ArrayList不存储对象

Question

Hello I am having trouble printing out items in my ArrayList. I can print it out in my PatronBorrow method but in PatronList and PatronReturn it does not print anything. Can anyone tell me what is wrong with the code? Thank you all very much

package proj1;

import java.util.ArrayList;
import java.util.List;


public class Patron {

private int id;
private Book book;
private List<Book> books;

public Patron(int id){
    this.id = id;
    books = new ArrayList<Book>();
}

public int getID(){
    return id;
}

public List<Book> getBooks(){
    return books;
}

public void PatronBorrow(String b){
    book = new Book(b);
    books.add(book);
    System.out.println("Patron " + id + " has borrowed " + book.getTitle());
}

public void PatronReturn(String b){
    for(Book book : books){
        if(book.getTitle().equals(b)){
            books.remove(book);
            System.out.println("Patron " + id + " has borrowed " +     book.getTitle());
        }
    }
}

public void PatronList(){
    for(Book b : books){
        System.out.println("Patron " + id + " has borrowed " + books.size() + " item(s)");
        System.out.println(b);
    }
}

}

package proj1;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Project1 {

public static boolean isNumeric(String str){
    for(char c : str.toCharArray()){
        if(Character.isDigit(c)){
            return true;
        }
    }
    return false;
}

public static void main(String[] args){

    String command;
    String line;
    Patron patron;
    int patronID;
    String title;
    String newTitle;
    String infile = args[0];

    if (args.length != 1){
        throw new IllegalArgumentException("Enter in file name");
    }

    try{
        Scanner file = new Scanner(new FileInputStream(infile));
        while(file.hasNext()){
            command = file.next();
            if(isNumeric(command)){
                patronID = Integer.parseInt(command);
                patron = new Patron(patronID);
                command = file.next();
                if(command.equals("borrow")){
                    title = file.nextLine();
                    newTitle = title.substring(2, title.length() - 1);
                    patron.PatronBorrow(newTitle);
                }else if(command.equals("return")){
                    title = file.nextLine();
                    newTitle = title.substring(2, title.length() - 1);
                    patron.PatronReturn(newTitle);
                }else if(command.equals("list")){
                    patron.PatronList();
                }
            }else{

            }
        }

    }catch (FileNotFoundException e) {
        System.out.println("File not found" + e.getMessage());
        System.exit(0);
    }


}

}

Answer 1

In the loop that uses the Patron class, you're creating a new (blank) Patron each time.

What you'll want to do, if you want to switch between patrons, is have a Map<Integer, Patron> patrons or the like in your main function. Instead of creating a new Patron(patronID) each time, retrieve it from patrons , and only create one (and store it in the map) if there's not already one in there.

(As far as your Patron class, though, you might find if you do some real testing of that class that PatronReturn often throws an exception when you remove a book. ConcurrentModificationException , namely, cause you're removing from a list you're iterating over at the time. Just a heads up.)