在对象中实现二进制搜索

collections binary-search java search

2022-09-01 18:05:54

有没有办法在数组列表中实现具有对象的二进制搜索？在此示例中，ArrayList 将使用字段 'id' 进行排序。

class User{
 public int id;
 public string name;
}

ArrayList<User> users = new ArrayList<User>();

sortById(users);

int id = 66
User searchuser = getUserById(users,id);

“User getUserById（ ArrayList users， int userid ）”如果我应该使用二进制搜索返回具有指定 id 的用户，它会是什么样子？这有可能吗？

答案 1

Java 教程的对象排序一文提供了一个示例，该示例编写了自己的比较器，以便对自定义类型执行比较。

然后，要查找的密钥（或任何其他）可以传递到 Collections.binarySearch 方法中。ArrayListListComparator

下面是一个示例：

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(20, "B"));
    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.

    int index = Collections.binarySearch(l, new User(20, null), c);
    System.out.println(index);    // Output: 1

    index = Collections.binarySearch(l, new User(10, null), c);
    System.out.println(index);    // Output: 0

    index = Collections.binarySearch(l, new User(42, null), c);
    System.out.println(index);    // Output: -4
                                  // See javadoc for meaning of return value.
  }
}

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);
  }
}

答案 2

您还可以将比较器放在 User 类中：

public class User implements Comparable<User>, Comparator<User>
{
  public User(int id, String name)
  {
    this.id = id;
    this.name = name;
  }
  @Override
  public int compareTo(User u)
  {
    return id - u.getID();
  }
  @Override
  public int compare(User u1, User u2)
  {
    return u1.getID() - u2.getID();
  }

  public int getID() { return id; }
  public String getName() { return name; }
  private int id;
  private String name;
}

然后，您将对名为 users 的 ArrayList 执行以下操作：

ArrayList<User> users = new ArrayList<User>();
users.add(new User(3, "Fred"));
users.add(new User(42, "Joe"));
users.add(new User(5, "Mary"));
users.add(new User(17, "Alice"));

Collections.sort(users);
int index = Collections.binarySearch(users, new User(5, null));
if(index >= 0)
  System.out.println("The user name of id 5 is: "+users.get(index).getName());
else
  System.out.println("ID 5 is not in the list");