Java - How to create new Entry (key, value)

Question

I'd like to create new item that similarly to Util.Map.Entry that will contain the structure key, value.

The problem is that I can't instantiate a Map.Entry because it's an interface.

Does anyone know how to create a new generic key/value object for Map.Entry?

Answer 1

org.apache.commons.lang3.tuple.Pair implements java.util.Map.Entry and can also be used standalone.

Also as others mentioned Guava's com.google.common.collect.Maps.immutableEntry(K, V) does the trick.

I prefer Pair for its fluent Pair.of(L, R) syntax.

Answer 2

If you are using Clojure, you have another option:

(defn map-entry
  [k v]
  (clojure.lang.MapEntry/create k v))

Answer 3

I defined a generic Pair class that I use all the time. It's great. As a bonus, by defining a static factory method (Pair.create) I only have to write the type arguments half as often.

public class Pair<A, B> {

    private A component1;
    private B component2;

    public Pair() {
            super();
    }

    public Pair(A component1, B component2) {
            this.component1 = component1;
            this.component2 = component2;
    }

    public A fst() {
            return component1;
    }

    public void setComponent1(A component1) {
            this.component1 = component1;
    }

    public B snd() {
            return component2;
    }

    public void setComponent2(B component2) {
            this.component2 = component2;
    }

    @Override
    public String toString() {
            return "<" + component1 + "," + component2 + ">";
    }

    @Override
    public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result
                            + ((component1 == null) ? 0 : component1.hashCode());
            result = prime * result
                            + ((component2 == null) ? 0 : component2.hashCode());
            return result;
    }

    @Override
    public boolean equals(Object obj) {
            if (this == obj)
                    return true;
            if (obj == null)
                    return false;
            if (getClass() != obj.getClass())
                    return false;
            final Pair<?, ?> other = (Pair<?, ?>) obj;
            if (component1 == null) {
                    if (other.component1 != null)
                            return false;
            } else if (!component1.equals(other.component1))
                    return false;
            if (component2 == null) {
                    if (other.component2 != null)
                            return false;
            } else if (!component2.equals(other.component2))
                    return false;
            return true;
    }

    public static <A, B> Pair<A, B> create(A component1, B component2) {
            return new Pair<A, B>(component1, component2);
    }

}

Answer 4

There's public static class AbstractMap.SimpleEntry<K,V>. Don't let the Abstract part of the name mislead you: it is in fact NOT an abstract class (but its top-level AbstractMap is).

The fact that it's a static nested class means that you DON'T need an enclosing AbstractMap instance to instantiate it, so something like this compiles fine:

Map.Entry<String,Integer> entry =
    new AbstractMap.SimpleEntry<String, Integer>("exmpleString", 42);

As noted in another answer, Guava also has a convenient static factory method Maps.immutableEntry that you can use.

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You said:

I can't use Map.Entry itself because apparently it's a read-only object that I can't instantiate new instanceof

That's not entirely accurate. The reason why you can't instantiate it directly (i.e. with new) is because it's an interface Map.Entry.

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Caveat and tip

As noted in the documentation, AbstractMap.SimpleEntry is @since 1.6, so if you're stuck to 5.0, then it's not available to you.

To look for another known class that implements Map.Entry, you can in fact go directly to the javadoc. From the Java 6 version

Interface Map.Entry

All Known Implementing Classes:

• AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry

Unfortunately the 1.5 version does not list any known implementing class that you can use, so you may have be stuck with implementing your own.