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Interface java.util.Map
- public interface Map
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 Dictionary, which was a totally abstract
class rather than an interface. This Interface is implemented by HashMap,
ArrayMap, TreeMap and Hashtable.
The Map interface provides three Collection views, which allow
a Map's contents to be viewed as a Collection of keys, values, or
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 TreeMap and ArrayMap, make specific
guarantees as to their order; others, like HashMap, do not.
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.
- Since:
- JDK1.2
- See Also:
- HashMap, ArrayMap, TreeMap, Hashtable, Collection, Set
Map.Entry- A Map entry (key-value pair).
clear()
- Removes all mappings from this Map (optional operation).
containsKey(Object)
- Returns true if this Map contains a mapping for the specified key.
containsValue(Object)
- Returns true if this Map maps one or more keys to the specified value.
entries()
- Returns a Collection view of the mappings contained in this Map.
equals(Object)
- Compares the specified Object with this Map for equality.
get(Object)
- Returns the value to which this Map maps the specified key.
hashCode()
- Returns the hash code value for this Map.
isEmpty()
- Returns true if this Map contains no key-value mappings.
keySet()
- Returns a Set view of the keys contained in this Map.
put(Object, Object)
- Associates the specified value with the specified key in this Map
(optional operation).
putAll(Map)
- Copies all of the mappings from the specified Map to this Map
(optional operation).
remove(Object)
- Removes the mapping for this key from this Map if present (optional
operation).
size()
- Returns the number of key-value mappings in this Map.
values()
- Returns a Collection view of the values contained in this Map.
size
public abstract int size()
- Returns the number of key-value mappings in this Map.
isEmpty
public abstract boolean isEmpty()
- Returns true if this Map contains no key-value mappings.
containsKey
public abstract boolean containsKey(Object key)
- Returns true if this Map contains a mapping for the specified key.
- Parameters:
- key - key whose presence in this Map is to be tested.
- Throws:
ClassCastException
- key is of an inappropriate type for
this Map.
- Throws:
NullPointerException
- key is null and this Map does not
not permit null keys.
containsValue
public abstract boolean containsValue(Object value)
- 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
(value==null ? v==null : value.equals(v))
.
This operation will probably require time linear in the Map size for
most implementations of Map.
- Parameters:
- value - value whose presence in this Map is to be tested.
get
public abstract Object get(Object key)
- Returns the value to which this Map maps the specified key.
Returns null if the Map contains no mapping for this key. 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.
- Parameters:
- key - key whose associated value is to be returned.
- Throws:
ClassCastException
- key is of an inappropriate type for
this Map.
- Throws:
NullPointerException
- key is null and this Map does not
not permit null keys.
- See Also:
- containsKey
put
public abstract Object put(Object key,
Object value)
- Associates the specified value with the specified key in this Map
(optional operation). If the Map previously contained a mapping for
this key, the old value is replaced.
- Parameters:
- key - key with which the specified value is to be associated.
- value - value to be associated with the specified key.
- Returns:
- previous value associated with specified key, or null if there
was no mapping for key. A null return can also indicate that
the Map previously associated null with the specified key,
if the implementation supports null values.
- Throws:
UnsupportedOperationException
- put operation is not
supported by this Map.
- Throws:
ClassCastException
- class of the specified key or value
prevents it from being stored in this Map.
- Throws:
IllegalArgumentException
- some aspect of this key or value
prevents it from being stored in this Map.
- Throws:
NullPointerException
- this Map does not permit null keys
or values, and the specified key or value is null.
remove
public abstract Object remove(Object key)
- Removes the mapping for this key from this Map if present (optional
operation).
- Parameters:
- key - key whose mapping is to be removed from the Map.
- Returns:
- previous value associated with specified key, or null if there
was no mapping for key. A null return can also indicate that
the Map previously associated null with the specified key,
if the implementation supports null values.
- Throws:
UnsupportedOperationException
- remove is not supported
by this Map.
putAll
public abstract void putAll(Map t)
- Copies all of the mappings from the specified Map to this Map
(optional operation). These mappings will replace any mappings that
this Map had for any of the keys currently in the specified Map.
- Parameters:
- t - Mappings to be stored in this Map.
- Throws:
UnsupportedOperationException
- putAll is not supported
by this Map.
- Throws:
ClassCastException
- class of a key or value in the specified
Map prevents it from being stored in this Map.
- Throws:
IllegalArgumentException
- some aspect of a key or value in the
specified Map prevents it from being stored in this Map.
- Throws:
NullPointerException
- this Map does not permit null keys
or values, and the specified key or value is null.
clear
public abstract void clear()
- Removes all mappings from this Map (optional operation).
- Throws:
UnsupportedOperationException
- clear is not supported
by this Map.
keySet
public abstract Set keySet()
- 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 while an iteration over
the Set is in progress, 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.
values
public abstract Collection values()
- 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 while an iteration over the Collection is in progress,
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.
entries
public abstract Collection entries()
- Returns a Collection view of the mappings contained in this Map.
Each element in the returned collection is a Map.Entry. 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 while an iteration over the Collection is in
progress, 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.
equals
public abstract boolean equals(Object o)
- 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
t1
and t2
represent the same mappings
if t1.keySet().equals(t2.keySet())
and for every
key k
in t1.keySet()
,
(t1.get(k)==null ? t2.get(k)==null : t1.get(k).equals(t2.get(k)))
. This ensures that the equals method works properly across
different implementations of the Map interface.
- Parameters:
- o - Object to be compared for equality with this Map.
- Returns:
- true if the specified Object is equal to this Map.
- Overrides:
- equals in class Object
hashCode
public abstract int hashCode()
- Returns the hash code value for this Map. The hash code of a Map
is defined to be the sum of the hashCodes of each Entry in the Map's
entries view. This ensures that
t1.equals(t2)
implies
that t1.hashCode()==t2.hashCode()
for any two Maps
t1
and t2
, as required by the general
contract of Object.hashCode.
- Overrides:
- hashCode in class Object
- See Also:
- hashCode, hashCode, equals, equals
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