Java 用时间概念实现地图

我最近被要求使用以下两种方法实现数据结构：

设置键、值、时间：在指定时间将键设置为值。 getkey，time：获取指定时间或更早时间的key值

如果我们在某个特定的时间设置了一个键，它将永远保持该值，或者直到稍后设置为止

例子 例1

例2

例3

---

以下是我的实现：

import java.util.HashMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.TreeMap;

/**
 * An Implementation a Map with the notion of Time.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @param <T> the type of time tracked by this map.
 *            T MUST implement {@link Comparable} to have a notion of comparing times.
 * @author Hari Krishnan
 * @see Map
 * @see NavigableMap
 */
public class TimedMap<K, V, T extends Comparable<T>> {

    private final Map<K, NavigableMap<T, V>> map = new HashMap<>();

    /**
     * Associates the specified value with the specified key at the specified time in this map.
     * If the map previously contained a mapping for the key at this time,
     * the old value is replaced by the specified value.
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @param time  time at which said association should occur
     */
    public void set(K key, V value, T time) {
        if (!map.containsKey(key))
            map.put(key, new TreeMap<>());
        map.get(key).put(time, value);
    }

    /**
     * Returns the value to which the specified key is mapped at the specified time or earlier,
     * or {@code null} if this map contains no mapping for the key at the specified time or earlier.
     *
     * @param key  the key whose associated value is to be returned
     * @param time the time(or earlier) at which the values associated with the key is to be returned.
     * @return The value to which the specified key is mapped at the specified time or earlier, or
     * {@code null} if this map contains no mapping for the key at the specified time or earlier.
     */
    public V get(K key, T time) {
        try {
            return map.get(key).floorEntry(time).getValue();
        } catch (NullPointerException ex) {
            return null;
        }
    }

}

---

欢迎改进和建议

以下是我的实现：

import java.util.HashMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.TreeMap;

/**
 * An Implementation a Map with the notion of Time.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @param <T> the type of time tracked by this map.
 *            T MUST implement {@link Comparable} to have a notion of comparing times.
 * @author Hari Krishnan
 * @see Map
 * @see NavigableMap
 */
public class TimedMap<K, V, T extends Comparable<T>> {

    private final Map<K, NavigableMap<T, V>> map = new HashMap<>();

    /**
     * Associates the specified value with the specified key at the specified time in this map.
     * If the map previously contained a mapping for the key at this time,
     * the old value is replaced by the specified value.
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @param time  time at which said association should occur
     */
    public void set(K key, V value, T time) {
        if (!map.containsKey(key))
            map.put(key, new TreeMap<>());
        map.get(key).put(time, value);
    }

    /**
     * Returns the value to which the specified key is mapped at the specified time or earlier,
     * or {@code null} if this map contains no mapping for the key at the specified time or earlier.
     *
     * @param key  the key whose associated value is to be returned
     * @param time the time(or earlier) at which the values associated with the key is to be returned.
     * @return The value to which the specified key is mapped at the specified time or earlier, or
     * {@code null} if this map contains no mapping for the key at the specified time or earlier.
     */
    public V get(K key, T time) {
        try {
            return map.get(key).floorEntry(time).getValue();
        } catch (NullPointerException ex) {
            return null;
        }
    }

}

---

欢迎改进和建议

而不是整个如果！map.containskey。。。您可以用一行代码替换set方法中的代码：map.computeIfAbsentkey，k->new TreeMap.puttime，value。如果您使用的是Java8+Oh-wow。我正在学习数据结构，但不知道它的存在！谢谢不要以这种方式捕获NullPointerException。这将隐藏bug。使用显式null检查并不复杂。虽然我同意NullPointerException的全面捕获是不好的，但在我的例子中，返回null的唯一可能位置是getkey或floorEntrytime。在这两种情况下，我都没有通过手动检查a并返回null来获得额外的清晰度。这是我的考虑，因此我觉得像这样的一行程序更具可读性，而且不失清晰。通过查看您的代码，您如何判断HashMap.get调用的hashCode和equals方法K和TreeMap.floorEntry调用的compareTo方法T永远不会抛出NullPointerException？除此之外，您的代码不是一行代码。您的代码是跨越五行的try-catch块。它并不比，例如，var e=map.getOrDefaultkey，Collections.emptyNavigableMap.floorEntrytime简单；返回e==null？null:e.getValue；而不是整个如果！map.containskey。。。您可以用一行代码替换set方法中的代码：map.computeIfAbsentkey，k->new TreeMap.puttime，value。如果您使用的是Java8+Oh-wow。我正在学习数据结构，但不知道它的存在！谢谢不要以这种方式捕获NullPointerException。这将隐藏bug。使用显式null检查并不复杂。虽然我同意NullPointerException的全面捕获是不好的，但在我的例子中，返回null的唯一可能位置是getkey或floorEntrytime。在这两种情况下，我都没有通过手动检查a并返回null来获得额外的清晰度。这是我的考虑，因此我觉得像这样的一行程序更具可读性，而且不失清晰。通过查看您的代码，您如何判断HashMap.get调用的hashCode和equals方法K和TreeMap.floorEntry调用的compareTo方法T永远不会抛出NullPointerException？除此之外，您的代码不是一行代码。您的代码是跨越五行的try-catch块。它并不比，例如，var e=map.getOrDefaultkey，Collections.emptyNavigableMap.floorEntrytime简单；返回e==null？null:e.getValue；所以你想保留这些变化的历史记录？如果是这样的话，如果你明确地说出来，你的问题会更清楚。不，我追踪的不是历史。假设一个传统地图有一个键值。如果现在引入新值，则覆盖现有值。跟踪这些旧值就是跟踪历史。我的要求与此无关。在这里，同一个键可以在不同的时间具有不同的值。在这里，时间可以是任何可比较的类型。所以您想保留更改的历史记录吗？如果是这样的话，如果你明确地说出来，你的问题会更清楚。不，我追踪的不是历史。假设一个传统地图有一个键值。如果现在引入新值，则覆盖现有值。跟踪这些旧值就是跟踪历史。我的要求与此无关。在这里，同一个键可以在不同的时间具有不同的值。在这里，时间可以是任何可比较的类型。

d.set(1, 1, 0) // set key 1 to value 1 at time 0
d.set(1, 2, 0) // set key 1 to value 2 at time 0
d.get(1, 0) // get key 1 at time 0 should be 2

import java.util.HashMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.TreeMap;

/**
 * An Implementation a Map with the notion of Time.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 * @param <T> the type of time tracked by this map.
 *            T MUST implement {@link Comparable} to have a notion of comparing times.
 * @author Hari Krishnan
 * @see Map
 * @see NavigableMap
 */
public class TimedMap<K, V, T extends Comparable<T>> {

    private final Map<K, NavigableMap<T, V>> map = new HashMap<>();

    /**
     * Associates the specified value with the specified key at the specified time in this map.
     * If the map previously contained a mapping for the key at this time,
     * the old value is replaced by the specified value.
     *
     * @param key   key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @param time  time at which said association should occur
     */
    public void set(K key, V value, T time) {
        if (!map.containsKey(key))
            map.put(key, new TreeMap<>());
        map.get(key).put(time, value);
    }

    /**
     * Returns the value to which the specified key is mapped at the specified time or earlier,
     * or {@code null} if this map contains no mapping for the key at the specified time or earlier.
     *
     * @param key  the key whose associated value is to be returned
     * @param time the time(or earlier) at which the values associated with the key is to be returned.
     * @return The value to which the specified key is mapped at the specified time or earlier, or
     * {@code null} if this map contains no mapping for the key at the specified time or earlier.
     */
    public V get(K key, T time) {
        try {
            return map.get(key).floorEntry(time).getValue();
        } catch (NullPointerException ex) {
            return null;
        }
    }

}