Java: как хранить данные три раза в списке?

Ответ 1

public class Triplet<T, U, V> {

    private final T first;
    private final U second;
    private final V third;

    public Triplet(T first, U second, V third) {
        this.first = first;
        this.second = second;
        this.third = third;
    }

    public T getFirst() { return first; }
    public U getSecond() { return second; }
    public V getThird() { return third; }
}

И для создания экземпляра списка:

List<Triplet<String, Integer, Integer>> = new ArrayList<>();

Ответ 2

public class Triple<F, S, T> {

    public final F first;
    public final S second;
    public final T third;

    public Triple(F first, S second, T third) {
        this.first = first;
        this.second = second;
        this.third = third;
    }

    @Override
    public boolean equals(Object o) {
        if (!(o instanceof Triple)) {
            return false;
        }
        Triple<?, ?, ?> p = (Triple<?, ?, ?>) o;
        return first.equals(p.first) && second.equals(p.second) && third.equals(p.third);
    }

    private static boolean equals(Object x, Object y) {
        return (x == null && y == null) || (x != null && x.equals(y));
    }

    @Override
    public int hashCode() {
        return (first == null ? 0 : first.hashCode()) ^ (second == null ? 0 : second.hashCode()) ^ (third == null ? 0 : third.hashCode());
    }

    public static <F, S, T> Triple <F, S, T> create(F f, S s, T t) {
        return new Triple<F, S, T>(f, s, t);
    }
}

Ответ 3

На основе Triple и ImmutableTriple без зависимостей импорта кода Apache. L, M и R должны реализовывать интерфейс Comparable.

Triple.java

import java.io.Serializable;

/**
 * <p>A triple consisting of three elements.</p>
 * <p/>
 * <p>This class is an abstract implementation defining the basic API. It refers to the elements as
 * 'left', 'middle' and 'right'.</p>
 * <p/>
 * <p>Subclass implementations may be mutable or immutable. However, there is no restriction on the
 * type of the stored objects that may be stored. If mutable objects are stored in the triple, then
 * the triple itself effectively becomes mutable.</p>
 *
 * @param <L>
 *         the left element type
 * @param <M>
 *         the middle element type
 * @param <R>
 *         the right element type
 *
 * @version $Id: Triple.java 1557584 2014-01-12 18:26:49Z britter $
 * @since 3.2
 */
public abstract class Triple<L, M, R>
        implements Comparable<Triple<L, M, R>>, Serializable
{

    /**
     * Serialization version
     */
    private static final long serialVersionUID = 1L;


    /**
     * <p>Obtains an immutable triple of from three objects inferring the generic types.</p>
     * <p/>
     * <p>This factory allows the triple to be created using inference to obtain the generic
     * types.</p>
     *
     * @param <L>
     *         the left element type
     * @param <M>
     *         the middle element type
     * @param <R>
     *         the right element type
     * @param left
     *         the left element, may be null
     * @param middle
     *         the middle element, may be null
     * @param right
     *         the right element, may be null
     *
     * @return a triple formed from the three parameters, not null
     */
    public static <L, M, R> Triple<L, M, R> of(
            final L left,
            final M middle,
            final R right)
    {
        return new ImmutableTriple<L, M, R>(left, middle, right);
    }

    //-----------------------------------------------------------------------


    /**
     * <p>Gets the left element from this triple.</p>
     *
     * @return the left element, may be null
     */
    public abstract L getLeft();

    /**
     * <p>Gets the middle element from this triple.</p>
     *
     * @return the middle element, may be null
     */
    public abstract M getMiddle();

    /**
     * <p>Gets the right element from this triple.</p>
     *
     * @return the right element, may be null
     */
    public abstract R getRight();

    //-----------------------------------------------------------------------


    /**
     * <p>Compares the triple based on the left element, followed by the middle element, finally the
     * right element. The types must be {@code Comparable}.</p>
     *
     * @param other
     *         the other triple, not null
     *
     * @return negative if this is less, zero if equal, positive if greater
     */
    @Override
    public int compareTo(final Triple<L, M, R> other)
    {
        @SuppressWarnings("unchecked") // assume this can be done; if not throw CCE as per Javadoc
        final Comparable<Object> comparableLeft = (Comparable<Object>) getLeft();
        int cmpLeft = comparableLeft.compareTo(other.getLeft());

        if (cmpLeft != 0) {
            return cmpLeft;
        }

        @SuppressWarnings("unchecked") // assume this can be done; if not throw CCE as per Javadoc
        final Comparable<Object> comparableMidle = (Comparable<Object>) getMiddle();
        int cmpMidle = comparableMidle.compareTo(other.getMiddle());

        if (cmpMidle != 0) {
            return cmpMidle;
        }

        @SuppressWarnings("unchecked") // assume this can be done; if not throw CCE as per Javadoc
        final Comparable<Object> comparableRight = (Comparable<Object>) getRight();
        int cmpRight = comparableRight.compareTo(other.getRight());

        return cmpRight;
    }


    /**
     * <p>Compares this triple to another based on the three elements.</p>
     *
     * @param obj
     *         the object to compare to, null returns false
     *
     * @return true if the elements of the triple are equal
     */
    @Override
    public boolean equals(final Object obj)
    {
        if (obj == null) {
            return false;
        }
        if (obj == this) {
            return true;
        }
        if (obj instanceof Triple<?, ?, ?>) {
            final Triple<?, ?, ?> other = (Triple<?, ?, ?>) obj;


            return equals(getLeft(), other.getLeft()) &&
                    equals(getMiddle(), other.getMiddle()) &&
                    equals(getRight(), other.getRight());
        }
        return false;
    }


    private boolean equals(
            final Object object1,
            final Object object2)
    {
        return !(object1 == null || object2 == null) &&
                (object1 == object2 || object1.equals(object2));
    }


    /**
     * <p>Returns a suitable hash code.</p>
     *
     * @return the hash code
     */
    @Override
    public int hashCode()
    {
        return (getLeft() == null ? 0 : getLeft().hashCode()) ^
                (getMiddle() == null ? 0 : getMiddle().hashCode()) ^
                (getRight() == null ? 0 : getRight().hashCode());
    }


    /**
     * <p>Returns a String representation of this triple using the format {@code
     * ($left, $middle, $right)}.</p>
     *
     * @return a string describing this object, not null
     */
    @Override
    public String toString()
    {
        return new StringBuilder().append('(')
                .append(getLeft())
                .append(',')
                .append(getMiddle())
                .append(',')
                .append(getRight())
                .append(')')
                .toString();
    }


    /**
     * <p>Formats the receiver using the given format.</p>
     * <p/>
     * <p>This uses {@link java.util.Formattable} to perform the formatting. Three variables may be
     * used to embed the left and right elements. Use {@code %1$s} for the left element, {@code
     * %2$s} for the middle and {@code %3$s} for the right element. The default format used by
     * {@code toString()} is {@code (%1$s,%2$s,%3$s)}.</p>
     *
     * @param format
     *         the format string, optionally containing {@code %1$s}, {@code %2$s} and {@code %3$s},
     *         not null
     *
     * @return the formatted string, not null
     */
    public String toString(final String format)
    {
        return String.format(format, getLeft(), getMiddle(), getRight());
    }
}

ImmutableTriple.java

public final class ImmutableTriple<L, M, R> extends Triple<L, M, R> {

    /** Serialization version */
    private static final long serialVersionUID = 1L;

    /** Left object */
    public final L left;
    /** Middle object */
    public final M middle;
    /** Right object */
    public final R right;

    /**
     * <p>Obtains an immutable triple of from three objects inferring the generic types.</p>
     *
     * <p>This factory allows the triple to be created using inference to
     * obtain the generic types.</p>
     *
     * @param <L> the left element type
     * @param <M> the middle element type
     * @param <R> the right element type
     * @param left  the left element, may be null
     * @param middle  the middle element, may be null
     * @param right  the right element, may be null
     * @return a triple formed from the three parameters, not null
     */
    public static <L, M, R> ImmutableTriple<L, M, R> of(final L left, final M middle, final R right) {
        return new ImmutableTriple<L, M, R>(left, middle, right);
    }

    /**
     * Create a new triple instance.
     *
     * @param left  the left value, may be null
     * @param middle the middle value, may be null
     * @param right  the right value, may be null
     */
    public ImmutableTriple(final L left, final M middle, final R right) {
        super();
        this.left = left;
        this.middle = middle;
        this.right = right;
    }

    //-----------------------------------------------------------------------
    /**
     * {@inheritDoc}
     */
    @Override
    public L getLeft() {
        return left;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public M getMiddle() {
        return middle;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public R getRight() {
        return right;
    }
}

Ответ 4

Кажется, я согласен с whirlwin.

Общий консенсус, хотя я не могу найти доказательства сейчас, предусматривает, что параметризованные типы должны быть не более 2. Вот почему каждый класс, который я могу представить в библиотеке коллекций Java, имеет максимум два параметризованных типа.

Код становится загроможденным, когда у вас есть три параметризованных типа. Просто слишком много > , < иметь дело.

Я предлагаю, что сказал whirlwin и создал объект, который содержит три конкретных типа, которые вам нужны.

Ответ 5

Если вы ожидаете отслеживать четыре или более объектов вместе, найдите что-то, что масштабируется.

Если бы вы решили, что хотите понятие "тройка", чтобы получить некоторые идеи, вы можете следовать шаблону Sun для javac.util.Pair

public class Pair<A, B> {

    public final A fst;
    public final B snd;

    public Pair(A fst, B snd) {
        this.fst = fst;
        this.snd = snd;
    }

    public String toString() {
        return "Pair[" + fst + "," + snd + "]";
    }

    private static boolean equals(Object x, Object y) {
        return (x == null && y == null) || (x != null && x.equals(y));
    }

    public boolean equals(Object other) {
        return
            other instanceof Pair<?,?> &&
            equals(fst, ((Pair<?,?>)other).fst) &&
            equals(snd, ((Pair<?,?>)other).snd);
    }

    public int hashCode() {
        if (fst == null) return (snd == null) ? 0 : snd.hashCode() + 1;
        else if (snd == null) return fst.hashCode() + 2;
        else return fst.hashCode() * 17 + snd.hashCode();
    }

    public static <A,B> Pair<A,B> of(A a, B b) {
        return new Pair<A,B>(a,b);
    }
}

или следуйте Google android.util.Pair

public class Pair<F, S> {
    public final F first;
    public final S second;

    public Pair(F first, S second) {
        this.first = first;
        this.second = second;
    }

    public boolean equals(Object o) {
        if (o == this) return true;
        if (!(o instanceof Pair)) return false;
        final Pair<F, S> other;
        try {
            other = (Pair<F, S>) o;
        } catch (ClassCastException e) {
            return false;
        }
        return first.equals(other.first) && second.equals(other.second);
    }

    public int hashCode() {
        int result = 17;
        result = 31 * result + first.hashCode();
        result = 31 * result + second.hashCode();
        return result;
    }
    //...
}