哈希集迭代器打印

Question

实验课

import java.math.BigInteger;
import java.util.Scanner;

public class Lab {

    public static void main(String[] args) {

        Studente s;
        inserimento();
    }

    public static void inserimento() {

        Studente s = null;
        do {
            try {
                //inserimento matricola
                System.out.println("\nmatricola:");
                Scanner mat = new Scanner(System.in);
                String matrstring = mat.nextLine();
                if (matrstring.equals("")) {
                    break;
                }
                int matricola = Integer.parseInt(matrstring);

                //inserimento cognome
                System.out.println("\ncognome:");
                Scanner cog = new Scanner(System.in);
                String cognome = cog.next();

                //inserimento nome
                System.out.println("\nnome:");
                Scanner nom = new Scanner(System.in);
                String nome = nom.next();

                //caricamento studente
                s = new Studente(matricola, cognome, nome);
                //caricamento studenti nell'hashset
                s.addStudenteSet(s);

            } catch (Exception e) {
                System.out.println("Dati inseriti sbagliati");
            }
        } while (true);

        System.out.println("fine inserimento");

        s.print();

    }

}

在这门课中，我输入学生代码、姓氏和姓名，然后将它们放入 Studente 课中。

import java.util.*;


public class Studente {

    private int matricola;
    private String cognome;
    private String nome;
    private Set<Studente> studenti = new HashSet<Studente>();

    public Studente(int matricola, String cognome, String nome)  {
        
        this.matricola=matricola;
        this.cognome=cognome;
        this.nome=nome;
    }
    
    public void addStudenteSet(Studente s){
        this.studenti.add(s);
    }
    @Override
    public boolean equals(Object o){
        Studente st = (Studente) o;
        if(this.matricola==st.matricola){
            return true;
        }else return false; 
    }
    @Override
    public int hashCode(){
        return Integer.hashCode(matricola);
    }
    
    
    public void print(){
        Iterator<Studente> i = this.studenti.iterator();
        while(i.hasNext()){
            Studente student = i.next();
            System.out.println("matricola: " + student.matricola + "\ncognome: " +student.cognome+ "\nnome: " +student.nome);

        }
        
    }
}

在这里，我使用了一个 hashset 和打印方法，我想打印我带进实验室课的每个学生，但它只打印最后一个。我该如何解决这个问题？在 Lab 类中，我调用了 addStudenteSet(s) 方法；

Answer 1

以下行有问题。

s = new Studente(matricola, cognome, nome);
//caricamento studenti nell'hashset
s.addStudenteSet(s);

您正在创建一个新对象“s”并将“s”添加到该对象内的 HashSet 中。 所以每次迭代都会产生新的对象，并且所有对象的 HashSet 都是相同的。

您可以将 HashSet studenti 设为静态，然后它将对所有对象都相同。

public static Set studenti = new HashSet();

或者您可以检查以下代码（请注意，我只考虑您提到的问题，未修复任何其他实现问题）。

public class Lab {

    private static  Set<Studente> studenti = new HashSet<Studente>();

    public static void main(String[] args) {

        Studente s;
        inserimento();
    }

    public static void inserimento() {

        Scanner mat = new Scanner(System.in);
        Studente s = null;
        boolean isContinue=true;
        do {
            try {
                //inserimento matricola
                System.out.println("\nmatricola:");
                String matrstring = mat.nextLine();
                if (matrstring.equals("")) {
                    break;
                }
                int matricola = Integer.parseInt(matrstring);

                //inserimento cognome
                System.out.println("\ncognome:");
                String cognome = mat.nextLine();

                //inserimento nome
                System.out.println("\nnome:");
                String nome = mat.nextLine();

                //caricamento studente
                s = new Studente(matricola, cognome, nome);
                //caricamento studenti nell'hashset
                studenti.add(s);
                
                System.out.println("\nenter 1 to continue:");
                isContinue= Integer.parseInt(mat.nextLine())==1 ? true :false;
            } catch (Exception e) {
                System.out.println("Dati inseriti sbagliati");
                isContinue=false;
            }
        } while (isContinue);

        System.out.println("fine inserimento");

        print();

    }



 public static void print(){
        Iterator<Studente> i = studenti.iterator();
        while(i.hasNext()){
            Studente student = i.next();
            System.out.println("matricola: " + student.getMatricola() + "\tcognome: " +student.getCognome()+ "\tnome: " +student.getNome()+"\n");

        }
        
    }

}



public class Studente {

    private int matricola;
    private String cognome;
    private String nome;
    
    public int getMatricola() {
        return matricola;
    }

    public String getCognome() {
        return cognome;
    }

    public String getNome() {
        return nome;
    }
    public Studente(int matricola, String cognome, String nome)  {
            
        this.matricola=matricola;
        this.cognome=cognome;
        this.nome=nome;
    }

    @Override
    public boolean equals(Object o){
        Studente st = (Studente) o;
        if(this.matricola==st.matricola){
            return true;
        }else return false; 
    }
    @Override
    public int hashCode(){
        return Integer.hashCode(matricola);
    }
}

Answer 2

每个Studente 都有自己的Set<Studente>，它只包含一个Studente（即当前学生本身，因为您使用s = new Studente(matricola, cognome, nome); 创建了一个新学生，然后使用s.addStudenteSet(s); 将其添加到自己的集合中)。

您需要在 void inserimento() 方法中添加一个 Set<Studente>，而不是这个：

public static void inserimento() {

    Scanner scanner = new Scanner(System.in);
    Set<Studente> studenti = new HashSet<Studente>();
    do {
        try {
            //inserimento matricola
            System.out.println("\nmatricola:");
            String matrstring = scanner.nextLine();
            if (matrstring.equals("")) {
                break;
            }
            int matricola = Integer.parseInt(matrstring);

            //inserimento cognome
            System.out.println("\ncognome:");
            String cognome = scanner.next();

            //inserimento nome
            System.out.println("\nnome:");
            String nome = scanner.next();

            //caricamento studente
            s = new Studente(matricola, cognome, nome);
            //caricamento studenti nell'hashset
            studenti.add(s);
            scanner.nextLine(); // added to skip the pending newline after scanner.next();

        } catch (Exception e) {
            System.out.println("Dati inseriti sbagliati");
        }
    } while (true);

    System.out.println("fine inserimento");

    for (Studente s: studenti) {
        System.out.println("matricola: " + student.matricola + "\ncognome: " +student.cognome+ "\nnome: " +student.nome);
    }

}