java8.util.Maps Maven / Gradle / Ivy
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*
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*
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package java8.util;
import java.io.Serializable;
import java.util.Comparator;
import java.util.ConcurrentModificationException;
import java.util.Map;
import java.util.concurrent.ConcurrentMap;
import java8.util.Objects;
import java8.util.concurrent.ConcurrentMaps;
import java8.util.function.BiConsumer;
import java8.util.function.BiFunction;
import java8.util.function.Function;
/**
* A place for static default implementations of the new Java 8
* default interface methods and static interface methods in the
* {@link Map} interface.
*/
public final class Maps {
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}) associates it with the given value and returns
* {@code null}, else returns the current value.
*
* Implementation Requirements:
* The default implementation is equivalent to, for the {@code
* map}:
*
*
{@code
* V v = map.get(key);
* if (v == null)
* v = map.put(key, value);
*
* return v;
* }
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code putIfAbsent}
* operation.
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the key or value is of an inappropriate
* type for the map (optional)
* @throws NullPointerException if the specified key or value is null,
* and the map does not permit null keys or values (optional)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in the map (optional)
* @since 1.8
*/
public static V putIfAbsent(Map map, K key, V value) {
Objects.requireNonNull(map);
// safety measure
if (map instanceof ConcurrentMap) {
return ((ConcurrentMap) map).putIfAbsent(key, value);
}
V v = map.get(key);
if (v == null) {
v = map.put(key, value);
}
return v;
}
/**
* Returns the value to which the specified key is mapped, or
* {@code defaultValue} if the map contains no mapping for the key.
*
* Implementation Requirements:
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code getOrDefault}
* operation.
* @param key the key whose associated value is to be returned
* @param defaultValue the default mapping of the key
* @return the value to which the specified key is mapped, or
* {@code defaultValue} if the map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* the map (optional)
* @throws NullPointerException if the specified key is null and the map
* does not permit null keys (optional)
* @since 1.8
*/
public static V getOrDefault(Map map, Object key, V defaultValue) {
Objects.requireNonNull(map);
// safety measure
if (map instanceof ConcurrentMap) {
return ConcurrentMaps.getOrDefault((ConcurrentMap) map, key, defaultValue);
}
V v;
return (((v = map.get(key)) != null) || map.containsKey(key))
? v
: defaultValue;
}
/**
* 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 {@code map}:
*
{@code
* for (Map.Entry entry : map.entrySet())
* action.accept(entry.getKey(), entry.getValue());
* }
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code forEach}
* operation.
* @param action The action to be performed for each entry
* @throws NullPointerException if the specified map or action is null
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
public static void forEach(Map map, BiConsumer action) {
Objects.requireNonNull(map);
Objects.requireNonNull(action);
// safety measure
if (map instanceof ConcurrentMap) {
ConcurrentMaps.forEach((ConcurrentMap) map, action);
} else {
for (Map.Entry entry : map.entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
ConcurrentModificationException cme = new ConcurrentModificationException();
cme.initCause(ise);
throw cme;
}
action.accept(k, 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 {@code null}. This
* method may be of use when combining multiple mapped values for a key.
* For example, to either create or append a {@code String msg} to a
* value mapping:
*
* {@code
* map.merge(key, msg, String::concat)
* }
*
* If the remapping function returns {@code 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 {@code map}, then returning the current value or
* {@code null} if absent:
*
*
{@code
* V oldValue = map.get(key);
* V newValue = (oldValue == null) ? value :
* remappingFunction.apply(oldValue, value);
* if (newValue == null)
* map.remove(key);
* else
* map.put(key, newValue);
* }
*
* The default implementation makes no guarantees about detecting if the
* remapping function modifies the passed map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies the map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies the map during computation and as a result
* computation would never complete.
*
*
The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code merge} operation.
* @param key key with which the resulting value is to be associated
* @param 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
* @param remappingFunction the remapping function to recompute a value if present
* @return the new value associated with the specified key, or null if no
* value is associated with the key
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in the map (optional)
* @throws NullPointerException if the specified key is null and the map
* does not support null keys or the value or remappingFunction is
* null
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in the map (optional)
* @since 1.8
*/
public static V merge(Map map, K key, V value,
BiFunction remappingFunction) {
Objects.requireNonNull(map);
Objects.requireNonNull(remappingFunction);
Objects.requireNonNull(value);
// safety measure
if (map instanceof ConcurrentMap) {
return ConcurrentMaps.merge((ConcurrentMap) map, key, value, remappingFunction);
}
V oldValue = map.get(key);
V newValue = (oldValue == null) ? value :
remappingFunction.apply(oldValue, value);
if (newValue == null) {
map.remove(key);
} else {
map.put(key, newValue);
}
return newValue;
}
/**
* If the specified key is not already associated with a value (or is mapped
* to {@code null}), attempts to compute its value using the given mapping
* function and enters it into the passed {@code map} unless {@code null}.
*
* If the mapping function returns {@code 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 -> new Value(f(k)));
* }
*
* Or to implement a multi-value map, {@code Map>},
* supporting multiple values per key:
*
* {@code
* map.computeIfAbsent(key, k -> new HashSet()).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
* {@code map}, then returning the current value or {@code null} if now
* absent:
*
*
{@code
* if (map.get(key) == null) {
* V newValue = mappingFunction.apply(key);
* if (newValue != null)
* map.put(key, newValue);
* }
* }
*
* The default implementation makes no guarantees about detecting if the
* mapping function modifies the passed map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* mapping function modifies the map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* mapping function modifies the map during computation and as a result
* computation would never complete.
*
*
The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the mapping function is applied once atomically only if the value
* is not present.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code computeIfAbsent}
* operation.
* @param key key with which the specified value is to be associated
* @param mappingFunction the mapping function to compute a value
* @return 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
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in the map (optional)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in the map (optional)
* @since 1.8
*/
public static V computeIfAbsent(Map map, K key,
Function mappingFunction) {
Objects.requireNonNull(map);
Objects.requireNonNull(mappingFunction);
// safety measure
if (map instanceof ConcurrentMap) {
return ConcurrentMaps.computeIfAbsent((ConcurrentMap) map, key, mappingFunction);
}
V v;
if ((v = map.get(key)) == null) {
V newValue;
if ((newValue = mappingFunction.apply(key)) != null) {
map.put(key, newValue);
return newValue;
}
}
return v;
}
/**
* Replaces the entry for the specified key only if currently
* mapped to the specified value.
*
* Implementation Requirements:
* The default implementation is equivalent to, for the {@code map}:
*
*
{@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.put(key, newValue);
* return true;
* } else
* return false;
* }
*
* The default implementation does not throw NullPointerException
* for maps that do not support null values if oldValue is null unless
* newValue is also null.
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code replace} operation.
* @param key key with which the specified value is associated
* @param oldValue value expected to be associated with the specified key
* @param newValue value to be associated with the specified key
* @return {@code true} if the value was replaced
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of a specified key or value
* prevents it from being stored in the map
* @throws NullPointerException if a specified key or newValue is null,
* and the map does not permit null keys or values
* @throws NullPointerException if oldValue is null and the map does not
* permit null values (optional)
* @throws IllegalArgumentException if some property of a specified key
* or value prevents it from being stored in the map
* @since 1.8
*/
public static boolean replace(Map map, K key, V oldValue, V newValue) {
Objects.requireNonNull(map);
// safety measure
if (map instanceof ConcurrentMap) {
return ((ConcurrentMap) map).replace(key, oldValue, newValue);
}
Object curValue = map.get(key);
if (!Objects.equals(curValue, oldValue) ||
(curValue == null && !map.containsKey(key))) {
return false;
}
map.put(key, newValue);
return true;
}
/**
* Replaces the entry for the specified key only if it is
* currently mapped to some value.
*
* Implementation Requirements:
* The default implementation is equivalent to, for the {@code map}:
*
*
{@code
* if (map.containsKey(key)) {
* return map.put(key, value);
* } else
* return null;
* }
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code replace} operation.
* @param key key with which the specified value is associated
* @param value value to be associated with the specified key
* @return the previous value associated with the specified key, or
* {@code null} if there was no mapping for the key.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with the key,
* if the implementation supports null values.)
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in the map (optional)
* @throws NullPointerException if the specified key or value is null,
* and the map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in the map
* @since 1.8
*/
public static V replace(Map map, K key, V value) {
Objects.requireNonNull(map);
// safety measure
if (map instanceof ConcurrentMap) {
return ((ConcurrentMap) map).replace(key, value);
}
V curValue;
if (((curValue = map.get(key)) != null) || map.containsKey(key)) {
curValue = map.put(key, value);
}
return curValue;
}
/**
* 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 {@code map}:
*
{@code
* for (Map.Entry entry : map.entrySet())
* entry.setValue(function.apply(entry.getKey(), entry.getValue()));
* }
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code replaceAll}
* operation.
* @param function the function to apply to each entry
* @throws UnsupportedOperationException if the {@code set} operation
* is not supported by the map's entry set iterator.
* @throws ClassCastException if the class of a replacement value
* prevents it from being stored in the map
* @throws NullPointerException if the specified function is null, or the
* specified replacement value is null, and the map does not permit null
* values
* @throws ClassCastException if a replacement value is of an inappropriate
* type for the map (optional)
* @throws NullPointerException if function or a replacement value is null,
* and the map does not permit null keys or values (optional)
* @throws IllegalArgumentException if some property of a replacement value
* prevents it from being stored in the map (optional)
* @throws ConcurrentModificationException if an entry is found to be
* removed during iteration
* @since 1.8
*/
public static void replaceAll(Map map, BiFunction function) {
Objects.requireNonNull(map);
Objects.requireNonNull(function);
// safety measure
if (map instanceof ConcurrentMap) {
ConcurrentMaps.replaceAll((ConcurrentMap) map, function);
} else {
for (Map.Entry entry : map.entrySet()) {
K k;
V v;
try {
k = entry.getKey();
v = entry.getValue();
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
ConcurrentModificationException cmex = new ConcurrentModificationException();
cmex.initCause(ise);
throw cmex;
}
// ise thrown from function is not a cme.
v = function.apply(k, v);
try {
entry.setValue(v);
} catch (IllegalStateException ise) {
// this usually means the entry is no longer in the map.
ConcurrentModificationException cmex = new ConcurrentModificationException();
cmex.initCause(ise);
throw cmex;
}
}
}
}
/**
* Attempts to compute a mapping for the specified key and its current
* mapped value (or {@code null} if there is no current mapping). For
* example, to either create or append a {@code String} msg to a value
* mapping:
*
* {@code
* map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}
* (Method {@link #merge merge()} is often simpler to use for such purposes.)
*
* If the remapping function returns {@code 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 {@code map}, then returning the current value or
* {@code null} if absent:
*
*
{@code
* V oldValue = map.get(key);
* V newValue = remappingFunction.apply(key, oldValue);
* if (oldValue != null ) {
* if (newValue != null)
* map.put(key, newValue);
* else
* map.remove(key);
* } else {
* if (newValue != null)
* map.put(key, newValue);
* else
* return null;
* }
* }
*
* The default implementation makes no guarantees about detecting if the
* remapping function modifies the passed map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies the map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies the map during computation and as a result
* computation would never complete.
*
*
The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code compute} operation.
* @param key key with which the specified value is to be associated
* @param remappingFunction the remapping function to compute a value
* @return 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
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in the map (optional)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map (optional)
* @since 1.8
*/
public static V compute(Map map, K key,
BiFunction remappingFunction) {
Objects.requireNonNull(map);
Objects.requireNonNull(remappingFunction);
// safety measure
if (map instanceof ConcurrentMap) {
return ConcurrentMaps.compute((ConcurrentMap) map, key, remappingFunction);
}
V oldValue = map.get(key);
V newValue = remappingFunction.apply(key, oldValue);
if (newValue == null) {
// delete mapping
if (oldValue != null || map.containsKey(key)) {
// something to remove
map.remove(key);
return null;
} else {
// nothing to do. Leave things as they were.
return null;
}
} else {
// add or replace old mapping
map.put(key, newValue);
return newValue;
}
}
/**
* 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 {@code 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 {@code map}, then returning the current value or
* {@code null} if now absent:
*
*
{@code
* if (map.get(key) != null) {
* V oldValue = map.get(key);
* V newValue = remappingFunction.apply(key, oldValue);
* if (newValue != null)
* map.put(key, newValue);
* else
* map.remove(key);
* }
* }
*
* The default implementation makes no guarantees about detecting if the
* remapping function modifies the passed map during computation and, if
* appropriate, reporting an error. Non-concurrent implementations should
* override this method and, on a best-effort basis, throw a
* {@code ConcurrentModificationException} if it is detected that the
* remapping function modifies the map during computation. Concurrent
* implementations should override this method and, on a best-effort basis,
* throw an {@code IllegalStateException} if it is detected that the
* remapping function modifies the map during computation and as a result
* computation would never complete.
*
*
The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties. In particular, all implementations of
* subinterface {@link java.util.concurrent.ConcurrentMap} must document
* whether the remapping function is applied once atomically only if the
* value is not present.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code computeIfPresent}
* operation.
* @param key key with which the specified value is to be associated
* @param remappingFunction the remapping function to compute a value
* @return 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
* @throws UnsupportedOperationException if the {@code put} operation
* is not supported by the map (optional)
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in the map (optional)
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map (optional)
* @since 1.8
*/
public static V computeIfPresent(Map map, K key,
BiFunction remappingFunction) {
Objects.requireNonNull(map);
Objects.requireNonNull(remappingFunction);
// safety measure
if (map instanceof ConcurrentMap) {
return ConcurrentMaps.computeIfPresent((ConcurrentMap) map, key, remappingFunction);
}
V oldValue;
if ((oldValue = map.get(key)) != null) {
V newValue = remappingFunction.apply(key, oldValue);
if (newValue != null) {
map.put(key, newValue);
return newValue;
} else {
map.remove(key);
return null;
}
} else {
return null;
}
}
/**
* Removes the entry for the specified key only if it is currently
* mapped to the specified value.
*
* Implementation Requirements:
* The default implementation is equivalent to, for the {@code map}:
*
*
{@code
* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
* map.remove(key);
* return true;
* } else
* return false;
* }
*
* The default implementation makes no guarantees about synchronization
* or atomicity properties of this method. Any implementation providing
* atomicity guarantees must override this method and document its
* concurrency properties.
*
* @param the type of keys maintained by the passed map
* @param the type of mapped values in the passed map
* @param map the {@code Map} on which to execute the {@code remove} operation.
* @param key key with which the specified value is associated
* @param value value expected to be associated with the specified key
* @return {@code true} if the value was removed
* @throws UnsupportedOperationException if the {@code remove} operation
* is not supported by the map (optional)
* @throws ClassCastException if the key or value is of an inappropriate
* type for the map (optional)
* @throws NullPointerException if the specified key or value is null,
* and the map does not permit null keys or values (optional)
* @since 1.8
*/
public static boolean remove(Map map, Object key, Object value) {
Objects.requireNonNull(map);
// safety measure
if (map instanceof ConcurrentMap) {
return ((ConcurrentMap) map).remove(key, value);
}
Object curValue = map.get(key);
if (!Objects.equals(curValue, value) ||
(curValue == null && !map.containsKey(key))) {
return false;
}
map.remove(key);
return true;
}
/**
* A place for the static interface methods of the Java 8 {@link Map.Entry}
* interface.
*/
public static final class Entry {
/**
* Returns a comparator that compares {@link Map.Entry} in natural order
* on key.
*
*
* The returned comparator is serializable and throws
* {@link NullPointerException} when comparing an entry with a null key.
*
* @param
* the {@link Comparable} type of then map keys
* @param
* the type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order
* on key.
* @see Comparable
* @since 1.8
*/
public static , V> Comparator> comparingByKey() {
return (Comparator> & Serializable) (c1, c2) -> c1
.getKey().compareTo(c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} in natural order
* on value.
*
*
* The returned comparator is serializable and throws
* {@link NullPointerException} when comparing an entry with null
* values.
*
* @param
* the type of the map keys
* @param
* the {@link Comparable} type of the map values
* @return a comparator that compares {@link Map.Entry} in natural order
* on value.
* @see Comparable
* @since 1.8
*/
public static > Comparator> comparingByValue() {
return (Comparator> & Serializable) (c1, c2) -> c1
.getValue().compareTo(c2.getValue());
}
/**
* Returns a comparator that compares {@link Map.Entry} by key using the
* given {@link Comparator}.
*
*
* The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param
* the type of the map keys
* @param
* the type of the map values
* @param cmp
* the key {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the key.
* @since 1.8
*/
public static Comparator> comparingByKey(
Comparator cmp) {
Objects.requireNonNull(cmp);
return (Comparator> & Serializable) (c1, c2) -> cmp
.compare(c1.getKey(), c2.getKey());
}
/**
* Returns a comparator that compares {@link Map.Entry} by value using
* the given {@link Comparator}.
*
*
* The returned comparator is serializable if the specified comparator
* is also serializable.
*
* @param
* the type of the map keys
* @param
* the type of the map values
* @param cmp
* the value {@link Comparator}
* @return a comparator that compares {@link Map.Entry} by the value.
* @since 1.8
*/
public static Comparator> comparingByValue(
Comparator cmp) {
Objects.requireNonNull(cmp);
return (Comparator> & Serializable) (c1, c2) -> cmp
.compare(c1.getValue(), c2.getValue());
}
private Entry() {
}
}
private Maps() {
}
}