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Class AbstractMap<K,V>
java.util.AbstractMap<K,V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Implemented Interfaces:
Direct Known Subclasses:
public abstract class AbstractMap<K,V>
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.
Since:
1.2
Author:
Josh Bloch, Neal Gafter
See Also:
Nested Class Summary |
static class |
An Entry maintaining a key and a value.
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static class |
An unmodifiable Entry maintaining a key and a value.
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Nested classes/interfaces inherited from interface java.util. Map |
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Constructor Summary |
protected |
Sole constructor.
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Method Summary |
void |
Removes all of the mappings from this map (optional operation).
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Returns a shallow copy of this AbstractMap instance: the keys
and values themselves are not cloned.
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boolean |
Returns true if this map contains a mapping for the specified
key.
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boolean |
Returns true if this map maps one or more keys to the
specified value.
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Returns a Set view of the mappings contained in this map.
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boolean |
Compares the specified object with this map for equality.
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Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
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int |
Returns the hash code value for this map.
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boolean |
Returns true if this map contains no key-value mappings.
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Returns a Set view of the keys contained in this map.
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Associates the specified value with the specified key in this map
(optional operation).
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void |
Copies all of the mappings from the specified map to this map
(optional operation).
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Removes the mapping for a key from this map if it is present
(optional operation).
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int |
Returns the number of key-value mappings in this map.
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Returns a string representation of this map.
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Returns a Collection view of the values contained in this map.
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Methods inherited from class java.lang. Object |
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Methods inherited from interface java.util. Map |
compute, computeIfAbsent, computeIfPresent, copyOf, entry, forEach, getOrDefault, merge, of, of, of, of, of, of, of, of, of, of, of, ofEntries, putIfAbsent, remove, replace, replace, replaceAll |
Sole constructor. (For invocation by subclass constructors, typically
implicit.)
Returns the number of key-value mappings in this map. If the
map contains more than
Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
Implementation Requirements:
This implementation returns entrySet().size().
Specified by:
Returns:
the number of key-value mappings in this map
public boolean isEmpty |
() |
Returns
true if this map contains no key-value mappings.
Implementation Requirements:
This implementation returns size() == 0.
Specified by:
Returns:
true if this map contains no key-value mappings
public boolean containsValue |
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Returns
true if this map maps one or more keys to the
specified value. More formally, returns
true if and only if
this map contains at least one mapping to a value
v such that
Objects.equals(value, v). This operation
will probably require time linear in the map size for most
implementations of the
Map interface.
Implementation Requirements:
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.
Specified by:
Parameters:
value - value whose presence in this map is to be tested
Returns:
true if this map maps one or more keys to the
specified value
Throws:
public boolean containsKey |
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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.)
Implementation Requirements:
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.
Specified by:
Parameters:
key - key whose presence in this map is to be tested
Returns:
true if this map contains a mapping for the specified
key
Throws:
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 operation may be used to distinguish these two cases.
Implementation Requirements:
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.
Specified by:
Parameters:
key - the key whose associated value is to be returned
Returns:
the value to which the specified key is mapped, or
null if this map contains no mapping for the key
Throws:
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
m.containsKey(k) would return
true.)
Implementation Requirements:
This implementation always throws an
UnsupportedOperationException.
Specified by:
Parameters:
key - key with which the specified value is to be associated
value - value to be associated with the specified key
Returns:
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.)
Throws:
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
k to value
v such that
Objects.equals(key, k), that mapping
is removed. (The map can contain at most one such mapping.)
Returns the value to which this map previously associated the key,
or null if the map contained no mapping for the key.
If this map permits null values, then a return value of
null does not necessarily indicate that the map
contained no mapping for the key; it's also possible that the map
explicitly mapped the key to null.
The map will not contain a mapping for the specified key once the
call returns.
Implementation Requirements:
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.
Specified by:
Parameters:
key - key whose mapping is to be removed from the map
Returns:
the previous value associated with key, or
null if there was no mapping for key.
Throws:
public void putAll |
( Map<? extends K,? extends V> m) |
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(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.
Implementation Requirements:
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.
Specified by:
Parameters:
m - mappings to be stored in this map
Throws:
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
Removes all of the mappings from this map (optional operation).
The map will be empty after this call returns.
Implementation Requirements:
This implementation calls
entrySet().clear().
Note that this implementation throws an
UnsupportedOperationException if the entrySet
does not support the clear operation.
Specified by:
Throws:
Returns a
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
remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove,
Set.remove,
removeAll,
retainAll, and
clear
operations. It does not support the
add or
addAll
operations.
Implementation Requirements:
This implementation returns a set that subclasses
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.
Specified by:
Returns:
a set view of the keys contained in this map
Returns a
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
remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the
Iterator.remove,
Collection.remove,
removeAll,
retainAll and
clear operations. It does not
support the
add or
addAll operations.
Implementation Requirements:
This implementation returns a collection that subclasses
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.
Specified by:
Returns:
a collection view of the values contained in this map
Description copied from interface:
Map
Returns a
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
remove operation, or through the
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
Iterator.remove,
Set.remove,
removeAll,
retainAll and
clear operations. It does not support the
add or
addAll operations.
Specified by:
Returns:
a set view of the mappings contained in this map
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.
Implementation Requirements:
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.
Specified by:
Overrides:
Parameters:
o - object to be compared for equality with this map
Returns:
true if the specified object is equal to this map
See Also:
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
Object.hashCode().
Implementation Requirements:
This implementation iterates over
entrySet(), calling
hashCode() on each element (entry) in the
set, and adding up the results.
Specified by:
Overrides:
Returns:
the hash code value for this map
See Also:
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
String.valueOf(Object).
Overrides:
Returns:
a string representation of this map
Returns a shallow copy of this
AbstractMap instance: the keys
and values themselves are not cloned.
Overrides:
Returns:
a shallow copy of this map
Throws:
CloneNotSupportedException - if the object's class does not
support the
Cloneable interface. Subclasses
that override the
clone method can also
throw this exception to indicate that an instance cannot
be cloned.
See Also:
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