Added in API level 1

ConcurrentMap

interface ConcurrentMap<K : Any!, V : Any!> : MutableMap<K, V>
java.util.concurrent.ConcurrentMap

A Map providing thread safety and atomicity guarantees.

To maintain the specified guarantees, default implementations of methods including putIfAbsent inherited from Map must be overridden by implementations of this interface. Similarly, implementations of the collections returned by methods keySet, values, and entrySet must override methods such as removeIf when necessary to preserve atomicity guarantees.

Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread.

This interface is a member of the Java Collections Framework.

Summary

Public methods
open V?
compute(key: K, remappingFunction: BiFunction<in K, in V?, out V?>)

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).

open V
computeIfAbsent(key: K, mappingFunction: Function<in K, out V>)

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

open V?
computeIfPresent(key: K, remappingFunction: BiFunction<in K, in V, out V?>)

If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

open Unit
forEach(action: BiConsumer<in K, in V>)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.

open V
getOrDefault(key: K, defaultValue: V)

Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

open V?
merge(key: K, value: V, remappingFunction: BiFunction<in V, in V, out V?>)

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.

abstract V?
putIfAbsent(key: K, value: V)

If the specified key is not already associated with a value, associates it with the given value.

abstract Boolean
remove(key: K, value: V)

Removes the entry for a key only if currently mapped to a given value.

abstract Boolean
replace(key: K, oldValue: V, newValue: V)

Replaces the entry for a key only if currently mapped to a given value.

abstract V?
replace(key: K, value: V)

Replaces the entry for a key only if currently mapped to some value.

open Unit
replaceAll(function: BiFunction<in K, in V, out V>)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

Public methods

compute

Added in API level 24
open fun compute(
    key: K,
    remappingFunction: BiFunction<in K, in V?, out V?>
): V?

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping:

<code>map.compute(key, (k, v) -&gt; (v == null) ? msg : v.concat(msg))</code>
(Method merge() is often simpler to use for such purposes.)

If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key K: key with which the specified value is to be associated
remappingFunction BiFunction<in K, in V?, out V?>: the remapping function to compute a value
Return
V? the new value associated with the specified key, or null if none
Exceptions
java.lang.NullPointerException if the specified key is null and this map does not support null keys, or the remappingFunction is null
java.lang.UnsupportedOperationException if the put operation is not supported by this map (optional)
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map (optional)
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map (optional)

computeIfAbsent

Added in API level 24
open fun computeIfAbsent(
    key: K,
    mappingFunction: Function<in K, out V>
): V

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:

<code>map.computeIfAbsent(key, k -&gt; new Value(f(k)));
  </code>

Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key:

<code>map.computeIfAbsent(key, k -&gt; new HashSet&lt;V&gt;()).add(v);
  </code>

The mapping function should not modify this map during computation.

Parameters
key K: key with which the specified value is to be associated
mappingFunction Function<in K, out V>: the mapping function to compute a value
Return
V the current (existing or computed) value associated with the specified key, or null if the computed value is null
Exceptions
java.lang.NullPointerException if the specified key is null and this map does not support null keys, or the mappingFunction is null
java.lang.UnsupportedOperationException if the put operation is not supported by this map (optional)
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map (optional)
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map (optional)

computeIfPresent

Added in API level 24
open fun computeIfPresent(
    key: K,
    remappingFunction: BiFunction<in K, in V, out V?>
): V?

If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key K: key with which the specified value is to be associated
remappingFunction BiFunction<in K, in V, out V?>: the remapping function to compute a value
Return
V? the new value associated with the specified key, or null if none
Exceptions
java.lang.NullPointerException if the specified key is null and this map does not support null keys, or the remappingFunction is null
java.lang.UnsupportedOperationException if the put operation is not supported by this map (optional)
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map (optional)
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map (optional)

forEach

Added in API level 24
open fun forEach(action: BiConsumer<in K, in V>): Unit

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.

Parameters
action BiConsumer<in K, in V>: The action to be performed for each entry
Exceptions
java.lang.NullPointerException if the specified action is null
java.util.ConcurrentModificationException if an entry is found to be removed during iteration

getOrDefault

Added in API level 24
open fun getOrDefault(
    key: K,
    defaultValue: V
): V

Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

Parameters
key K: the key whose associated value is to be returned
defaultValue V: the default mapping of the key
Return
V the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key
Exceptions
java.lang.ClassCastException if the key is of an inappropriate type for this map (optional)
java.lang.NullPointerException if the specified key is null and this map does not permit null keys (optional)

merge

Added in API level 24
open fun merge(
    key: K,
    value: V,
    remappingFunction: BiFunction<in V, in V, out V?>
): V?

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping:

<code>map.merge(key, msg, String::concat)
  </code>

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

Parameters
key K: key with which the resulting value is to be associated
value V: the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
remappingFunction BiFunction<in V, in V, out V?>: the remapping function to recompute a value if present
Return
V? the new value associated with the specified key, or null if no value is associated with the key
Exceptions
java.lang.UnsupportedOperationException if the put operation is not supported by this map (optional)
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map (optional)
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map (optional)
java.lang.NullPointerException if the specified key is null and this map does not support null keys or the value or remappingFunction is null

putIfAbsent

Added in API level 1
abstract fun putIfAbsent(
    key: K,
    value: V
): V?

If the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for this map:

<code>if (!map.containsKey(key))
    return map.put(key, value);
  else
    return map.get(key);</code>
except that the action is performed atomically.

Parameters
key K: key with which the specified value is to be associated
value V: value to be associated with the specified key
Return
V? the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
Exceptions
java.lang.UnsupportedOperationException if the put operation is not supported by this map
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map
java.lang.NullPointerException if the specified key or value is null, and this map does not permit null keys or values
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map

remove

Added in API level 1
abstract fun remove(
    key: K,
    value: V
): Boolean

Removes the entry for a key only if currently mapped to a given value. This is equivalent to, for this map:

<code>if (map.containsKey(key)
      &amp;&amp; Objects.equals(map.get(key), value)) {
    map.remove(key);
    return true;
  } else {
    return false;
  }</code>
except that the action is performed atomically.

Parameters
key K: key with which the specified value is associated
value V: value expected to be associated with the specified key
Return
Boolean true if the value was removed
Exceptions
java.lang.UnsupportedOperationException if the remove operation is not supported by this map
java.lang.ClassCastException if the key or value is of an inappropriate type for this map (optional)
java.lang.NullPointerException if the specified key or value is null, and this map does not permit null keys or values (optional)

replace

Added in API level 1
abstract fun replace(
    key: K,
    oldValue: V,
    newValue: V
): Boolean

Replaces the entry for a key only if currently mapped to a given value. This is equivalent to, for this map:

<code>if (map.containsKey(key)
      &amp;&amp; Objects.equals(map.get(key), oldValue)) {
    map.put(key, newValue);
    return true;
  } else {
    return false;
  }</code>
except that the action is performed atomically.

Parameters
key K: key with which the specified value is associated
oldValue V: value expected to be associated with the specified key
newValue V: value to be associated with the specified key
Return
Boolean true if the value was replaced
Exceptions
java.lang.UnsupportedOperationException if the put operation is not supported by this map
java.lang.ClassCastException if the class of a specified key or value prevents it from being stored in this map
java.lang.NullPointerException if a specified key or value is null, and this map does not permit null keys or values
java.lang.IllegalArgumentException if some property of a specified key or value prevents it from being stored in this map

replace

Added in API level 1
abstract fun replace(
    key: K,
    value: V
): V?

Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for this map:

<code>if (map.containsKey(key))
    return map.put(key, value);
  else
    return null;</code>
except that the action is performed atomically.

Parameters
key K: key with which the specified value is associated
value V: value to be associated with the specified key
Return
V? the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
Exceptions
java.lang.UnsupportedOperationException if the put operation is not supported by this map
java.lang.ClassCastException if the class of the specified key or value prevents it from being stored in this map
java.lang.NullPointerException if the specified key or value is null, and this map does not permit null keys or values
java.lang.IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map

replaceAll

Added in API level 24
open fun replaceAll(function: BiFunction<in K, in V, out V>): Unit

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.

Parameters
function BiFunction<in K, in V, out V>: the function to apply to each entry
Exceptions
java.lang.UnsupportedOperationException if the set operation is not supported by this map's entry set iterator.
java.lang.ClassCastException if the class of a replacement value prevents it from being stored in this map
java.lang.NullPointerException if the specified function is null, or the specified replacement value is null, and this map does not permit null values
java.lang.IllegalArgumentException if some property of a replacement value prevents it from being stored in this map (optional)
java.util.ConcurrentModificationException if an entry is found to be removed during iteration