如何在 Java 中为 TreeMap 编写自定义比较器？

Question

我想将键值对存储在 TreeMap 中，并根据以下逻辑根据 Key 的值对条目进行排序：按密钥长度排序。如果两个键的长度相同，则按字母顺序对其进行排序。例如，对于以下键值对。IBARAKI MitoCityTOCHIGI UtunomiyaCityGUNMA MaehashiCitySAITAMA SaitamaCityCHIBA ChibaCityTOKYO SinjyukuKANAGAWA YokohamaCity预期的输出是这样的。CHIBA : ChibaCityGUNMA : MaehashiCityTOKYO : SinjyukuIBARAKI : MitoCitySAITAMA : SaitamaCityTOCHIGI : UtunomiyaCityKANAGAWA : YokohamaCity

Answer 1

您可以将 Comparator 作为参数传递给 Map 的构造函数。根据文档，它仅用于键：

/**

 * Constructs a new, empty tree map, ordered according to the given

 * comparator.  All keys inserted into the map must be <em>mutually

 * comparable</em> by the given comparator: {@code comparator.compare(k1,

 * k2)} must not throw a {@code ClassCastException} for any keys

 * {@code k1} and {@code k2} in the map.  If the user attempts to put

 * a key into the map that violates this constraint, the {@code put(Object

 * key, Object value)} call will throw a

 * {@code ClassCastException}.

 *

 * @param comparator the comparator that will be used to order this map.

 *        If {@code null}, the {@linkplain Comparable natural

 *        ordering} of the keys will be used.

 */

public TreeMap(Comparator<? super K> comparator) {

    this.comparator = comparator;

}

通过这种方式，您可以通过密钥长度传递比较器，如下所示：

new TreeMap<>(Comparator.comparingInt(String::length).thenComparing(Comparator.naturalOrder()))

Answer 2

您需要为此编写自己comparator的并在 中使用它TreeMap，例如：

public class StringComparator implements Comparator<String> {

    @Override

    public int compare(String s1, String s2) {

        return s1.length() == s2.length() ? s1.compareTo(s2) : s1.length() - s2.length();

    }

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {

        Map<String, String> map = new TreeMap<>(new StringComparator());

        map.put("IBARAKI", "MitoCity");

        map.put("TOCHIGI", "UtunomiyaCity");

        map.put("GUNMA", "MaehashiCity");

        map.put("SAITAMA", "SaitamaCity");

        map.put("CHIBA", "ChibaCity");

        map.put("TOKYO", "Sinjyuku");

        map.put("KANAGAWA", "YokohamaCity");

        System.out.println(map);

    }

}

这不处理null值，但如果您null在用例中期望值，您可以添加处理。

Answer 3

您可以按如下方式执行此操作。

  public static void main(String[] args) {

      Map<String, String> map = new TreeMap<>(new CustomSortComparator());

      map.put("IBARAKI", "MitoCity");

      map.put("TOCHIGI", "UtunomiyaCity");

      map.put("GUNMA", "MaehashiCity");

      map.put("SAITAMA", "SaitamaCity");

      map.put("CHIBA", "ChibaCity");

      map.put("TOKYO", "Sinjyuku");

      map.put("KANAGAWA", "YokohamaCity");

      System.out.println(map);

  }

CustomSortComparator 的定义如下。

public class CustomSortComparator implements Comparator<String> {

  @Override

  public int compare(String o1, String o2) {

    if (o1.length() > o2.length()) {

      return 1;

    }

    if (o1.length() < o2.length()) {

      return -1;

    }

    return returnCompareBytes(o1, o2);

  }

  private int returnCompareBytes(String key1, String key2) {

    for (int i = 0; i < key1.length() - 1; i++) {

      if (key1.charAt(i) > key2.charAt(i)) {

        return 1;

      }

      if (key1.charAt(i) < key2.charAt(i)) {

        return -1;

      }

    }

    return 0;

  }

}