JAVA-问题打印ArrayList

Question

I'm currently attempting to make a lottery program through the use of a GUI. I can not figure out why the method getTicketNumbers() is not returning anything. It is simply printing [].

And so too is:

String output = "Name: " + name + "\nNumbers: " + ticketNumbers + "\n\n";

tickNumbers is outputting [] but name successfully prints out the name.

I added a System.out.print to the Ticket class in the constructor to confirm the ArrayList is being passed successfully and it is:

public Ticket(ArrayList<Integer> ticketNumbers, String name) {
    this.ticketNumbers = ticketNumbers;
    this.name = name;
    System.out.print("Are the numbers being passed:" + ticketNumbers + "\n");
}

The method toString is successfully printing the name but again, it is not printing the ArrayList called ticketNumbers.

private void enterLottoButtonActionPerformed(java.awt.event.ActionEvent evt) {                                                 
    if (nameInput.getText().equalsIgnoreCase("")) {
        JOptionPane.showMessageDialog(null, "Please Enter Your Name", "Error", JOptionPane.ERROR_MESSAGE);
    } else if (ticketNumbers.size() < 4) {
        JOptionPane.showMessageDialog(null, "Please Enter Four Numbers", "Error", JOptionPane.ERROR_MESSAGE);
    } else {
        ticketList.add(new Ticket(ticketNumbers, nameInput.getText()));

        JOptionPane.showMessageDialog(null, "You Successfully Entered! \n\nName: " + nameInput.getText() + "\nNumbers: " + ticketNumbers.toString());

        ticketNumbers.clear();
        numbersTextField.setText("");
        nameInput.setText("");     
        numberOfPeopleLabel.setText("           People Entered: " + ticketList.size());
    }

Ticket Class:

import java.util.ArrayList;
import java.util.Collections;
import javax.swing.JOptionPane;

public class Ticket {
    private String name;
    private ArrayList<Integer> ticketNumbers = new ArrayList<>();

public Ticket(ArrayList<Integer> ticketNumbers, String name) {
    this.ticketNumbers = ticketNumbers;
    this.name = name;
    System.out.print("Are the numbers being passed:" + ticketNumbers + "\n");
}

public String getName() {
    return name;
}

public ArrayList<Integer> getTicketNumbers() {
    return ticketNumbers;
}

public ArrayList<Integer> getSortedTicketNumbers() {
    Collections.sort(ticketNumbers);
    return ticketNumbers;
}

@Override
public String toString() {
    String output = "Name: " + name + "\nNumbers: " + ticketNumbers + "\n\n";

    return output;
}
}

If needed, code in its entirety: http://pastebin.com/7i8VWQLk and http://pastebin.com/iRd49Nc7

Answer 1

It looks like you're copying the reference to the list, and so when you clear it in one place (or modify it) it changes/clears everywhere.

Try making a defensive copy. Like:

ArrayList<Integer> tickets = new ArrayList<>(ticketNumbers)

Answer 2

ticketNumbers.clear()

This line cause the problem. As you said, during the construction of Ticket object, the data been passed in correctly in this line: ticketList.add(new Ticket(ticketNumbers, nameInput.getText())); . But you cleared them afterwards. You will need a clone of ticketList . Regardless of the best practice, you can do it inside Ticket class.

private List<Integer> clonedTicketNumbersList = new ArrayList<Integer>();

public Ticket(ArrayList<Integer> ticketNumbers, String name) {
    this.ticketNumbers = ticketNumbers;
    this.name = name;
    for(Integer ticketNum : ticketNumbers) {
      clonedTicketNumbersList.add(ticketNum );
    }
    System.out.print("Are the numbers being passed:" + ticketNumbers + "\n");
}

@Override
public String toString() {
    String output = "Name: " + name + "\nNumbers: " + clonedTicketNumbersList + "\n\n";

    return output;
}