【发布时间】：2011-05-15 19:55:48
【问题描述】：
在 java 中将数据三元组存储在列表中的最佳方法是什么？
[a, b, c]
[a, b, c]
...
我通常将 HashMap 用于一对数据键 + 值。我应该使用 HashMap + Arraylist 吗？还是 ArrayList + ArrayList ？
谢谢
【问题讨论】：
-
或者创建一个包含三个对象的新类。你用它做什么？
标签： java
【发布时间】：2011-05-15 19:55:48
【问题描述】：
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; }
}
并实例化列表：
List<Triplet<String, Integer, Integer>> = new ArrayList<>();
【讨论】：
-
您应该将
a、b和c设为final。 -
equals 和 hashCode 在这里可能还不错
-
如果 a、b、c 不是私有的，为什么还要创建 getter？
-
@SteveKuo 为什么它必须是最终的？
-
@AmiHollander 在之前的编辑中，这些字段不是私有的。现在它有点多余，但可能仍然有助于传达它们是最终的（而不是暗示缺乏 setter）。
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);
}
}
【讨论】：
-
这个类的hashCode方法很容易发现冲突。考虑创建三个实例（
）。第一个是 [1,0,0]，第二个是 [0,1,0]，第三个是 [0,0,1]。尽管它们是不同的三元组，但它们的 hashCode 都等于 1。
基于Triple 和ImmutableTriple，不依赖Apache 代码的导入。 L、M 和 R 类型必须实现接口 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;
}
}
【讨论】：
如果您希望同时跟踪四个或更多对象，请寻找可以缩放的对象。
如果您决定要使用 Triple 的概念，为了获得一些想法，您可以遵循 Sun 的 javac.util.Pair 模式：
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);
}
}
或关注谷歌的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;
}
//...
}
【讨论】：
我似乎同意 whirlwin。
虽然我现在找不到证据，但普遍的共识规定参数化类型不应超过 2。这就是为什么我能想到的 Java 集合库中的每个类都最多有两个参数化类型。
当您拥有三种参数化类型时，代码会变得混乱。太多了 >,
我建议 whirlwin 所说的并创建一个包含您需要的三种特定类型的对象。
【讨论】：
Kotlin 实现了一个名为 Triple 的类：
/**
* Represents a triad of values
*
* There is no meaning attached to values in this class, it can be used for any purpose.
* Triple exhibits value semantics, i.e. two triples are equal if all three components are equal.
* An example of decomposing it into values:
* @sample samples.misc.Tuples.tripleDestructuring
*
* @param A type of the first value.
* @param B type of the second value.
* @param C type of the third value.
* @property first First value.
* @property second Second value.
* @property third Third value.
*/
public data class Triple<out A, out B, out C>(
public val first: A,
public val second: B,
public val third: C
) : Serializable {
/**
* Returns string representation of the [Triple] including its [first], [second] and [third] values.
*/
public override fun toString(): String = "($first, $second, $third)"
}
【讨论】：