AbstractMappublic abstract class AbstractMap extends Object implements Map| 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 methods 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. |
| Fields Summary |
|---|
| volatile transient Set | keySetEach of these fields are initialized to contain an instance of the
appropriate view the first time this view is requested. The views are
stateless, so there's no reason to create more than one of each. | | volatile transient Collection | values |
| Constructors Summary |
|---|
protected AbstractMap()Sole constructor. (For invocation by subclass constructors, typically
implicit.)
|
| Methods Summary |
|---|
| public void | clear()Removes all mappings from this map (optional operation).
This implementation calls entrySet().clear().
Note that this implementation throws an
UnsupportedOperationException if the entrySet
does not support the clear operation.
entrySet().clear();
| | protected java.lang.Object | clone()Returns a shallow copy of this AbstractMap instance: the keys
and values themselves are not cloned.
AbstractMap<K,V> result = (AbstractMap<K,V>)super.clone();
result.keySet = null;
result.values = null;
return result;
| | public boolean | containsKey(java.lang.Object key)Returns true if this map contains a mapping for the specified
key.
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.
Iterator<Map.Entry<K,V>> i = entrySet().iterator();
if (key==null) {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (e.getKey()==null)
return true;
}
} else {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (key.equals(e.getKey()))
return true;
}
}
return false;
| | public boolean | containsValue(java.lang.Object value)Returns true if this map maps one or more keys to this 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.
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.
Iterator<Entry<K,V>> i = entrySet().iterator();
if (value==null) {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (e.getValue()==null)
return true;
}
} else {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (value.equals(e.getValue()))
return true;
}
}
return false;
| | public abstract java.util.Set | entrySet()Returns a set view of the mappings contained in this map. Each element
in this 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 an iteration over the set is in progress, the results of
the iteration are undefined.) The set supports element removal, which
removes the corresponding entry from the map, via the
Iterator.remove, Set.remove, removeAll,
retainAll and clear operations. It does not support
the add or addAll operations.
| | 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.
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 set; 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.
if (o == this)
return true;
if (!(o instanceof Map))
return false;
Map<K,V> t = (Map<K,V>) o;
if (t.size() != size())
return false;
try {
Iterator<Entry<K,V>> i = entrySet().iterator();
while (i.hasNext()) {
Entry<K,V> e = i.next();
K key = e.getKey();
V value = e.getValue();
if (value == null) {
if (!(t.get(key)==null && t.containsKey(key)))
return false;
} else {
if (!value.equals(t.get(key)))
return false;
}
}
} catch(ClassCastException unused) {
return false;
} catch(NullPointerException unused) {
return false;
}
return true;
| | public V | 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.
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.
Iterator<Entry<K,V>> i = entrySet().iterator();
if (key==null) {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (e.getKey()==null)
return e.getValue();
}
} else {
while (i.hasNext()) {
Entry<K,V> e = i.next();
if (key.equals(e.getKey()))
return e.getValue();
}
}
return null;
| | 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 hash codes 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.
This implementation iterates over entrySet(), calling
hashCode on each element (entry) in the Collection, and adding
up the results.
int h = 0;
Iterator<Entry<K,V>> i = entrySet().iterator();
while (i.hasNext())
h += i.next().hashCode();
return h;
| | public boolean | isEmpty()Returns true if this map contains no key-value mappings.
This implementation returns size() == 0.
return size() == 0;
| | public java.util.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 an iteration over
the Set is in progress, the results of the iteration are undefined.)
The Set supports element removal, which removes the corresponding entry
from the map, via the Iterator.remove, Set.remove, removeAll
retainAll, and clear operations. It does not support the add or
addAll operations.
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.
if (keySet == null) {
keySet = new AbstractSet<K>() {
public Iterator<K> iterator() {
return new Iterator<K>() {
private Iterator<Entry<K,V>> i = entrySet().iterator();
public boolean hasNext() {
return i.hasNext();
}
public K next() {
return i.next().getKey();
}
public void remove() {
i.remove();
}
};
}
public int size() {
return AbstractMap.this.size();
}
public boolean contains(Object k) {
return AbstractMap.this.containsKey(k);
}
};
}
return keySet;
| | public V | put(K key, V 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.
This implementation always throws an
UnsupportedOperationException.
throw new UnsupportedOperationException();
| | public void | putAll(java.util.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.
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.
Iterator<? extends Entry<? extends K, ? extends V>> i = t.entrySet().iterator();
while (i.hasNext()) {
Entry<? extends K, ? extends V> e = i.next();
put(e.getKey(), e.getValue());
}
| | public V | remove(java.lang.Object key)Removes the mapping for this key from this map if present (optional
operation).
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.
Iterator<Entry<K,V>> i = entrySet().iterator();
Entry<K,V> correctEntry = null;
if (key==null) {
while (correctEntry==null && i.hasNext()) {
Entry<K,V> e = i.next();
if (e.getKey()==null)
correctEntry = e;
}
} else {
while (correctEntry==null && i.hasNext()) {
Entry<K,V> e = i.next();
if (key.equals(e.getKey()))
correctEntry = e;
}
}
V oldValue = null;
if (correctEntry !=null) {
oldValue = correctEntry.getValue();
i.remove();
}
return oldValue;
| | public int | size()Returns the number of key-value mappings in this map. If the map
contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
This implementation returns entrySet().size().
return entrySet().size();
| | public java.lang.String | toString()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).
This implementation creates an empty string buffer, appends a left
brace, and iterates over the map's entrySet view, appending
the string representation of each map.entry in turn. After
appending each entry except the last, the string ", " is
appended. Finally a right brace is appended. A string is obtained
from the stringbuffer, and returned.
StringBuffer buf = new StringBuffer();
buf.append("{");
Iterator<Entry<K,V>> i = entrySet().iterator();
boolean hasNext = i.hasNext();
while (hasNext) {
Entry<K,V> e = i.next();
K key = e.getKey();
V value = e.getValue();
if (key == this)
buf.append("(this Map)");
else
buf.append(key);
buf.append("=");
if (value == this)
buf.append("(this Map)");
else
buf.append(value);
hasNext = i.hasNext();
if (hasNext)
buf.append(", ");
}
buf.append("}");
return buf.toString();
| | public java.util.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 an
iteration over the collection is in progress, the results of the
iteration are undefined.) The collection supports element removal,
which removes the corresponding entry from the map, via the
Iterator.remove, Collection.remove,
removeAll, retainAll and clear operations.
It does not support the add or addAll operations.
This implementation returns a collection that subclasses abstract
collection. 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.
if (values == null) {
values = new AbstractCollection<V>() {
public Iterator<V> iterator() {
return new Iterator<V>() {
private Iterator<Entry<K,V>> i = entrySet().iterator();
public boolean hasNext() {
return i.hasNext();
}
public V next() {
return i.next().getValue();
}
public void remove() {
i.remove();
}
};
}
public int size() {
return AbstractMap.this.size();
}
public boolean contains(Object v) {
return AbstractMap.this.containsValue(v);
}
};
}
return values;
|
|
