nicely worked on Java8

Pair.java

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

public class Pair<Left, Right> {
    private static final List<Pair<?, ?>> cache = new ArrayList<>();
    
    public final Left left;
    public final Right right;
    private Pair(Left left, Right right) {
        this.left = left;
        this.right = right;
    }
    
    public static synchronized <Left, Right> Pair<Left, Right> of(Left left, Right right) {
        Pair<Left, Right> pair = new Pair<>(
            Objects.requireNonNull(left),
            Objects.requireNonNull(right));
        return pair.reallocate();
    }
    
    @SuppressWarnings("unchecked")
    private Pair<Left, Right> reallocate() {
        return (Pair<Left, Right>) cache.stream()
            .filter(this::equals)
            .findFirst()
            .orElseGet(() -> {
                cache.add(this);
                return this;
            });
    }
    
    @Override
    public int hashCode() {
        return left.hashCode() ^ right.hashCode();
    }
    @Override
    public boolean equals(Object other) {
        if (other instanceof Pair) {
            Pair<?, ?> p = (Pair<?, ?>) other;
            return left.equals(p.left) && right.equals(p.right);
        }
        return false;
    }
}