Java : Map.Entry with Examples

final var entry1 = Map.entry("a", 10);
System.out.println(entry1); // a=10

final var entry2 = Map.entry("b", 20);
System.out.println(entry2); // b=20

final var entry3 = Map.entry("c", 30);
System.out.println(entry3); // c=30

final var map = Map.ofEntries(entry1, entry2, entry3);
System.out.println(map); // {b=20, c=30, a=10}

final var map = new HashMap<String, Integer>();
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);

System.out.println("-- entries --");
for (final var entry : map.entrySet()) {
    System.out.printf("key = %s : value = %d%n",
            entry.getKey(), entry.getValue());
}

// Result
// ↓
//-- entries --
//key = a : value = 10
//key = b : value = 20
//key = c : value = 30

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.

final var map = Map.ofEntries(
        Map.entry("b", 10),
        Map.entry("a", 20),
        Map.entry("d", 30),
        Map.entry("c", 40)
);

{
    final var comparator = Map.Entry.<String, Integer>comparingByKey();

    final var sorted = map.entrySet().stream().sorted(comparator).toList();
    System.out.println(sorted); // [a=20, b=10, c=40, d=30]
}
{
    final var comparator = Map.Entry.<String, Integer>comparingByKey(
            Comparator.reverseOrder());

    final var sorted = map.entrySet().stream().sorted(comparator).toList();
    System.out.println(sorted); // [d=30, c=40, b=10, a=20]
}

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.

Please see comparingByKey().

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.

final var map = Map.ofEntries(
        Map.entry("a", 20),
        Map.entry("b", 10),
        Map.entry("c", 40),
        Map.entry("d", 30)
);

{
    final var comparator = Map.Entry.<String, Integer>comparingByValue();

    final var sorted = map.entrySet().stream().sorted(comparator).toList();
    System.out.println(sorted); // [b=10, a=20, d=30, c=40]
}
{
    final var comparator = Map.Entry.<String, Integer>comparingByValue(
            Comparator.reverseOrder());

    final var sorted = map.entrySet().stream().sorted(comparator).toList();
    System.out.println(sorted); // [c=40, d=30, a=20, b=10]
}

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.

Please see comparingByValue().

static <K, V> Map.Entry<K,V> copyOf (Map.Entry<? extends K,? extends V> e)

Returns a copy of the given Map.Entry.

final var entry = new AbstractMap.SimpleEntry<>("a", 10);
System.out.println(entry); // a=10

final var copied = Map.Entry.copyOf(entry);
System.out.println(copied); // a=10

entry.setValue(999);

System.out.println(entry); // a=999
System.out.println(copied); // a=10

boolean equals (Object o)

Compares the specified object with this entry for equality.

final var entry1 = Map.entry("a", 10);
final var entry2 = Map.entry("a", 10);

System.out.println(entry1.equals(entry2)); // true

final var entry1 = Map.entry("a", 10);
final var entry2 = Map.entry("a", 20);
final var entry3 = Map.entry("b", 20);

System.out.println(entry1.equals(entry2)); // false
System.out.println(entry1.equals(entry3)); // false
System.out.println(entry2.equals(entry3)); // false

K getKey ()

Returns the key corresponding to this entry.

final var map = new HashMap<String, Integer>();
map.put("a", 10);
map.put("b", 20);
map.put("c", 30);

System.out.println("-- entries --");
for (final var entry : map.entrySet()) {
    System.out.printf("key = %s : value = %d%n",
            entry.getKey(), entry.getValue());
}

// Result
// ↓
//-- entries --
//key = a : value = 10
//key = b : value = 20
//key = c : value = 30

V getValue ()

Returns the value corresponding to this entry.

Please see getKey().

int hashCode ()

Returns the hash code value for this map entry.

final var entry1 = Map.entry("a", 10);
System.out.println(entry1.hashCode()); // 107

final var entry2 = Map.entry("a", 20);
System.out.println(entry2.hashCode()); // 117

final var entry3 = Map.entry("b", 20);
System.out.println(entry3.hashCode()); // 118

V setValue (V value)

Replaces the value corresponding to this entry with the specified value (optional operation).

final var entry = Map.entry("a", 10);
System.out.println(entry); // a=10

try {
    final var ret = entry.setValue(999);
} catch (UnsupportedOperationException e) {
    System.out.println("UnsupportedOperationException! : " + e.getMessage());
}

// Result
// ↓
//UnsupportedOperationException! : not supported

final var entry = new AbstractMap.SimpleEntry<>("a", 10);
System.out.println(entry); // a=10

final var ret = entry.setValue(999);
System.out.println(ret); // 10
System.out.println(entry); // a=999