How Can I Store Value Pairs in Java Without Unique Keys?

A Java Collection for Value Pairs: Tuples?

Introduction:
When working with data structures in Java, one often encounters the need for a collection that stores pairs of values with distinct types. While Java provides Maps for this purpose, they enforce a unique key for each entry, which may not always be desired. This article delves into an alternative solution for storing value pairs: AbstractMap.SimpleEntry.

Answer:

AbstractMap.SimpleEntry is a convenient class for representing pairs of values. It implements the Map.Entry interface, providing methods for accessing and modifying the key and value. However, unlike Maps, SimpleEntry does not require a unique key for each pair.

Implementation:

To use AbstractMap.SimpleEntry, you can create an ArrayList of SimpleEntry objects, specifying the types of the key and value at instantiation. Here's an example:

java.util.List<java.util.Map.Entry<String, Integer>> pairList = new java.util.ArrayList<>();

// Create and add pairs to the list
java.util.Map.Entry<String, Integer> pair1 = new java.util.AbstractMap.SimpleEntry<>("Not Unique key1", 1);
java.util.Map.Entry<String, Integer> pair2 = new java.util.AbstractMap.SimpleEntry<>("Not Unique key2", 2);
pairList.add(pair1);
pairList.add(pair2);

Customization:

For added convenience, you can create a subclass of ArrayList that provides an "of" method for easily creating and adding pairs to the list. This further simplifies the syntax:

public class TupleList<T> extends ArrayList<T> {
    public static <S, U> TupleList<Map.Entry<S, U>> of(S key, U value) {
        Map.Entry<S, U> pair = new AbstractMap.SimpleEntry<>(key, value);
        TupleList<Map.Entry<S, U>> tupleList = new TupleList<>();
        tupleList.add(pair);
        return tupleList;
    }
}

// Usage
TupleList<Map.Entry<String, Integer>> pair = new TupleList<>();
pair.of("Not Unique key1", 1);
pair.of("Not Unique key2", 2);

By utilizing AbstractMap.SimpleEntry, developers can efficiently store data in pairs with varying types, maintaining order without key uniqueness constraints.

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