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Java 9+

In Java 9, you can simply write: Map.entry(key, value) to create an immutable pair.

Note: this method does not allow keys or values to be null. If you want to allow null values, for example, you‘d want to change this to: Map.entry(key, Optional.ofNullable(value)).

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Java 8+

In Java 8, you can use the more general-purpose javafx.util.Pair

to create an immutable, serializable pair. This class does allow null keys and null values. (In Java 9, this class is included in the javafx.base module). EDIT: As of Java 11, JavaFX has been decoupled from the JDK, so you‘d need the additional maven artifact org.openjfx:javafx-base.

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Java 6+

In Java 6 and up, you can use the more verbose AbstractMap.SimpleImmutableEntry

for an immutable pair, or AbstractMap.SimpleEntry for a pair whose value can be changed. These classes also allow null keys and null values, and are serializable.

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Android

If you‘re writing for Android, just use Pair.create(key, value) to create an immutable pair.

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Apache Commons

Apache Commons Lang

provides the helpful Pair.of(key, value) to create an immutable, comparable, serializable pair.

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Eclipse Collections

If you‘re using pairs that contain primitives, Eclipse Collections provides some very efficient primitive pair classes that will avoid all the inefficient auto-boxing and auto-unboxing.

For instance, you could use PrimitiveTuples.pair(int, int) to create an IntIntPair, or PrimitiveTuples.pair(float, long) to create a FloatLongPair.

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Project Lombok

Using Project Lombok, you can create an immutable pair class simply by writing:

@Value
public class Pair<K, V> {
    K key;
    V value;
}

Lombok will fill in the constructor, getters, equals(), hashCode(), and toString() methods for you automatically in the generated bytecode. If you want a static factory method instead of a constructor, e.g., a Pair.of(k, v), simply change the annotation to: @Value(staticConstructor = "of").

在java中如何表示pairs。map要求key是不重復的