons while the
* object is a key in the map. A special case of this prohibition is that it
* is not permissible for a map to contain itself as a key. While it is
* permissible for a map to contain itself as a value, extreme caution is
* advised: the {@code equals} and {@code hashCode} methods are no longer
* well defined on such a map.
*
* <p>All general-purpose map implementation classes should provide two
* "standard" constructors: a void (no arguments) constructor which creates an
* empty map, and a constructor with a single argument of type {@code Map},
* which creates a new map with the same key-value mappings as its argument.
* In effect, the latter constructor allows the user to copy any map,
* producing an equivalent map of the desired class. There is no way to
* enforce this recommendation (as interfaces cannot contain constructors) but
* all of the general-purpose map implementations in the JDK comply.
*
* <p>The "destructive" methods contained in this interface, that is, the
* methods that modify the map on which they operate, are specified to throw
* {@code UnsupportedOperationException} if this map does not support the
* operation. If this is the case, these methods may, but are not required
* to, throw an {@code UnsupportedOperationException} if the invocation would
* have no effect on the map. For example, invoking the {@link #putAll(Map)}
* method on an unmodifiable map may, but is not required to, throw the
* exception if the map whose mappings are to be "superimposed" is empty.
*
* <p>Some map implementations have restrictions on the keys and values they
* may contain. For example, some implementations prohibit null keys and
* values, and some have restrictions on the types of their keys. Attempting
* to insert an ineligible key or value throws an unchecked exception,
* typically {@code NullPointerException} or {@code ClassCastException}.
* Attempting to query the presence of an ineligible key or value may throw an
* exception, or it may simply return false; some implementations will exhibit
* the former behavior and some will exhibit the latter. More generally,
* attempting an operation on an ineligible key or value whose completion
* would not result in the insertion of an ineligible element into the map may
* throw an exception or it may succeed, at the option of the implementation.
* Such exceptions are marked as "optional" in the specification for this
* interface.
*
* <p>Many methods in Collections Framework interfaces are defined
* in terms of the {@link Object#equals(Object) equals} method. For
* example, the specification for the {@link #containsKey(Object)
* containsKey(Object key)} method says: "returns {@code true} if and
* only if this map contains a mapping for a key {@code k} such that
* {@code (key==null ? k==null : key.equals(k))}." This specification should
* <i>not</i> be construed to imply that invoking {@code Map.containsKey}
* with a non-null argument {@code key} will cause {@code key.equals(k)} to
* be invoked for any key {@code k}. Implementations are free to
* implement optimizations whereby the {@code equals} invocation is avoided,
* for example, by first comparing the hash codes of the two keys. (The
* {@link Object#hashCode()} specification guarantees that two objects with
* unequal hash codes cannot be equal.) More generally, implementations of
* the various Collections Framework interfaces are free to take advantage of
* the specified behavior of underlying {@link Object} methods wherever the
* implementor deems it appropriate.
*
* <p>Some map operations which perform recursive traversal of the map may fail
* with an exception for self-referential instances where the map directly or
* indirectly contains itself. This includes the {@code clone()},
* {@code equals()}, {@code hashCode()} and {@code toString()} methods.
* Implementations may optionally handle the self-referential scenario, however
* most current implementations do not do so.
*
* <h2><a id="unmodifiable">Unmodifiable Maps</a></h2>
* <p>The {@link Map#of() Map.of},
* {@link Map#ofEntries(Map.Entry...) Map.ofEntries},
* {@link Map#copyOf Map.copyOf}, and
* {@link Map#ofLazy(Set, Function)}
* static factory methods provide a convenient way to create unmodifiable maps.
* The {@code Map}
* instances created by these methods have the following characteristics:
*
* <ul>
* <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Keys and values
* cannot be added, removed, or updated. Calling any mutator method on the Map
* will always cause {@code UnsupportedOperationException} to be thrown.
* However, if the contained keys or values are themselves mutable, this may cause the
* Map to behave inconsistently or its contents to appear to change.
* <li>They disallow {@code null} keys and values. Attempts to create them with
* {@code null} keys or values result in {@code NullPointerException}.
* <li>Unless otherwise specified, they are serializable if all keys and values
* are serializable.
* <li>They reject duplicate keys at creation time. Duplicate keys
* passed to a static factory method result in {@code IllegalArgumentException}.
* <li>The iteration order of mappings is unspecified and is subject to change.
* <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
* Programmers should treat instances that are {@linkplain #equals(Object) equal}
* as interchangeable and should not use them for synchronization, or
* unpredictable behavior may occur. For example, in a future release,
* synchronization may fail. Callers should make no assumptions
* about the identity of the returned instances. Factories are free to
* create new instances or reuse existing ones.
* <li>They are serialized as specified on the
* <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
* page.
* </ul>
*
* <p>This interface is a member of the
* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
* Java Collections Framework</a>.
*
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
*
* @author Josh Bloch
* @see HashMap
* @see TreeMap
* @see Hashtable
* @see SortedMap
* @see Collection
* @see Set
* @since 1.2
*/
public interface Map<K, V> {
// Query Operations
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than {@code Integer.MAX_VALUE} elements, returns
* {@code Integer.MAX_VALUE}.
*
* @return the number of key-value mappings in this map
*/
int size();
/**
* Returns {@code true} if this map contains no key-value mappings.
*
* @return {@code true} if this map contains no key-value mappings
*/
boolean isEmpty();
/**
* Returns {@code true} if this map contains a mapping for the specified
* key. More formally, returns {@code true} if and only if
* this map contains a mapping for a key {@code k} such that
* {@code Objects.equals(key, k)}. (There can be
* at most one such mapping.)
*
* @param key key whose presence in this map is to be tested
* @return {@code true} if this map contains a mapping for the specified
* key
* @throws ClassCastException if the key is of an inappropriate type for
* this map ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys ({@linkplain Collection##optional-restrictions optional})
*/
boolean containsKey(Object key);
/**
* Returns {@code true} if this map maps one or more keys to the
* specified value. More formally, returns {@code true} if and only if
* this map contains at least one mapping to a value {@code v} such that
* {@code Objects.equals(value, v)}. This operation
* will probably require time linear in the map size for most
* implementations of the {@code Map} interface.
*
* @param value value whose presence in this map is to be tested
* @return {@code true} 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 ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified value is null and this
* map does not permit null values ({@linkplain Collection##optional-restrictions optional})
*/
boolean containsValue(Object value);
/**
* 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 Objects.equals(key, 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 ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys ({@linkplain Collection##optional-restrictions optional})
*/
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
* {@code m} is said to contain a mapping for a key {@code k} if and only
* if {@link #containsKey(Object) m.containsKey(k)} would return
* {@code true}.)
*
* @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 {@code key}, or
* {@code null} if there was no mapping for {@code key}.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with {@code key},
* if the implementation supports {@code null} values.)
* @throws UnsupportedOperationException if the {@code put} 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 {@code k} to value {@code v} such that
* {@code Objects.equals(key, k)}, 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 {@code null} if the map contained no mapping for the key.
*
* <p>If this map permits null values, then a return value of
* {@code null} 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 {@code null}.
*
* <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 {@code key}, or
* {@code null} if there was no mapping for {@code key}.
* @throws UnsupportedOperationException if the {@code remove} operation
* is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
* this map ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key is null and this
* map does not permit null keys ({@linkplain Collection##optional-restrictions optional})
*/
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 {@code k} to value {@code v} in the
* specified map. The behavior of this operation is undefined if the specified map
* is modified while the operation is in progress. If the specified map has a defined
* <a href="SequencedCollection.html#encounter">encounter order</a>,
* processing of its mappings generally occurs in that order.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the {@code putAll} 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 {@code clear} operation
* is not supported by this map
*/
void clear();
// Views
/**
* 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 {@code remove} operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* {@code Iterator.remove}, {@code Set.remove},
* {@code removeAll}, {@code retainAll}, and {@code clear}
* operations. It does not support the {@code add} or {@code addAll}
* 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 {@code remove} operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the {@code Iterator.remove},
* {@code Collection.remove}, {@code removeAll},
* {@code retainAll} and {@code clear} operations. It does not
* support the {@code add} or {@code addAll} operations.
*
* @return a collection view of the values contained in this map
*/
Collection<V> values();
/**
* 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 {@code remove} operation, or through the
* {@code setValue} 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 {@code Iterator.remove},
* {@code Set.remove}, {@code removeAll}, {@code retainAll} and
* {@code clear} operations. It does not support the
* {@code add} or {@code addAll} operations.
*
* @return a set view of the mappings contained in this map
*/
Set<Map.Entry<K, V>> entrySet();
/**
* A map entry (key-value pair). The Entry may be unmodifiable, or the
* value may be modifiable if the optional {@code setValue} method is
* implemented. The Entry may be independent of any map, or it may represent
* an entry of the entry-set view of a map.
* <p>
* An Entry maintains a connection to its underlying map if the Entry was obtained by
* iterating the {@link Map#entrySet} view of a map, either explicitly by using an
* {@link Iterator} or implicitly via the enhanced {@code for} statement. This connection
* to the backing map is valid <i>only</i> during iteration of the entry-set view. During
* the iteration, if supported by the backing map, a change to an Entry's value via
* the {@link Map.Entry#setValue setValue} method will be visible in the backing map.
* The behavior of such an Entry is undefined outside of iteration of the map's entry-set
* view. It is also undefined if the backing map has been modified after the Entry was
* returned by the iterator, except through the {@code setValue} method. In addition,
* a change to the value of a mapping in the backing map might or might not be
* visible in the corresponding Entry of the entry-set view.
* <p>
* An Entry may also be obtained from a map's entry-set view by other means, for
* example, using the
* {@link Set#parallelStream parallelStream},
* {@link Set#stream stream},
* {@link Set#spliterator spliterator} methods,
* any of the
* {@link Set#toArray toArray} overloads,
* or by copying the entry-set view into another collection. It is unspecified whether
* the obtained Entry instances are connected to the underlying map, whether changes
* to such an Entry will affect the underlying the map and vice-versa, and whether
* such an Entry supports the optional {@link Map.Entry#setValue setValue} method.
* <p>
* In addition, an Entry may be obtained directly from a map, for example via calls
* to methods directly on the {@link NavigableMap} interface. An entry thus obtained
* is generally not connected to the map and is an unmodifiable snapshot of the mapping
* as of the time of the call. Such an Entry also does not generally support the
* {@code setValue} method.
* <p>
* An Entry obtained by direct construction of the {@link AbstractMap.SimpleEntry}
* or {@link AbstractMap.SimpleImmutableEntry} classes or from a call to the
* {@link Map#entry Map.entry} or {@link Map.Entry#copyOf Map.Entry.copyOf} methods
* is not connected to any map.
*
* @apiNote
* The exact behavior of Entry instances obtained from a map's entry-set view other than
* via iteration varies across different map implementations; some are connected to the
* backing map, and some are not. To guarantee that an Entry is disconnected from its
* backing map, use the {@link Map.Entry#copyOf copyOf} method. For example, the following
* creates a snapshot of a map's entries that is guaranteed not to change even if the
* original map is modified:
* <pre> {@code
* var entries = map.entrySet().stream().map(Map.Entry::copyOf).toList()
* }</pre>
*
* @param <K> the type of the key
* @param <V> the type of the value
*
* @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
* {@code remove} 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 {@code remove} operation).
*
* @param value new value to be stored in this entry
* @return old value corresponding to the entry
* @throws UnsupportedOperationException if the {@code put} 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 {@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<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 {@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
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry {@code e} is defined to be: <pre>
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* 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 {@code Object.hashCode}.
*
* @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());
}
/**
* Returns a copy of the given {@code Map.Entry}. The returned instance is not
* associated with any map. The returned instance has the same characteristics
* as instances returned by the {@link Map#entry Map::entry} method.
*
* @apiNote
* An instance obtained from a map's entry-set view has a connection to that map.
* The {@code copyOf} method may be used to create a {@code Map.Entry} instance,
* containing the same key and value, that is independent of any map.
*
* @implNote
* If the given entry was obtained from a call to {@code copyOf} or {@code Map::entry},
* calling {@code copyOf} will generally not create another copy.
*
* @param <K> the type of the key
* @param <V> the type of the value
* @param e the entry to be copied
* @return a map entry equal to the given entry
* @throws NullPointerException if e is null or if either of its key or value is null
* @since 17
*/
@SuppressWarnings("unchecked")
public static <K, V> Map.Entry<K, V> copyOf(Map.Entry<? extends K, ? extends V> e) {
Objects.requireNonNull(e);
if (e instanceof KeyValueHolder) {
return (Map.Entry<K, V>) e;
} else {
return Map.entry(e.getKey(), e.getValue());
}
}
}
// Comparison and hashing
/**
* Compares the specified object with this map for equality. Returns
* {@code true} if the given object is also a map and the two maps
* represent the same mappings. More formally, two maps {@code m1} and
* {@code m2} represent the same mappings if
* {@code m1.entrySet().equals(m2.entrySet())}. This ensures that the
* {@code equals} method works properly across different implementations
* of the {@code Map} interface.
*
* @param o object to be compared for equality with this map
* @return {@code true} if the specified object is equal to this map
*/
boolean equals(Object o);
/**
* 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
* {@code entrySet()} view. This ensures that {@code m1.equals(m2)}
* implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
* {@code m1} and {@code m2}, as required by the general contract of
* {@link Object#hashCode}.
*
* @return the hash code value for this map
* @see Map.Entry#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
// Defaultable methods
/**
* Returns the value to which the specified key is mapped, or
* {@code defaultValue} if this map contains no mapping for the key.
*
* @implSpec
* 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 the key whose associated value is to be returned
* @param defaultValue the default mapping of the key
* @return the value to which the specified key is mapped, or
* {@code defaultValue} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys ({@linkplain Collection##optional-restrictions optional})
* @since 1.8
*/
default V getOrDefault(Object key, V defaultValue) {
V v;
return (((v = get(key)) != null) || containsKey(key))
? v
: defaultValue;
}
/**
* Performs the given action for each entry in this map until all entries
* have been processed or the action throws an exception. Unless
* otherwise specified by the implementing class, actions are performed in
* the order of entry set iteration (if an iteration order is specified.)
* Exceptions thrown by the action are relayed to the caller.
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
* <pre> {@code
* for (Map.Entry<K, V> entry : map.entrySet())
* action.accept(entry.getKey(), entry.getValue());
* }</pre>
*
* 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 action The action to be performed for each entry
* @throws NullPointerException if the specified action is null
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
default void forEach(BiConsumer<? super K, ? super V> action) {
Objects.requireNonNull(action);
for (Map.Entry<K, V> entry : entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
action.accept(k, v);
}
}
/**
* Replaces each entry's value with the result of invoking the given
* function on that entry until all entries have been processed or the
* function throws an exception (optional operation). Exceptions thrown
* by the function are relayed to the caller.
*
* @implSpec
* <p>The default implementation is equivalent to, for this {@code map}:
* <pre> {@code
* for (Map.Entry<K, V> entry : map.entrySet())
* entry.setValue(function.apply(entry.getKey(), entry.getValue()));
* }</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 function the function to apply to each entry
* @throws UnsupportedOperationException if the {@code replaceAll} operation
* is not supported by this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws ClassCastException if the class of a replacement value
* prevents it from being stored in this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified function is null, or if a
* replacement value is null and this map does not permit null values
* ({@linkplain Collection##optional-restrictions optional})
* @throws IllegalArgumentException if some property of a replacement value
* prevents it from being stored in this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
Objects.requireNonNull(function);
for (Map.Entry<K, V> entry : entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
// ise thrown from function is not a cme.
v = function.apply(k, v);
try {
entry.setValue(v);
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
throw new ConcurrentModificationException(ise);
}
}
}
/**
* 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 (optional operation).
*
* @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 putIfAbsent} operation
* is not supported by this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* ({@linkplain Collection##optional-restrictions optional})
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
* ({@linkplain Collection##optional-restrictions optional})
* @since 1.8
*/
default V putIfAbsent(K key, V value) {
V v = get(key);
if (v == null) {
v = put(key, value);
}
return v;
}
/**
* Removes the entry for the specified key only if it is currently
* mapped to the specified value (optional operation).
*
* @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
* ({@linkplain Collection##optional-restrictions optional})
* @throws ClassCastException if the key or value is of an inappropriate
* type for this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws NullPointerException if the specified key or value is null,
* and this map does not permit null keys or values
* ({@linkplain Collection##optional-restrictions optional})
* @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;
}
/**
* Replaces the entry for the specified key only if currently
* mapped to the specified value (optional operation).
*
* @implSpec
* The default implementation is equivalent to, for this {@code map}:
*
* <pre> {@code
* if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
* 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 replace} operation
* is not supported by this map
* ({@linkplain Collection##optional-restrictions optional})
* @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 ({@linkplain Collection##optional-restrictions optional})
* @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;
}
/**
* Replaces the entry for the specified key only if it is
* currently mapped to some value (optional operation).
*
* @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 replace} operation
* is not supported by this map
* ({@linkplain Collection##optional-restrictions optional})
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* ({@linkplain Collection##optional-restrictions optional})
* @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;
}
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}), attempts to compute its value using the given mapping
* function and enters it into this map unless {@code null} (optional operation).
*
* <p>If the mapping function returns {@code null}, no mapping is recorded.
* If the mapping function itself throws an (unchecked) exception, the
* exception is rethrown, and no mapping is recorded. The most
* common usage is to construct a new object serving as an initial