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)).
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.
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.
Android
If you're writing for Android, just use Pair.create(key, value) to create an immutable pair.
Apache Commons
Apache Commons Lang provides the helpful Pair.of(key, value) to create an immutable, comparable, serializable pair.
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.
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").