When Should I Use `AbstractMap.SimpleEntry` Instead of Java's `Map` for Value Pairs?

An Alternative to Java's Map for Storing Value Pairs

Java provides the Map data structure for managing key-value pairs, allowing you to define specific types for each entry. However, when you need a collection of value pairs where each element consists of two values with distinct types, the Map may not be the ideal solution.

To address this, Java offers an alternative: the AbstractMap.SimpleEntry class. This class enables you to create an entry containing a pair of values, where each value can have its own type.

Using AbstractMap.SimpleEntry

To utilize AbstractMap.SimpleEntry, follow these steps:

1. Create an instance: Instantiate a SimpleEntry object, specifying the types of the values it will hold.
2. Add entries: Use the add() method of your collection to add SimpleEntry instances to your collection.
3. Access values: Retrieve the values from the SimpleEntry using the getKey() and getValue() methods.

For example:

// Create a list of SimpleEntry pairs
List<Map.Entry<String, Integer>> pairList = new ArrayList<>();

// Add some pairs to the list
pairList.add(new AbstractMap.SimpleEntry<>("Not Unique Key 1", 1));
pairList.add(new AbstractMap.SimpleEntry<>("Not Unique Key 2", 2));

Convenience Methods and Subclassing

To simplify the process, you can use the createEntry() method to create SimpleEntry instances with the desired types. Furthermore, you can subclass the ArrayList class and expose an of() method to make the syntax even more succinct.

// Create a TupleList class extending ArrayList
public class TupleList<E> extends ArrayList<E> {

    public TupleList() {}

    // Exposed method for creating and adding SimpleEntry pairs
    public void of(Object... args) {
        if (args.length % 2 != 0) {
            throw new IllegalArgumentException("Number of arguments must be even.");
        }

        for (int i = 0; i < args.length; i += 2) {
            add(new SimpleEntry<>(args[i], args[i + 1]));
        }
    }
}

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

This allows for a more concise syntax for creating and adding pairs to your collection.

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