Java List of Pair

Sometimes in programming, there is a need to store data in pairs or key-value pairs. In Java, a pair can be represented using the Map.Entry class. However, if we need to store multiple pairs, we can use a List of Pair.

Implementation

To implement a List of Pair in Java, we can make use of the SimpleEntry class in the java.util.AbstractMap package. This class extends the AbstractMap.SimpleImmutableEntry class and allows us to create a simple key-value pair.

List<Map.Entry<String, Integer>> pairs = new ArrayList<>();
pairs.add(new AbstractMap.SimpleEntry<>("John", 25));
pairs.add(new AbstractMap.SimpleEntry<>("Mary", 30));

We can also create a Pair class to represent a pair of values.

public class Pair<T, U> {
    public final T first;
    public final U second;

    public Pair(T first, U second) {
        this.first = first;
        this.second = second;
    }
}

With this class, we can create a List of Pair as follows:

List<Pair<String, Integer>> pairs = new ArrayList<>();
pairs.add(new Pair<>("John", 25));
pairs.add(new Pair<>("Mary", 30));

Usage

We can use the List of Pair in various scenarios. For example, to store the frequency of elements in a list.

List<String> list = Arrays.asList("John", "Mary", "John", "Mary", "Mary");
Map<String, Integer> frequency = new HashMap<>();
for (String str : list) {
    frequency.put(str, frequency.getOrDefault(str, 0) + 1);
}
List<Map.Entry<String, Integer>> pairs = new ArrayList<>(frequency.entrySet());

In the above code, we first create a Map to store the frequency of elements in the list. We then create a List of Pair from the entrySet of the Map.

Conclusion

In this article, we learned how to implement and use a List of Pair in Java. This data structure can be very useful in various scenarios where we need to store data in pairs.