使用 Collections.sort() 对自定义对象进行排序

Question

I am trying to sort custom objects using Collections.sort in java and I am running into this error, with the following code:

import java.util.ArrayList;
import java.util.Collections;

public class StudentClient {
   public static void main(String[] args){
      ArrayList<Student> students = new ArrayList<Student>();
      students.add(new Student("Jasper Holton", 12));
      students.add(new Student("John Doe", 10));
      ArrayList<Student> sortedStudents = Collections.sort(students);
      System.out.println(sortedStudents);
   }
}
public class Student implements Comparable<Student> {
   public String name;
   public int id;
   public Student(String name){
      this.name = name;
   }
   public Student(String name, int id){
      this.name = name;
      this.id = id;
   }
   ...
   @Override
   public int compareTo(Student s){
      return s.id - this.id;
   }
}

This is the error:

StudentClient.java:14: error: incompatible types: void cannot be converted to ArrayList<Student>
      ArrayList<Student> sortedStudents = Collections.sort(students);

How do I fix this error? Why is collections.sort returning a void type?

Answer 1

This is a compile time error. sort works on the same list you provide, it doesn't return a new one.

So just change this:

ArrayList<Student> sortedStudents = Collections.sort(students);
System.out.println(sortedStudents);

For this

Collections.sort(students);
System.out.println(students);

Update : Reference documentation

All java classes are well documented on what's named javadoc (java documentation). There you can find every specification of classes, methods, return types, etc, etc, with explanations and sometimes even examples.

In your case, a quick look here would have been really helpful:

https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html

Answer 2

Have a look at the documentation of Collections.sort(List) . It says Sorts the specified list into ascending order . It won't return a new sorted list, but instead sort the existing list.

If you want a new sorted list you can use Stream or copy the list and then sort the copy.

//Stream
List<Student> sortedStudents = students.stream().sorted().collect(Collectors.toList());

//Copy
List <Student> sortedStudents = new ArrayList<>(students);
Collections.sort(sortedStudents);