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Immutable Clojure collections and a Transformation abstraction for Java 8+, immutably, type-safely, and with good performance. Name will change to "Paguro" in November 2016.

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package org.organicdesign.fp.collections;

import java.util.Map;
import java.util.Objects;

import org.organicdesign.fp.tuple.Tuple2;

import static org.organicdesign.fp.FunctionUtils.stringify;

/**
 Represents an immutable and serializable Key and Value pair.  This uses the Serialization Proxy
 pattern (Josh Bloch) so if you want to subclass, serialization may be difficult.
 */
public class KeyVal extends Tuple2 implements Map.Entry,
        UnmodMap.UnEntry {

    /** Public static Key/Value factory method */
    public static  KeyVal of(K a, V b) { return new KeyVal<>(a, b); }

    /** Public static Map.Entry factory method */
    public static  KeyVal of(Map.Entry entry) { return new KeyVal<>(entry); }

    // ======================================= Constructors =======================================

    /** Key/Value Constructor */
    public KeyVal(K k, V v) { super(k, v); }

    /** Map.Entry Constructor */
    public KeyVal(Map.Entry entry) { super(entry.getKey(), entry.getValue()); }

    // ===================================== Instance Methods =====================================

    @Override public String toString() {
        return "kv(" + stringify(_1) + "," + stringify(_2) + ")";
    }

    @Override
    public boolean equals(Object other) {
        // Cheapest operation first...
        if (this == other) { return true; }
        if (!(other instanceof Map.Entry)) { return false; }
        // Details...
        final Map.Entry that = (Map.Entry) other;
        return Objects.equals(_1, that.getKey()) && Objects.equals(_2, that.getValue());
    }

    @Override
    public int hashCode() {
        // This is specified in java.util.Map as part of the map contract.
        return  (_1 == null ? 0 : _1.hashCode()) ^
                (_2 == null ? 0 : _2.hashCode());
    }

    // Inherited from Map.Entry
    /** Returns the first field of the tuple.  To implement Map.Entry. */
    @Override public K getKey() { return _1; }
    /** Returns the second field of the tuple.  To implement Map.Entry. */
    @Override public V getValue() { return _2; }

    /** This method is required to implement Map.Entry, but calling it only issues an exception */
    @SuppressWarnings("deprecation")
    @Override @Deprecated public V setValue(V value) {
        throw new UnsupportedOperationException("KeyVal is immutable");
    }
}