Returns true
if this map contains a mapping for the specified
key. More formally, returns true
if and only if
this map contains a mapping for a key k
such that
Objects.equals(key, k)
. (There can be
at most one such mapping.)
key whose presence in this map is to be tested
true
if this map contains a mapping for the specified
key
ClassCastException if the key is of an inappropriate type for this map (optional)
NullPointerException if the specified key is null and this map does not permit null keys (optional)
Returns the value to which the specified key is mapped,
or null
if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k
to a value v
such that
Objects.equals(key, k)
,
then this method returns v
; otherwise
it returns null
. (There can be at most one such mapping.)
If this map permits null values, then a return value of
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 null
. The #containsKey containsKey operation may be used to distinguish these two cases.
the key whose associated value is to be returned
the value to which the specified key is mapped, or
null
if this map contains no mapping for the key
ClassCastException if the key is of an inappropriate type for this map (optional)
NullPointerException if the specified key is null and this map does not permit null keys (optional)
Returns the value to which the specified key is mapped, or
defaultValue
if this map contains no mapping for the key.
the key whose associated value is to be returned
the default mapping of the key
the value to which the specified key is mapped, or
defaultValue
if this map contains no mapping for the key
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.
ClassCastException if the key is of an inappropriate type for this map (optional)
NullPointerException if the specified key is null and this map does not permit null keys (optional)
1.8
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
m
is said to contain a mapping for a key k
if and only
if #containsKey(Object) m.containsKey(k) would return
true
.)
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
,
if the implementation supports null
values.)
UnsupportedOperationException if the put
operation
is not supported by this map
ClassCastException if the class of the specified key or value prevents it from being stored in this map
NullPointerException if the specified key or value is null and this map does not permit null keys or values
IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map
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 #put(Object,Object) put(k, v) on this map once
for each mapping from key k
to value v
in the
specified map. The behavior of this operation is undefined if the
specified map is modified while the operation is in progress.
UnsupportedOperationException if the putAll
operation
is not supported by this map
ClassCastException if the class of a key or value in the specified map prevents it from being stored in this map
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
IllegalArgumentException if some property of a key or value in the specified map prevents it from being stored in this map
If the specified key is not already associated with a value (or is mapped
to null
) associates it with the given value and returns
null
, else returns the current value.
the previous value associated with the specified key, or
null
if there was no mapping for the key.
(A null
return can also indicate that the map
previously associated null
with the key,
if the implementation supports null values.)
The default implementation is equivalent to, for this * map
:
{@code V v = map.get(key); if (v == null) v = map.put(key, value); return v;}
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.
UnsupportedOperationException if the put
operation
is not supported by this map
(optional)
ClassCastException if the key or value is of an inappropriate type for this map (optional)
NullPointerException if the specified key or value is null, and this map does not permit null keys or values (optional)
IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map (optional)
1.8
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
This interface takes the place of the
Dictionary
class, which was a totally abstract class rather than an interface.The
Map
interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order 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 theTreeMap
class, make specific guarantees as to their order; others, like theHashMap
class, do not.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
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: theequals
andhashCode
methods are no longer well defined on such a map.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
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.The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException
if this map does not support the operation. If this is the case, these methods may, but are not required to, throw anUnsupportedOperationException
if the invocation would have no effect on the map. For example, invoking the #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.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
NullPointerException
orClassCastException
. 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.Many methods in Collections Framework interfaces are defined in terms of the Object#equals(Object) equals method. For example, the specification for the #containsKey(Object) containsKey(Object key) method says: "returns
true
if and only if this map contains a mapping for a keyk
such that(key==null ? k==null : key.equals(k))
." This specification should not be construed to imply that invokingMap.containsKey
with a non-null argumentkey
will causekey.equals(k)
to be invoked for any keyk
. Implementations are free to implement optimizations whereby theequals
invocation is avoided, for example, by first comparing the hash codes of the two keys. (The 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 Object methods wherever the implementor deems it appropriate.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
clone()
,equals()
,hashCode()
andtoString()
methods. Implementations may optionally handle the self-referential scenario, however most current implementations do not do so.Unmodifiable Maps The Map#of() Map.of, Map#ofEntries(Map.Entry...) Map.ofEntries, and Map#copyOf Map.copyOf static factory methods provide a convenient way to create unmodifiable maps. The
Map
instances created by these methods have the following characteristics:They are unmodifiable. Keys and values cannot be added, removed, or updated. Calling any mutator method on the Map will always cause
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. They disallownull
keys and values. Attempts to create them withnull
keys or values result inNullPointerException
. They are serializable if all keys and values are serializable. They reject duplicate keys at creation time. Duplicate keys passed to a static factory method result inIllegalArgumentException
. The iteration order of mappings is unspecified and is subject to change. They are value-based. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (==
), identity hash code, and synchronization) are unreliable and should be avoided. They are serialized as specified on the Serialized Form page.This interface 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
See
Since
1.2