Module java.base
Package java.util

Interface Map.Entry<K,V>

Type Parameters:
K - the type of the key
V - the type of the value
All Known Implementing Classes:
AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry
Enclosing interface:
Map<K,V>
public static interface Map.Entry<K,V>
A map entry (key-value pair). The Entry may be unmodifiable, or the value may be modifiable if the optional setValue method is implemented. The Entry may be independent of any map, or it may represent an entry of the entry-set view of a map.

An Entry maintains a connection to its underlying map if the Entry was obtained by iterating the Map.entrySet() view of a map, either explicitly by using an Iterator or implicitly via the enhanced for statement. This connection to the backing map is valid only during iteration of the entry-set view. During the iteration, if supported by the backing map, a change to an Entry's value via the setValue method will be visible in the backing map. The behavior of such an Entry is undefined outside of iteration of the map's entry-set view. It is also undefined if the backing map has been modified after the Entry was returned by the iterator, except through the setValue method. In addition, a change to the value of a mapping in the backing map might or might not be visible in the corresponding Entry of the entry-set view.

An Entry may also be obtained from a map's entry-set view by other means, for example, using the parallelStream, stream, spliterator methods, any of the toArray overloads, or by copying the entry-set view into another collection. It is unspecified whether the obtained Entry instances are connected to the underlying map, whether changes to such an Entry will affect the underlying the map and vice-versa, and whether such an Entry supports the optional setValue method.

In addition, an Entry may be obtained directly from a map, for example via calls to methods directly on the NavigableMap interface. An entry thus obtained is generally not connected to the map and is an unmodifiable snapshot of the mapping as of the time of the call. Such an Entry also does not generally support the setValue method.

An Entry obtained by direct construction of the AbstractMap.SimpleEntry or AbstractMap.SimpleImmutableEntry classes or from a call to the Map.entry or Map.Entry.copyOf methods is not connected to any map.

API Note:
The exact behavior of Entry instances obtained from a map's entry-set view other than via iteration varies across different map implementations; some are connected to the backing map, and some are not. To guarantee that an Entry is disconnected from its backing map, use the copyOf method. For example, the following creates a snapshot of a map's entries that is guaranteed not to change even if the original map is modified: 
 
 var entries = map.entrySet().stream().map(Map.Entry::copyOf).toList()
Since:
1.2
See Also:

  • Method Details

    • getKey

      K getKey()
      Returns the key corresponding to this entry.
      Returns:
      the key corresponding to this entry
      Throws:
      IllegalStateException - implementations may, but are not required to, throw this exception if the entry has been removed from the backing map.

    • getValue

      V getValue()
      Returns the value corresponding to this entry. If the mapping has been removed from the backing map (by the iterator's remove operation), the results of this call are undefined.
      Returns:
      the value corresponding to this entry
      Throws:
      IllegalStateException - implementations may, but are not required to, throw this exception if the entry has been removed from the backing map.

    • setValue

      V setValue(V value)
      Replaces the value corresponding to this entry with the specified value (optional operation). (Writes through to the map.) The behavior of this call is undefined if the mapping has already been removed from the map (by the iterator's remove operation).
      Parameters:
      value - new value to be stored in this entry
      Returns:
      old value corresponding to the entry
      Throws:
      UnsupportedOperationException - if the put operation is not supported by the backing map
      ClassCastException - if the class of the specified value prevents it from being stored in the backing map
      NullPointerException - if the backing map does not permit null values, and the specified value is null
      IllegalArgumentException - if some property of this value prevents it from being stored in the backing map
      IllegalStateException - implementations may, but are not required to, throw this exception if the entry has been removed from the backing map.

    • equals

      boolean equals(Object o)
      Compares the specified object with this entry for equality. Returns true if the given object is also a map entry and the two entries represent the same mapping. More formally, two entries e1 and e2 represent the same mapping if
           (e1.getKey()==null ?
      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
     (e1.getValue()==null ?
      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
      This ensures that the equals method works properly across different implementations of the Map.Entry interface.
      Overrides:
      equals in class Object
      Parameters:
      o - object to be compared for equality with this map entry
      Returns:
      true if the specified object is equal to this map entry
      See Also:

    • hashCode

      int hashCode()
      Returns the hash code value for this map entry. The hash code of a map entry e is defined to be: 
           (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
     (e.getValue()==null ? 0 : e.getValue().hashCode())
      This ensures that e1.equals(e2) implies that e1.hashCode()==e2.hashCode() for any two Entries e1 and e2, as required by the general contract of Object.hashCode.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this map entry
      See Also:

    • comparingByKey

      static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey()
      Returns a comparator that compares Map.Entry in natural order on key.

      The returned comparator is serializable and throws NullPointerException when comparing an entry with a null key.

      Type Parameters:
      K - the Comparable type of then map keys
      V - the type of the map values
      Returns:
      a comparator that compares Map.Entry in natural order on key.
      Since:
      1.8
      See Also:

    • comparingByValue

      static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue()
      Returns a comparator that compares Map.Entry in natural order on value.

      The returned comparator is serializable and throws NullPointerException when comparing an entry with null values.

      Type Parameters:
      K - the type of the map keys
      V - the Comparable type of the map values
      Returns:
      a comparator that compares Map.Entry in natural order on value.
      Since:
      1.8
      See Also:

    • comparingByKey

      static <K, V> Comparator<Map.Entry<K,V>> comparingByKey(Comparator<? super K> cmp)
      Returns a comparator that compares Map.Entry by key using the given Comparator.

      The returned comparator is serializable if the specified comparator is also serializable.

      Type Parameters:
      K - the type of the map keys
      V - the type of the map values
      Parameters:
      cmp - the key Comparator
      Returns:
      a comparator that compares Map.Entry by the key.
      Since:
      1.8

    • comparingByValue

      static <K, V> Comparator<Map.Entry<K,V>> comparingByValue(Comparator<? super V> cmp)
      Returns a comparator that compares Map.Entry by value using the given Comparator.

      The returned comparator is serializable if the specified comparator is also serializable.

      Type Parameters:
      K - the type of the map keys
      V - the type of the map values
      Parameters:
      cmp - the value Comparator
      Returns:
      a comparator that compares Map.Entry by the value.
      Since:
      1.8

    • copyOf

      static <K, V> Map.Entry<K,V> copyOf(Map.Entry<? extends K,? extends V> e)
      Returns a copy of the given Map.Entry. The returned instance is not associated with any map. The returned instance has the same characteristics as instances returned by the Map::entry method.
      API Note:
      An instance obtained from a map's entry-set view has a connection to that map. The copyOf method may be used to create a Map.Entry instance, containing the same key and value, that is independent of any map.
      Implementation Note:
      If the given entry was obtained from a call to copyOf or Map::entry, calling copyOf will generally not create another copy.
      Type Parameters:
      K - the type of the key
      V - the type of the value
      Parameters:
      e - the entry to be copied
      Returns:
      a map entry equal to the given entry
      Throws:
      NullPointerException - if e is null or if either of its key or value is null
      Since:
      17