java8.util.concurrent

Class ConcurrentMaps

java.lang.Object

java8.util.concurrent.ConcurrentMaps

public final class ConcurrentMaps
extends Object

A place for static default implementations of the new Java 8 default interface methods and static interface methods in the ConcurrentMap interface.

Method Summary

Modifier and Type	Method and Description
static <K,V> V	compute(ConcurrentMap<K,V> map, K key, BiFunction<? super K,? super V,? extends V> remappingFunction) Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
static <K,V> V	computeIfAbsent(ConcurrentMap<K,V> map, K key, Function<? super K,? extends V> mappingFunction) 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 the passed map unless null.
static <K,V> V	computeIfPresent(ConcurrentMap<K,V> map, K key, BiFunction<? super K,? super V,? extends V> remappingFunction) 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.
static <K,V> void	forEach(ConcurrentMap<K,V> map, BiConsumer<? super K,? super V> action) Performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
static <K,V> V	getOrDefault(ConcurrentMap<K,V> map, Object key, V defaultValue) Returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
static <K,V> V	merge(ConcurrentMap<K,V> map, K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
static <K,V> void	replaceAll(ConcurrentMap<K,V> map, BiFunction<? super K,? super V,? extends V> function) 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.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

merge

public static <K,V> V merge(ConcurrentMap<K,V> map,
                            K key,
                            V value,
                            BiFunction<? super V,? super V,? extends V> remappingFunction)

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:

 
 map.merge(key, msg, String::concat)
 

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 itself should not modify the passed map during computation.

Implementation Requirements:
The default implementation is equivalent to performing the following steps for the map:

 
 for (;;) {
   V oldValue = map.get(key);
   if (oldValue != null) {
     V newValue = remappingFunction.apply(oldValue, value);
     if (newValue != null) {
       if (map.replace(key, oldValue, newValue))
         return newValue;
     } else if (map.remove(key, oldValue)) {
       return null;
     }
   } else if (map.putIfAbsent(key, value) == null) {
     return value;
   }
 }

When multiple threads attempt updates, map operations and the remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override the default implementation.

The default implementation makes no guarantees about detecting if the remapping function modifies the passed map during computation and, if appropriate, reporting an error. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the remapping function modifies the map during computation and as a result computation would never complete.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the merge operation.

key - key with which the resulting value is to be associated

value - 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 - the function to recompute a value if present

Returns:

the new value associated with the specified key, or null if no value is associated with the key

Throws:

UnsupportedOperationException - if the put operation is not supported by the map (optional)

ClassCastException - if the class of the specified key or value prevents it from being stored in the map (optional)

NullPointerException - if the specified key is null and the map does not support null keys or the value or remappingFunction is null

IllegalArgumentException - if some property of the specified key or value prevents it from being stored in the map (optional)

Since:

1.8

computeIfAbsent

public static <K,V> V computeIfAbsent(ConcurrentMap<K,V> map,
                                      K key,
                                      Function<? super K,? extends V> mappingFunction)

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 the passed 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:

 
 map.computeIfAbsent(key, k -> new Value(f(k)));
 

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

 
 map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
 

The mapping function itself should not modify the passed map during computation.

Implementation Requirements:
The default implementation is equivalent to the following steps for the map:

 
 V oldValue, newValue;
 return ((oldValue = map.get(key)) == null
         && (newValue = mappingFunction.apply(key)) != null
         && (oldValue = map.putIfAbsent(key, newValue)) == null)
   ? newValue
   : oldValue;

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override the default implementation.

The default implementation makes no guarantees about detecting if the mapping function modifies the passed map during computation and, if appropriate, reporting an error. Concurrent implementations should override this method and, on a best-effort basis, throw an IllegalStateException if it is detected that the mapping function modifies the map during computation and as a result computation would never complete.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the computeIfAbsent operation.

key - key with which the specified value is to be associated

mappingFunction - the mapping function to compute a value

Returns:

the current (existing or computed) value associated with the specified key, or null if the computed value is null

Throws:

NullPointerException - if the specified key is null and the map does not support null keys, or the mappingFunction is null

UnsupportedOperationException - if the put operation is not supported by the map (optional)

ClassCastException - if the class of the specified key or value prevents it from being stored in the map (optional)

IllegalArgumentException - if some property of the specified key or value prevents it from being stored in the map (optional)

Since:

1.8

computeIfPresent

public static <K,V> V computeIfPresent(ConcurrentMap<K,V> map,
                                       K key,
                                       BiFunction<? super K,? super V,? extends V> remappingFunction)

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 itself should not modify the passed map during computation.

Implementation Requirements:
The default implementation is equivalent to performing the following steps for the map:

 
 for (V oldValue; (oldValue = map.get(key)) != null; ) {
   V newValue = remappingFunction.apply(key, oldValue);
   if ((newValue == null)
       ? map.remove(key, oldValue)
       : map.replace(key, oldValue, newValue))
     return newValue;
 }
 return null;

When multiple threads attempt updates, map operations and the remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the computeIfPresent operation.

key - key with which the specified value is to be associated

remappingFunction - the remapping function to compute a value

Returns:

the new value associated with the specified key, or null if none

Throws:

NullPointerException - if the specified key is null and the map does not support null keys, or the remappingFunction is null

UnsupportedOperationException - if the put operation is not supported by the map (optional)

ClassCastException - if the class of the specified key or value prevents it from being stored in the map (optional)

IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)

Since:

1.8

compute

public static <K,V> V compute(ConcurrentMap<K,V> map,
                              K key,
                              BiFunction<? super K,? super V,? extends V> remappingFunction)

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:

 
 map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

(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 itself should not modify the passed map during computation.

Implementation Requirements:
The default implementation is equivalent to performing the following steps for the map:

 
 for (;;) {
   V oldValue = map.get(key);
   V newValue = remappingFunction.apply(key, oldValue);
   if (newValue != null) {
     if ((oldValue != null)
       ? map.replace(key, oldValue, newValue)
       : map.putIfAbsent(key, newValue) == null)
       return newValue;
   } else if (oldValue == null || map.remove(key, oldValue)) {
     return null;
   }
 }

When multiple threads attempt updates, map operations and the remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the compute operation.

key - key with which the specified value is to be associated

remappingFunction - the remapping function to compute a value

Returns:

the new value associated with the specified key, or null if none

Throws:

NullPointerException - if the specified key is null and the map does not support null keys, or the remappingFunction is null

UnsupportedOperationException - if the put operation is not supported by the map (optional)

ClassCastException - if the class of the specified key or value prevents it from being stored in the map (optional)

IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map (optional)

Since:

1.8

replaceAll

public static <K,V> void replaceAll(ConcurrentMap<K,V> map,
                                    BiFunction<? super K,? super V,? extends V> function)

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.

Implementation Requirements:

The default implementation is equivalent to, for the map:

 
 for ((Map.Entry<K, V> entry : map.entrySet())
     do {
        K k = entry.getKey();
        V v = entry.getValue();
     } while(!replace(k, v, function.apply(k, v)));
 

The default implementation may retry these steps when multiple threads attempt updates including potentially calling the function repeatedly for a given key.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override the default implementation.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the replaceAll operation.

function - the function to apply to each entry

Throws:

UnsupportedOperationException - if the set operation is not supported by the map's entry set iterator.

ClassCastException - if the class of a replacement value prevents it from being stored in the map

NullPointerException - if the specified function is null, or the specified replacement value is null, and the map does not permit null values

ClassCastException - if a replacement value is of an inappropriate type for the map (optional)

NullPointerException - if function or a replacement value is null, and the map does not permit null keys or values (optional)

IllegalArgumentException - if some property of a replacement value prevents it from being stored in the map (optional)

ConcurrentModificationException - if an entry is found to be removed during iteration

Since:

1.8

getOrDefault

public static <K,V> V getOrDefault(ConcurrentMap<K,V> map,
                                   Object key,
                                   V defaultValue)

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

Implementation Note:
This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the getOrDefault operation.

key - the key whose associated value is to be returned

defaultValue - the default mapping of the key

Returns:

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

Throws:

ClassCastException - if the key is of an inappropriate type for the map (optional)

NullPointerException - if the specified key is null and the map does not permit null keys (optional)

Since:

1.8

forEach

public static <K,V> void forEach(ConcurrentMap<K,V> map,
                                 BiConsumer<? super K,? super V> action)

Performs the given action for each entry in the 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.

Implementation Requirements:
The default implementation is equivalent to, for the map:

 
 for ((Map.Entry<K, V> entry : map.entrySet())
     action.accept(entry.getKey(), entry.getValue());
 

Implementation Note:
The default implementation assumes that IllegalStateException thrown by getKey() or getValue() indicates that the entry has been removed and cannot be processed. Operation continues for subsequent entries.

Type Parameters:

K - the type of keys maintained by the passed map

V - the type of mapped values in the passed map

Parameters:

map - the ConcurrentMap on which to execute the forEach operation.

action - The action to be performed for each entry

Throws:

NullPointerException - if the specified map or action is null

ConcurrentModificationException - if an entry is found to be removed during iteration

Since:

1.8