AbstractMap (Java Platform SE 8 )
Keywords: java.util.AbstractMap class
Date: 2026-01-21

compact1, compact2, compact3
java.util

## Class AbstractMap<K,V>

- java.lang.Object
- - 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:
    Map<K,V>
  Direct Known Subclasses:
    ConcurrentHashMap, ConcurrentSkipListMap, EnumMap, HashMap, IdentityHashMap, TreeMap, WeakHashMap

      public abstract class AbstractMap<K,V>
      extends Object
      implements Map<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
  See Also:
    `Map`, `Collection`

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `AbstractMap.SimpleEntry<K,V>` An Entry maintaining a key and a value.
    `static class` | `AbstractMap.SimpleImmutableEntry<K,V>` An Entry maintaining an immutable key and value.

    - ### Nested classes/interfaces inherited from interface java.util.Map

      `Map.Entry<K,V>`

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `AbstractMap()` Sole constructor.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `clear()` Removes all of the mappings from this map (optional operation).
    `protected Object` | `clone()` Returns a shallow copy of this `AbstractMap` instance: the keys and values themselves are not cloned.
    `boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
    `boolean` | `containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
    `abstract Set<Map.Entry<K,V>>` | `entrySet()` Returns a `Set` view of the mappings contained in this map.
    `boolean` | `equals(Object o)` Compares the specified object with this map for equality.
    `V` | `get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
    `int` | `hashCode()` Returns the hash code value for this map.
    `boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
    `Set<K>` | `keySet()` Returns a `Set` view of the keys contained in this map.
    `V` | `put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
    `void` | `putAll(Map<? extends K,? extends V> m)` Copies all of the mappings from the specified map to this map (optional operation).
    `V` | `remove(Object key)` Removes the mapping for a key from this map if it is present (optional operation).
    `int` | `size()` Returns the number of key-value mappings in this map.
    `String` | `toString()` Returns a string representation of this map.
    `Collection<V>` | `values()` Returns a `Collection` view of the values contained in this map.

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Map

      `compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll`

- - ### Constructor Detail

    - #### AbstractMap

          protected AbstractMap()

      Sole constructor. (For invocation by subclass constructors, typically implicit.)

  - ### Method Detail

    - #### size

          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`.
      Specified by:
        `size` in interface `Map<K,V>`
      Implementation Requirements:
        This implementation returns `entrySet().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.
      Specified by:
        `isEmpty` in interface `Map<K,V>`
      Implementation Requirements:
        This implementation returns `size() == 0`.
      Returns:
        `true` if this map contains no key-value mappings

    - #### containsValue

          public 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 the `Map` interface.
      Specified by:
        `containsValue` in interface `Map<K,V>`
      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.
      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:
        `ClassCastException` - if the value is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified value is null and this map does not permit null values (optional)

    - #### containsKey

          public boolean containsKey(Object key)

      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 `(key==null ? k==null : key.equals(k))`. (There can be at most one such mapping.)
      Specified by:
        `containsKey` in interface `Map<K,V>`
      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.
      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:
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### get

          public V get(Object key)

      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 `(key==null ? k==null : key.equals(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.

      Specified by:
        `get` in interface `Map<K,V>`
      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.
      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:
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### put

          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 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`.)
      Specified by:
        `put` in interface `Map<K,V>`
      Implementation Requirements:
        This implementation always throws an `UnsupportedOperationException`.
      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:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map
        `NullPointerException` - if the specified key or value is null and this map does not permit null keys or values
        `IllegalArgumentException` - if some property of the specified key or value prevents it from being stored in this map

    - #### remove

          public V remove(Object key)

      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 `(key==null ? k==null : key.equals(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.

      Specified by:
        `remove` in interface `Map<K,V>`
      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.

      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:
        `UnsupportedOperationException` - if the `remove` operation is not supported by this map
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### putAll

          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.
      Specified by:
        `putAll` in interface `Map<K,V>`
      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.

      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

    - #### clear

          public void clear()

      Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.
      Specified by:
        `clear` in interface `Map<K,V>`
      Implementation Requirements:
        This implementation calls `entrySet().clear()`.

        Note that this implementation throws an `UnsupportedOperationException` if the `entrySet` does not support the `clear` operation.

      Throws:
        `UnsupportedOperationException` - if the `clear` operation is not supported by this map

    - #### keySet

          public Set<K> 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 (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.
      Specified by:
        `keySet` in interface `Map<K,V>`
      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.

      Returns:
        a set view of the keys contained in this map

    - #### values

          public Collection<V> 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 (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.
      Specified by:
        `values` in interface `Map<K,V>`
      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.

      Returns:
        a collection view of the values contained in this map

    - #### entrySet

          public abstract Set<Map.Entry<K,V>> entrySet()

      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:
        `entrySet` in interface `Map<K,V>`
      Returns:
        a set view of the mappings contained in this map

    - #### equals

          public 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 `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.
      Specified by:
        `equals` in interface `Map<K,V>`
      Overrides:
        `equals` in class `Object`
      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.
      Parameters:
        `o` - object to be compared for equality with this map
      Returns:
        `true` if the specified object is equal to this map
      See Also:
        `Object.hashCode()`, `HashMap`

    - #### 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 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()`.
      Specified by:
        `hashCode` in interface `Map<K,V>`
      Overrides:
        `hashCode` in class `Object`
      Implementation Requirements:
        This implementation iterates over `entrySet()`, calling `hashCode()` on each element (entry) in the set, and adding up the results.
      Returns:
        the hash code value for this map
      See Also:
        `Map.Entry.hashCode()`, `Object.equals(Object)`, `Set.equals(Object)`

    - #### toString

          public 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)`.
      Overrides:
        `toString` in class `Object`
      Returns:
        a string representation of this map

    - #### clone

          protected Object clone()
                          throws CloneNotSupportedException

      Returns a shallow copy of this `AbstractMap` instance: the keys and values themselves are not cloned.
      Overrides:
        `clone` in class `Object`
      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:
        `Cloneable`

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