Java中的 Collections.binarySearch() 和示例

Java.util.Collections.binarySearch() 方法是一个Java.util.Collections 类方法，它返回对象在排序列表中的位置。

// Returns index of key in sorted list sorted in
// ascending order
public static int binarySearch(List slist, T key)

// Returns index of key in sorted list sorted in
// order defined by Comparator c.
public static int binarySearch(List slist, T key, Comparator c)

If key is not present, the it returns "(-(insertion point) - 1)". 
The insertion point is defined as the point at which the key 
would be inserted into the list.

如果列表的元素不能使用指定的比较器进行比较，或者搜索键与元素不能比较，则该方法将抛出ClassCastException 。
在按升序排序的列表中搜索 int 键：

Java

// Java program to demonstrate working of Collections.
// binarySearch()
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
public class GFG {
    public static void main(String[] args)
    {
        List al = new ArrayList();
        al.add(1);
        al.add(2);
        al.add(3);
        al.add(10);
        al.add(20);
 
        // 10 is present at index 3.
        int index = Collections.binarySearch(al, 10);
        System.out.println(index);
 
        // 13 is not present. 13 would have been inserted
        // at position 4. So the function returns (-4-1)
        // which is -5.
        index = Collections.binarySearch(al, 13);
        System.out.println(index);
    }
}

输出 ：

3
-5

在按降序排序的列表中搜索 int 键。

Java

// Java program to demonstrate working of Collections.
// binarySearch() in an array sorted in descending order.
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
 
public class GFG {
    public static void main(String[] args)
    {
        List al = new ArrayList();
        al.add(100);
        al.add(50);
        al.add(30);
        al.add(10);
        al.add(2);
 
        // The last parameter specifies the comparator
        // method used for sorting.
        int index = Collections.binarySearch(
            al, 50, Collections.reverseOrder());
 
        System.out.println("Found at index " + index);
    }
}

输出 ：

Found at index 1

在用户定义的类对象列表中搜索：

Java

// Java program to demonstrate working of Collections.
// binarySearch() in a list of user defined objects
import java.util.*;
 
class Binarysearch {
    public static void main(String[] args)
    {
        // Create a list
        List l = new ArrayList();
        l.add(new Domain(10, "quiz.geeksforgeeks.org"));
        l.add(new Domain(20, "practice.geeksforgeeks.org"));
        l.add(new Domain(30, "code.geeksforgeeks.org"));
        l.add(new Domain(40, "www.geeksforgeeks.org"));
 
        Comparator c = new Comparator() {
            public int compare(Domain u1, Domain u2)
            {
                return u1.getId().compareTo(u2.getId());
            }
        };
 
        // Searching a domain with key value 10. To search
        // we create an object of domain with key 10.
        int index = Collections.binarySearch(
            l, new Domain(10, null), c);
        System.out.println("Found at index  " + index);
 
        // Searching an item with key 5
        index = Collections.binarySearch(
            l, new Domain(5, null), c);
        System.out.println(index);
    }
}
 
// A user-defined class to store domains with id and url
class Domain {
    private int id;
    private String url;
 
    // Constructor
    public Domain(int id, String url)
    {
        this.id = id;
        this.url = url;
    }
 
    public Integer getId() { return Integer.valueOf(id); }
}

输出 ：

0
-1

数组.binarysearch() 与 Collections.binarySearch()
Arrays.binarysearch() 适用于也可以是原始数据类型的数组。 Collections.binarysearch() 适用于对象集合，如 ArrayList 和 LinkedList。
要点：

• 如果输入列表未排序，则结果未定义。
• 如果有重复，则无法保证会找到哪一个。
• 对于像 ArrayList 这样的“随机访问”列表，此方法在 log(n) 时间内运行。如果指定的列表没有实现 RandomAccess 接口并且很大，则此方法将执行基于迭代器的二进制搜索，执行 O(n) 链接遍历和 O(log n) 元素比较。