Java错误日食排序

Question

我有以下行错误：

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

类STUDENTE必须实现Comparable接口，以便进行排序！

否则.sort(...)不知道如何比较Studente对象。

Answer 2

只要将您的Arraylist更改为List即可。

ArrayList<Studente> l=new ArrayList<Studente>();

应该

List<Studente> l=new ArrayList<Studente>();

Collections.sort method accepts an列表and not an ArrayList`

从规格

见 api

片段

Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

你Studente类应该实现Comparable界面为你sort正常工作。

ImplementComparable接口在Studente类

class Studente implements Comparable<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; 
    } 

    @Override 
    public int compareTo(Studente o) { 
     // Logic for compare has to go here 
     return 0; 
    } 

Answer 3

变化

1）在学生类中实现Comparable接口

public class Studente implements Comparable<Studente>{ 
} 

2.）更改重写方法compareTo

@Override 
    public int compareTo(Studente o) { 
     Studente p = (Studente) o; 
     return cognome.compareTo(p.cognome); 
    } 

从Javadoc中，

Sorts the specified list into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

参考Collections#sort更多细节

Answer 4

class Studente implements Comparable<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(Studente x) { 
     //how to compare 
     return 0; 
    } 

你必须让你的类实现可比。

Answer 5

您只需implements Comparable<T>来解决这个问题，或者你可以传递一个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; 
      } 
     }); 

这样你的类并不需要实现Comparable接口