极客教程Java Collections.sort()方法及实例
java.util.Collections.sort() 方法存在于java.util.Collections类中。它用于将指定的集合列表中的元素按升序排序。它的工作原理类似于java.util.Arrays.sort()方法，但它比它更好，因为它可以对数组中的元素进行排序，也可以对链接列表、队列和许多其他元素进行排序。
public static void sort(List myList)
myList : 一个我们想要排序的列表类型对象。
该方法不返回任何东西
例子。
Let us suppose that our list contains
{"Geeks For Geeks", "Friends", "Dear", "Is", "Superb"}
After using Collection.sort(), we obtain a sorted list as
{"Dear", "Friends", "Geeks For Geeks", "Is", "Superb"}
以升序对数组列表进行排序
// Java program to demonstrate working of Collections.sort()
import java.util.*;
public class Collectionsorting
{
public static void main(String[] args)
{
// Create a list of strings
ArrayList<String> al = new ArrayList<String>();
al.add("Geeks For Geeks");
al.add("Friends");
al.add("Dear");
al.add("Is");
al.add("Superb");
/* Collections.sort method is sorting the
elements of ArrayList in ascending order. */
Collections.sort(al);
// Let us print the sorted list
System.out.println("List after the use of" +
" Collection.sort() :\n" + al);
}
}
输出
List after the use of Collection.sort() :
[Dear, Friends, Geeks For Geeks, Is, Superb]
时间复杂度 ：O(N log N)，因为Collections.sort()的时间复杂度是O(nlog(n))。
辅助空间 ：O(1)
对ArrayList进行降序排序
// Java program to demonstrate working of Collections.sort()
// to descending order.
import java.util.*;
public class Collectionsorting
{
public static void main(String[] args)
{
// Create a list of strings
ArrayList<String> al = new ArrayList<String>();
al.add("Geeks For Geeks");
al.add("Friends");
al.add("Dear");
al.add("Is");
al.add("Superb");
/* Collections.sort method is sorting the
elements of ArrayList in ascending order. */
Collections.sort(al, Collections.reverseOrder());
// Let us print the sorted list
System.out.println("List after the use of" +
" Collection.sort() :\n" + al);
}
}
输出
List after the use of Collection.sort() :
[Superb, Is, Geeks For Geeks, Friends, Dear]
时间复杂度： O(N log N)，因为Collections.sort()的时间复杂度是O(nlog(n))。
辅助空间： O(1)
根据用户定义的标准对ArrayList进行排序 我们可以使用比较器接口来实现这一目的。
// Java program to demonstrate working of Comparator
// interface and Collections.sort() to sort according
// to user defined criteria.
import java.util.*;
import java.lang.*;
import java.io.*;
// A class to represent a student.
class Student
{
int rollno;
String name, address;
// Constructor
public Student(int rollno, String name,
String address)
{
this.rollno = rollno;
this.name = name;
this.address = address;
}
// Used to print student details in main()
public String toString()
{
return this.rollno + " " + this.name +
" " + this.address;
}
}
class Sortbyroll implements Comparator<Student>
{
// Used for sorting in ascending order of
// roll number
public int compare(Student a, Student b)
{
return a.rollno - b.rollno;
}
}
// Driver class
class Main
{
public static void main (String[] args)
{
ArrayList<Student> ar = new ArrayList<Student>();
ar.add(new Student(111, "bbbb", "london"));
ar.add(new Student(131, "aaaa", "nyc"));
ar.add(new Student(121, "cccc", "jaipur"));
System.out.println("Unsorted");
for (int i=0; i<ar.size(); i++)
System.out.println(ar.get(i));
Collections.sort(ar, new Sortbyroll());
System.out.println("\nSorted by rollno");
for (int i=0; i<ar.size(); i++)
System.out.println(ar.get(i));
}
}
输出
Unsorted
111 bbbb london
131 aaaa nyc
121 cccc jaipur
Sorted by rollno
111 bbbb london
121 cccc jaipur
131 aaaa nyc
Arrays. sort() vs Collections.sort() Arrays.sort适用于数组，也可以是原始数据类型。Collections.sort()适用于对象集合，如ArrayList、LinkedList等。我们可以使用Collections.sort()在给定数组项目创建ArrayList后对数组进行排序。
// Using Collections.sort() to sort an array
import java.util.*;
public class Collectionsort
{
public static void main(String[] args)
{
// create an array of string objs
String domains[] = {"Practice", "Geeks",
"Code", "Quiz"};
// Here we are making a list named as Collist
List colList =
new ArrayList(Arrays.asList(domains));
// Collection.sort() method is used here
// to sort the list elements.
Collections.sort(colList);
// Let us print the sorted list
System.out.println("List after the use of" +
" Collection.sort() :\n" +
colList);
}
}
输出
List after the use of Collection.sort() :
[Code, Geeks, Practice, Quiz]
Arrays.sort() vs Collections.sort() 的时间复杂度
Arrays.sort()使用Dual-Pivot Quicksort算法，其时间复杂度为O(N.log N)，通常比传统Quicksort算法快。另一方面，Collections.sort()创建了一个列表元素的数组，使用自适应的Mergesort算法对它们进行排序，并在列表上进行迭代，将每个元素放在正确的位置。因此，对于原始数据类型如int、char、double等。Arrays.sort()被证明比Collections.sort()更节省时间。涉及原始数据类型的问题应该尝试使用Arrays.sort()来解决，以达到更好的优化效果。
下面的代码展示了这一区别。
/*package whatever //do not write package name here */
import java.io.*;
import java.util.*;
class GFG {
public static void main (String[] args) {
int len = 5000000;
// creating a large test array
int[] arr = new int[len];
for (int i = len; i > 0; i--)
arr[len - i] = i;
// creating a large test arraylist
ArrayList<Integer> list = new ArrayList<>();
for (int i = len; i > 0; i--)
list.add(i);
// calculating time used by arrays.sort()
long startA = System.currentTimeMillis();
Arrays.sort(arr);
long stopA = System.currentTimeMillis();
// calculating time used by collections.sort()
long startAL = System.currentTimeMillis();
Collections.sort(list);
long stopAL = System.currentTimeMillis();
System.out.println("Time taken by Arrays.sort(): " + (stopA - startA));
System.out.println("Time taken by Collections.sort(): " + (stopAL - startAL));
}
}
// This code is contributed by godcoder28
输出
Time taken by Arrays.sort(): 29
Time taken by Collections.sort(): 42