Java ArrayList - 检查列表是否为空 [英] Java ArrayList - Check if list is empty

问题描述

如何检查列表是否为空?如果是这样，系统必须给出一个消息说列表为空.如果不是，系统必须给出一个消息，说列表不为空.用户可以输入数字，-1 来停止程序.这是我现在拥有的代码，但这不起作用，它总是说列表不是空的".

How can I check if a list is empty? If so, the system has to give a message saying List is empty. If not, the system has to give a message saying List is not empty. Users can enter numbers, -1 to stop the program. This is the code I now have, but this doesn't work, it always says 'List isn't empty'.

import java.util.*;
import javax.swing.JOptionPane;

public class ArrayListEmpty 
{
    public static void main(String[] args) 
    {
        List<Integer> numbers = new ArrayList<Integer>();
        int number;
        do {
            number = Integer.parseInt(JOptionPane.showInputDialog("Enter a number (-1 to stop)"));
            numbers.add(number);
        } while (number != -1);
        giveList(numbers);
    }

    public static void giveList(List<Integer> numbers)
    {
        if (numbers != null)
            JOptionPane.showMessageDialog(null, "List isn't empty");
        else
            JOptionPane.showMessageDialog(null, "List is empty!");
    }
}

推荐答案

就像:

if (numbers.isEmpty()) {...}

请注意，快速浏览文档 会给你这些信息.

Note that a quick look at the documentation would have given you that information.

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