Java集合binarySearch无法正常工作

Question

我只是想使用本机Java binarySearch，希望它总是可以找到第一个匹配项。但是它并不总是返回第一次出现的错误，我在这里做错了什么？

import java.util.*;

class BinarySearchWithComparator
{
  public static void main(String[] args)
  {
    // Please scroll down to see 'User' class implementation.
    List<User> l = new ArrayList<User>();
    l.add(new User(10, "A"));
    l.add(new User(10, "A"));
    l.add(new User(10, "A"));

    l.add(new User(20, "B"));
    l.add(new User(20, "B"));
    l.add(new User(20, "B"));
    l.add(new User(20, "B"));
    l.add(new User(20, "B"));
    l.add(new User(20, "B"));
    l.add(new User(20, "B"));


    l.add(new User(30, "C"));
    l.add(new User(30, "C"));
    l.add(new User(30, "C"));
    l.add(new User(30, "C"));

    Comparator<User> c = new Comparator<User>() {
      public int compare(User u1, User u2) {
        return u1.getId().compareTo(u2.getId());
      }
    };

    // Must pass in an object of type 'User' as the key.
    // The key is an 'User' with the 'id' which is been searched for.
    // The 'name' field is not used in the comparison for the binary search,
    // so it can be a dummy value -- here it is omitted with a null.
    //
    // Also note that the List must be sorted before running binarySearch,
    // in this case, the list is already sorted.
    Collections.sort(l,c);

    int index = Collections.binarySearch(l, new User(10, null), c);
    System.out.println(index);

    index = Collections.binarySearch(l, new User(20, null), c);
    System.out.println(index);

    index = Collections.binarySearch(l, new User(30, null), c);
    System.out.println(index);

    index = Collections.binarySearch(l, new User(42, null), c);
    System.out.println(index);
  }
}

class User {
  private int id;
  private String name;

  public User(int id, String name) {
    this.id = id;
    this.name = name;
  }

  public Integer getId() {
    return Integer.valueOf(id);
  }
}

========

Result:
2 //not 0 ?
6 //not 3?
10 //ok
-15  ok

Why first 10 is not index 0 ?
Why first 20 is not index 3 ?
OK , 30 first occurrence is index 10

=====================================

更新资料

现在看来，API并没有保证！谁能给我一个有关如何找到给定元素的第一个出现和最后一个出现的工作示例（例如User（10，null）？

非常感谢。

Answer 1

Java不保证它将返回相等元素中的哪个元素。当您在相等范围内有多个元素时，您需要向下移动列表以查找与您要查找的元素相等的第一个元素，如下所示：

User lookFor = new User(30, null);
index = Collections.binarySearch(l, lookFor, c);
while (index > 0 && c.compare(lookFor, l[index-1]) == 0) {
    index--;
}
// At this point the index is at the first element of the equal range

您可以将此逻辑包装在静态函数中，以避免每次都编写显式循环。

请注意，如果您的集合是随机访问，这将导致最坏情况下的性能（当有许多相同的元素时）从O（lg（n））降为O（n）。