HashMap (Java SE 19 & JDK 19) (original) (raw)

Type Parameters:

K - the type of keys maintained by this map

V - the type of mapped values

All Implemented Interfaces:

[Serializable](../io/Serializable.html "interface in java.io"), [Cloneable](../lang/Cloneable.html "interface in java.lang"), [Map](Map.html "interface in java.util")<K,V>

Direct Known Subclasses:

[LinkedHashMap](LinkedHashMap.html "class in java.util"), [PrinterStateReasons](../../../java.desktop/javax/print/attribute/standard/PrinterStateReasons.html "class in javax.print.attribute.standard")

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permitsnull values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of theHashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The_capacity_ is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The_load factor_ is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, includingget and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable, this class may use comparison order among keys to help break ties.

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using theCollections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

Map m = Collections.synchronizedMap(new HashMap(...));

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's ownremove method, the iterator will throw aConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

Since:

1.2

See Also:

• Object.hashCode()

• Collection

• Map

• TreeMap

• Hashtable

• Serialized Form

• Nested Class Summary

Nested classes/interfaces declared in interface java.util.Map

[Map.Entry](Map.Entry.html "interface in java.util")<[K](Map.Entry.html "type parameter in Map.Entry"),[V](Map.Entry.html "type parameter in Map.Entry")>

• Constructor Summary

Constructors
[HashMap](#%3Cinit%3E%28%29)()
Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
[HashMap](#%3Cinit%3E%28int%29)(int initialCapacity)
Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
[HashMap](#%3Cinit%3E%28int,float%29)(int initialCapacity, float loadFactor)
Constructs an empty HashMap with the specified initial capacity and load factor.
[HashMap](#%3Cinit%3E%28java.util.Map%29)([Map](Map.html "interface in java.util")<? extends [K](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> m)
Constructs a new HashMap with the same mappings as the specified Map.

• Method Summary

void
[clear](#clear%28%29)()
Removes all of the mappings from this map.
[clone](#clone%28%29)()
Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
[compute](#compute%28K,java.util.function.BiFunction%29)([K](HashMap.html "type parameter in HashMap") key,[BiFunction](function/BiFunction.html "interface in java.util.function")<? super [K](HashMap.html "type parameter in HashMap"),? super [V](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
[computeIfAbsent](#computeIfAbsent%28K,java.util.function.Function%29)([K](HashMap.html "type parameter in HashMap") key,[Function](function/Function.html "interface in java.util.function")<? super [K](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> 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 this map unless null.
[computeIfPresent](#computeIfPresent%28K,java.util.function.BiFunction%29)([K](HashMap.html "type parameter in HashMap") key,[BiFunction](function/BiFunction.html "interface in java.util.function")<? super [K](HashMap.html "type parameter in HashMap"),? super [V](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> 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.
boolean
Returns true if this map contains a mapping for the specified key.
boolean
Returns true if this map maps one or more keys to the specified value.
[entrySet](#entrySet%28%29)()
Returns a Set view of the mappings contained in this map.
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
boolean
[isEmpty](#isEmpty%28%29)()
Returns true if this map contains no key-value mappings.
[keySet](#keySet%28%29)()
Returns a Set view of the keys contained in this map.
[merge](#merge%28K,V,java.util.function.BiFunction%29)([K](HashMap.html "type parameter in HashMap") key,[V](HashMap.html "type parameter in HashMap") value,[BiFunction](function/BiFunction.html "interface in java.util.function")<? super [V](HashMap.html "type parameter in HashMap"),? super [V](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> 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.
[newHashMap](#newHashMap%28int%29)(int numMappings)
Creates a new, empty HashMap suitable for the expected number of mappings.
[put](#put%28K,V%29)([K](HashMap.html "type parameter in HashMap") key,[V](HashMap.html "type parameter in HashMap") value)
Associates the specified value with the specified key in this map.
void
[putAll](#putAll%28java.util.Map%29)([Map](Map.html "interface in java.util")<? extends [K](HashMap.html "type parameter in HashMap"),? extends [V](HashMap.html "type parameter in HashMap")> m)
Copies all of the mappings from the specified map to this map.
Removes the mapping for the specified key from this map if present.
int
[size](#size%28%29)()
Returns the number of key-value mappings in this map.
[values](#values%28%29)()
Returns a Collection view of the values contained in this map.

• Constructor Details

  • HashMap

  public HashMap(int initialCapacity, float loadFactor)
  Constructs an empty HashMap with the specified initial capacity and load factor.
  API Note:
  To create a HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.
  Parameters:
  initialCapacity - the initial capacity
  loadFactor - the load factor
  Throws:
  [IllegalArgumentException](../lang/IllegalArgumentException.html "class in java.lang") - if the initial capacity is negative or the load factor is nonpositive

  • HashMap

  public HashMap(int initialCapacity)
  Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
  API Note:
  To create a HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.
  Parameters:
  initialCapacity - the initial capacity.
  Throws:
  [IllegalArgumentException](../lang/IllegalArgumentException.html "class in java.lang") - if the initial capacity is negative.

  • HashMap

  public HashMap()
  Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

  • HashMap

  public HashMap(Map<? extends K,? extends V> m)
  Constructs a new HashMap with the same mappings as the specified Map. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified Map.
  Parameters:
  m - the map whose mappings are to be placed in this map
  Throws:
  [NullPointerException](../lang/NullPointerException.html "class in java.lang") - if the specified map is null

• Method Details

  • size

  public int size()
  Returns the number of key-value mappings in this map.
  Specified by:
  [size](Map.html#size%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [size](AbstractMap.html#size%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  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](Map.html#isEmpty%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [isEmpty](AbstractMap.html#isEmpty%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Returns:
  true if this map contains no key-value mappings

  • get

  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 keyk 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.)
  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](Map.html#get%28java.lang.Object%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [get](AbstractMap.html#get%28java.lang.Object%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  key - the key whose associated value is to be returned
  Returns:
  the value to which the specified key is mapped, ornull if this map contains no mapping for the key
  See Also:
  * put(Object, Object)

  • containsKey

  public boolean containsKey(Object key)
  Returns true if this map contains a mapping for the specified key.
  Specified by:
  [containsKey](Map.html#containsKey%28java.lang.Object%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [containsKey](AbstractMap.html#containsKey%28java.lang.Object%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  key - The key whose presence in this map is to be tested
  Returns:
  true if this map contains a mapping for the specified key.

  • put

  public V put(K key,V value)
  Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.
  Specified by:
  [put](Map.html#put%28K,V%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [put](AbstractMap.html#put%28K,V%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  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, ornull if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

  • putAll

  public void putAll(Map<? extends K,? extends V> m)
  Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
  Specified by:
  [putAll](Map.html#putAll%28java.util.Map%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [putAll](AbstractMap.html#putAll%28java.util.Map%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  m - mappings to be stored in this map
  Throws:
  [NullPointerException](../lang/NullPointerException.html "class in java.lang") - if the specified map is null

  • remove

  Removes the mapping for the specified key from this map if present.
  Specified by:
  [remove](Map.html#remove%28java.lang.Object%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [remove](AbstractMap.html#remove%28java.lang.Object%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  key - key whose mapping is to be removed from the map
  Returns:
  the previous value associated with key, ornull if there was no mapping for key. (A null return can also indicate that the map previously associated null with key.)

  • clear

  public void clear()
  Removes all of the mappings from this map. The map will be empty after this call returns.
  Specified by:
  [clear](Map.html#clear%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [clear](AbstractMap.html#clear%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>

  • containsValue

  public boolean containsValue(Object value)
  Returns true if this map maps one or more keys to the specified value.
  Specified by:
  [containsValue](Map.html#containsValue%28java.lang.Object%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [containsValue](AbstractMap.html#containsValue%28java.lang.Object%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  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

  • 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 theIterator.remove, Set.remove,removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
  Specified by:
  [keySet](Map.html#keySet%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [keySet](AbstractMap.html#keySet%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Returns:
  a set view of the keys contained in this map

  • 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](Map.html#values%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Overrides:
  [values](AbstractMap.html#values%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Returns:
  a view of the values contained in this map

  • entrySet

  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 thesetValue 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 andclear operations. It does not support theadd or addAll operations.
  Specified by:
  [entrySet](Map.html#entrySet%28%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Returns:
  a set view of the mappings contained in this map

  • computeIfAbsent

  public V computeIfAbsent(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 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:
  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 should not modify this map during computation.
  This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the mapping function modifies this map during computation.
  Specified by:
  [computeIfAbsent](Map.html#computeIfAbsent%28K,java.util.function.Function%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  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:
  [ConcurrentModificationException](ConcurrentModificationException.html "class in java.util") - if it is detected that the mapping function modified this map

  • computeIfPresent

  public V computeIfPresent(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 should not modify this map during computation.
  This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
  Specified by:
  [computeIfPresent](Map.html#computeIfPresent%28K,java.util.function.BiFunction%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  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:
  [ConcurrentModificationException](ConcurrentModificationException.html "class in java.util") - if it is detected that the remapping function modified this map

  • compute

  public V compute(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 should not modify this map during computation.
  This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
  Specified by:
  [compute](Map.html#compute%28K,java.util.function.BiFunction%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  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:
  [ConcurrentModificationException](ConcurrentModificationException.html "class in java.util") - if it is detected that the remapping function modified this map

  • merge

  public V merge(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 should not modify this map during computation.
  This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
  Specified by:
  [merge](Map.html#merge%28K,V,java.util.function.BiFunction%29) in interface [Map](Map.html "interface in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Parameters:
  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 remapping 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:
  [ConcurrentModificationException](ConcurrentModificationException.html "class in java.util") - if it is detected that the remapping function modified this map

  • clone

  Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
  Overrides:
  [clone](AbstractMap.html#clone%28%29) in class [AbstractMap](AbstractMap.html "class in java.util")<[K](HashMap.html "type parameter in HashMap"),[V](HashMap.html "type parameter in HashMap")>
  Returns:
  a shallow copy of this map
  See Also:
  * Cloneable

  • newHashMap

  public static <K,V> HashMap<K,V> newHashMap(int numMappings)
  Creates a new, empty HashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.
  Type Parameters:
  K - the type of keys maintained by the new map
  V - the type of mapped values
  Parameters:
  numMappings - the expected number of mappings
  Returns:
  the newly created map
  Throws:
  [IllegalArgumentException](../lang/IllegalArgumentException.html "class in java.lang") - if numMappings is negative
  Since:
  19