Java错误eclipse列表排序

Question

我在以下行中遇到错误:

Collections.sort(l);

你能解释一下为什么吗？

这是程序代码:

import java.io.*;
import java.util.*;

public class Esame{

    public static void main(String args[]) throws IOException{
        ArrayList<Studente> l=new ArrayList<Studente>();
        BufferedReader b=new BufferedReader(new InputStreamReader(System.in));
        String cognome = b.readLine();
        while(!cognome.equals("")){
            System.out.println("inserire nome");
            String nome = b.readLine();
            System.out.println("inserire matricola");
            String matricola = b.readLine();
            Studente s = new Studente(nome, cognome, matricola);
            l.add(s);
            System.out.println("inserire cognome");
            cognome = b.readLine();
        }
        b.close();
        Collections.sort(l);
        Iterator<Studente> i=l.iterator();
        PrintStream p=new PrintStream(new File("uscita.txt"));
        while(i.hasNext()){
            p.println(i.hasNext());
        }
        p.close();
    }

}

class Studente{

    protected String nome,cognome;
    protected int matricola;

    public Studente(String nome,String cognome,String matricola){
        this.nome=nome;
        this.cognome=cognome;
        this.matricola=Integer.parseInt(matricola);
    }

    public String toString(){
        return "\n NOME:  "+nome+"Cognome:  "+cognome+"MATRICOLA: "+matricola;
    }

    public int compareTo(Object x) {
         Studente p = (Studente) x;
         return cognome.compareTo(p.cognome);
    }
}

Answer 1

您只需 implements Comparable<T>要解决问题，或者您可以传递 Comparator到sort方法，如:

Collections.sort(l, new Comparator<Studente>() {
            @Override
            public int compare(Studente o1, Studente o2) {
                // some kind of comparation
                return o1.matricola - o2.matricola;
            }
        });

这样你的类就不需要实现 Comparable 接口(interface)