Map.Entry (Java Platform SE 8 )
Keywords: java.util.Map.Entry interface
Date: 2026-04-22

compact1, compact2, compact3
java.util

## Interface Map.Entry<K,V>

- 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 `Map.entrySet` method returns a collection-view of the map, whose elements are of this class. The only way to obtain a reference to a map entry is from the iterator of this collection-view. These `Map.Entry` objects are valid only for the duration of the iteration; more formally, the behavior of a map entry is undefined if the backing map has been modified after the entry was returned by the iterator, except through the `setValue` operation on the map entry.
  Since:
    1.2
  See Also:
    `Map.entrySet()`

- - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Default Methods
    Modifier and Type | Method and Description
    --- | ---
    `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.
    `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`.
    `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.
    `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`.
    `boolean` | `equals(Object o)` Compares the specified object with this entry for equality.
    `K` | `getKey()` Returns the key corresponding to this entry.
    `V` | `getValue()` Returns the value corresponding to this entry.
    `int` | `hashCode()` Returns the hash code value for this map entry.
    `V` | `setValue(V value)` Replaces the value corresponding to this entry with the specified value (optional operation).

- - ### Method Detail

    - #### 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:
        `Object.hashCode()`, `HashMap`

    - #### 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:
        `Object.hashCode()`, `Object.equals(Object)`, `equals(Object)`

    - #### 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:
        `Comparable`

    - #### 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:
        `Comparable`

    - #### 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

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