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java.util

Class AbstractMap

java.lang.Object
|
+--java.util.AbstractMap

All Implemented Interfaces:

Map


public abstract class AbstractMap

extends Object

implements Map

An abstract implementation of Map to make it easier to create your own implementations. In order to create an unmodifiable Map, subclass AbstractMap and implement the entrySet (usually via an AbstractSet). To make it modifiable, also implement put, and have entrySet().iterator() support remove.

It is recommended that classes which extend this support at least the no-argument constructor, and a constructor which accepts another Map. Further methods in this class may be overridden if you have a more efficient implementation.

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Constructor Summary

AbstractMap()

The main constructor, for use by subclasses.

Method Summary

voidclear()

Remove all entries from this Map (optional operation).
java.lang.Objectclone()

Create a shallow copy of this Map, no keys or values are copied.
booleancontainsKey(java.lang.Object key)

Returns true if this contains a mapping for the given key.
booleancontainsValue(java.lang.Object value)

Returns true if this contains at least one mapping with the given value.
java.util.SetentrySet()

Returns a set view of the mappings in this Map.
booleanequals(java.lang.Object o)

Compares the specified object with this map for equality.
java.lang.Objectget(java.lang.Object key)

Returns the value mapped by the given key.
inthashCode()

Returns the hash code for this map.
booleanisEmpty()

Returns true if the map contains no mappings.
java.util.SetkeySet()

Returns a set view of this map's keys.
java.lang.Objectput(java.lang.Object key, java.lang.Object value)

Associates the given key to the given value (optional operation).
voidputAll(java.util.Map m)

Copies all entries of the given map to this one (optional operation).
java.lang.Objectremove(java.lang.Object key)

Removes the mapping for this key if present (optional operation).
intsize()

Returns the number of key-value mappings in the map.
java.lang.StringtoString()

Returns a String representation of this map.
java.util.Collectionvalues()

Returns a collection or bag view of this map's values.

Constructor Details

AbstractMap

protected AbstractMap()

The main constructor, for use by subclasses.


Method Details

clear

public void clear()

Remove all entries from this Map (optional operation). This default implementation calls entrySet().clear(). NOTE: If the entry set does not permit clearing, then this will fail, too. Subclasses often override this for efficiency. Your implementation of entrySet() should not call AbstractMap.clear unless you want an infinite loop.

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clone

protected Object clone()

Create a shallow copy of this Map, no keys or values are copied. The default implementation simply calls super.clone().

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containsKey

public boolean containsKey(java.lang.Object key)

Returns true if this contains a mapping for the given key. This implementation does a linear search, O(n), over the entrySet(), returning true if a match is found, false if the iteration ends. Many subclasses can implement this more efficiently.

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containsValue

public boolean containsValue(java.lang.Object value)

Returns true if this contains at least one mapping with the given value. This implementation does a linear search, O(n), over the entrySet(), returning true if a match is found, false if the iteration ends. A match is defined as (value == null ? v == null : value.equals(v)) Subclasses are unlikely to implement this more efficiently.

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entrySet

public Set entrySet()

Returns a set view of the mappings in this Map. Each element in the set must be an implementation of Map.Entry. The set is backed by the map, so that changes in one show up in the other. Modifications made while an iterator is in progress cause undefined behavior. If the set supports removal, these methods must be valid: Iterator.remove, Set.remove, removeAll, retainAll, and clear. Element addition is not supported via this set.

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equals

public boolean equals(java.lang.Object o)

Compares the specified object with this map for equality. Returns true if the other object is a Map with the same mappings, that is,
o instanceof Map && entrySet().equals(((Map) o).entrySet();

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get

public Object get(java.lang.Object key)

Returns the value mapped by the given key. Returns null if there is no mapping. However, in Maps that accept null values, you must rely on containsKey to determine if a mapping exists. This iteration takes linear time, searching entrySet().iterator() of the key. Many implementations override this method.

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hashCode

public int hashCode()

Returns the hash code for this map. As defined in Map, this is the sum of all hashcodes for each Map.Entry object in entrySet, or basically entrySet().hashCode().

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isEmpty

public boolean isEmpty()

Returns true if the map contains no mappings. This is implemented by size() == 0.

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keySet

public Set keySet()

Returns a set view of this map's keys. The set is backed by the map, so changes in one show up in the other. Modifications while an iteration is in progress produce undefined behavior. The set supports removal if entrySet() does, but does not support element addition.

This implementation creates an AbstractSet, where the iterator wraps the entrySet iterator, size defers to the Map's size, and contains defers to the Map's containsKey. The set is created on first use, and returned on subsequent uses, although since no synchronization occurs, there is a slight possibility of creating two sets.

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put

public Object put(java.lang.Object key, java.lang.Object value)

Associates the given key to the given value (optional operation). If the map already contains the key, its value is replaced. This implementation simply throws an UnsupportedOperationException. Be aware that in a map that permits null values, a null return does not always imply that the mapping was created.

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putAll

public void putAll(java.util.Map m)

Copies all entries of the given map to this one (optional operation). If the map already contains a key, its value is replaced. This implementation simply iterates over the map's entrySet(), calling put, so it is not supported if puts are not.

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remove

public Object remove(java.lang.Object key)

Removes the mapping for this key if present (optional operation). This implementation iterates over the entrySet searching for a matching key, at which point it calls the iterator's remove method. It returns the result of getValue() on the entry, if found, or null if no entry is found. Note that maps which permit null values may also return null if the key was removed. If the entrySet does not support removal, this will also fail. This is O(n), so many implementations override it for efficiency.

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size

public int size()

Returns the number of key-value mappings in the map. If there are more than Integer.MAX_VALUE mappings, return Integer.MAX_VALUE. This is implemented as entrySet().size().

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toString

public String toString()

Returns a String representation of this map. This is a listing of the map entries (which are specified in Map.Entry as being getKey() + "=" + getValue()), separated by a comma and space (", "), and surrounded by braces ('{' and '}'). This implementation uses a StringBuffer and iterates over the entrySet to build the String. Note that this can fail with an exception if underlying keys or values complete abruptly in toString().

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values

public Collection values()

Returns a collection or bag view of this map's values. The collection is backed by the map, so changes in one show up in the other. Modifications while an iteration is in progress produce undefined behavior. The collection supports removal if entrySet() does, but does not support element addition.

This implementation creates an AbstractCollection, where the iterator wraps the entrySet iterator, size defers to the Map's size, and contains defers to the Map's containsValue. The collection is created on first use, and returned on subsequent uses, although since no synchronization occurs, there is a slight possibility of creating two collections.

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