Interface java.util.Map
- Subinterfaces:
- SortedMap
- Implementing Classes:
- AbstractMap, java.util.Collections.SynchronizedMap, java.util.Collections.UnmodifiableMap, HashMap, WeakHashMap, Hashtable
- public abstract 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,
TreeMap and Hashtable.
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 TreeMap, make specific guarantees as to
their order; others, like HashMap, 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
while the object is in use as a key in the 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.
- Since:
- JDK1.2
- Version:
- 1.25 06/29/98
- See Also:
- HashMap, TreeMap, Hashtable, SortedMap, Collection, Set
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Inner Class Summary
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static
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Map.Entry
A Map entry (key-value pair). |
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Method Summary
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void
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clear()
Removes all mappings from this Map (optional operation). |
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boolean
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containsKey(java.lang.Object key)
Returns true if this Map contains a mapping for the specified key. |
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boolean
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containsValue(java.lang.Object value)
Returns true if this Map maps one or more keys to the specified value.
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Set
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entrySet()
Returns a Set view of the mappings contained in this Map. |
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boolean
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equals(java.lang.Object o)
Compares the specified Object with this Map for equality.
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java.lang.Object
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get(java.lang.Object key)
Returns the value to which this Map maps the specified key.
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int
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hashCode()
Returns the hash code value for this Map. |
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boolean
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isEmpty()
Returns true if this Map contains no key-value mappings. |
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Set
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keySet()
Returns a Set view of the keys contained in this Map. |
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void
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putAll(Map t)
Copies all of the mappings from the specified Map to this Map
(optional operation). |
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java.lang.Object
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put(java.lang.Object key,
java.lang.Object value)
Associates the specified value with the specified key in this Map
(optional operation). |
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java.lang.Object
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remove(java.lang.Object key)
Removes the mapping for this key from this Map if present (optional
operation). |
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int
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size()
Returns the number of key-value mappings in this Map. |
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Collection
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values()
Returns a Collection view of the values contained in this Map. |
size
public int size()
- Returns the number of key-value mappings in this Map.
isEmpty
public boolean isEmpty()
- Returns true if this Map contains no key-value mappings.
containsKey
public boolean containsKey(java.lang.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.
- NullPointerException - key is null and this Map does not
not permit null keys.
containsValue
public boolean containsValue(java.lang.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 java.lang.Object get(java.lang.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.
- NullPointerException - key is null and this Map does not
not permit null keys.
- See Also:
- containsKey(Object)
put
public java.lang.Object put(java.lang.Object key,
java.lang.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.
- ClassCastException - class of the specified key or value
prevents it from being stored in this Map.
- java.lang.IllegalArgumentException - some aspect of this key or value
prevents it from being stored in this Map.
- NullPointerException - this Map does not permit null keys
or values, and the specified key or value is null.
remove
public java.lang.Object remove(java.lang.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 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.
- ClassCastException - class of a key or value in the specified
Map prevents it from being stored in this Map.
- java.lang.IllegalArgumentException - some aspect of a key or value in the
specified Map prevents it from being stored in this Map.
- NullPointerException - this Map does not permit null keys
or values, and the specified key or value is null.
clear
public void clear()
- Removes all mappings from this Map (optional operation).
- Throws:
- UnsupportedOperationException - clear is not supported
by this Map.
keySet
public 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 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.
entrySet
public Set entrySet()
- Returns a Set view of the mappings contained in this Map. Each element
in the returned Set is a Map.Entry. 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.
equals
public boolean equals(java.lang.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 java.lang.Object
hashCode
public 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
entrySet 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 java.lang.Object
- See Also:
- Entry#hashCode(), hashCode(), equals(Object), Map#equals()