A simple pair

Pair.java

public class Pair<L, R> {

    private L left;
    private R right;

    public Pair(L left, R right) {
        if (left == null) { throw new RuntimeException("Left must not be null"); }
        if (right == null) { throw new RuntimeException("Right must not be null"); }
        this.left = left;
        this.right = right;
    }

    public static <L, R> Pair<L, R> valueOf(L l, R r) {
        return new Pair<>(l, r);
    }

    public String leftString() {
        return left.toString();
    }

    public String rightString() {
        return right.toString();
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Pair<?, ?> pair = (Pair<?, ?>) o;

        if (left != null ? !left.equals(pair.left) : pair.left != null) return false;
        return !(right != null ? !right.equals(pair.right) : pair.right != null);

    }

    @Override
    public int hashCode() {
        int result = left != null ? left.hashCode() : 0;
        result = 31 * result + (right != null ? right.hashCode() : 0);
        return result;
    }

    @Override
    public String toString() {
        return left + ":" + right;
    }
}