For循环不适用于ArrayList

Question

Ok basically I am sort of new to ArrayList and i have to use it to make a program of getting some patients ID and then displaying it on the screen using objects and for loop. I did make the program but my loops aren't executing. this is my program:

import java.util.*;
import java.util.ArrayList;
import java.util.Scanner;

public class patient$ {
    public static void main(String[] args) {
        ArrayList<patientss> patient1 = new ArrayList<patientss>();
        Scanner src = new Scanner(System.in);
        int id, i, num;
        String name;
        int ages;
        System.out.println(" j");
        for (i = 0; i < patient1.size(); i++) {
            patientss xx = new patientss();
            System.out.println("Enter the patient's ID ");
            id = src.nextInt();
            xx.setId(id);
            patient1.add(xx);
        }
        patientss tt = new patientss();
        for (i = 0; i < patient1.size(); i++) {
            tt = patient1.get(i);
            System.out.println(tt.getId());
        }
    }
}

and this is my patientss class:

package samplee.java;
public class patientss {
    int id;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

Answer 1

您的循环在一个空的ArrayList上运行，因此size()方法将返回0。尝试将第一个for循环中的size()方法更改为要添加的许多patientss 。

Answer 2

Your first loop is iterating over an empty ArrayList.

Since the point of this loop seems to be to be adding things to that list, the number of iterations should be controlled by how many things you want to add to it, not how many are already in it. For example, the loop could continue as long as patient1.size()<N , where N is the number of items you want to add.

Answer 3

Here's one solution. Replace the first loop with my code.

Boolean leave = false;
while (!leave) {
    patientss xx = new patientss();
    System.out.println("Enter the patient's ID ");
    System.out.println("-1 to finish entering");
    id = src.nextInt();
    if (id != -1) {
        xx.setId(id);
        patient1.add(xx);
    } else { leave = true; } //Escape the while loop.
}