java - 为什么我的 isEmpty() 方法读取的不是空的？

Question

我在驱动程序中制作了两个框，一个带有名称，另一个带有数字。他们随机选择一个数字的名称并将其打印出来。如果名称框为空（真或假），它也会打印出来，但在第 5 个名称读出名称后，它也应该读出该框为空。相反，它会导致错误，因为所有框都是空的。我的 isEmpty 方法有什么问题？另外，我可以让它忽略错误，而是让它打印出“盒子是空的”吗？

司机

import java.util.*; 

public class DrawTester
{
/** two boxes within drawTester
* box of names (5)
* box of places at a table (5)
*/

public static void main ( String[] args)
{
String code;

Draw<String> myStringBox = new Draw<>();
Draw<Integer> myIntegerBox = new Draw<>(1, 2, 3, 4, 5);

//scanner to draw when user requests
Scanner scan = new Scanner(System.in);

System.out.print("Let's draw some names. Press 'd' to draw. ('q' to stop)");
code = scan.nextLine();

if (!code.equals("q"))
{
    if (code.equals("d"))
    {
        myStringBox = new Draw<>("Happy", "Lucky", "Freddy", "Muddy", "Fuddy");
        myIntegerBox = new Draw<>(1, 2, 3, 4, 5);
        for (int index = 0; index < 10 ; index++) {
            String person = myStringBox.drawItem();
            
            int seat = myIntegerBox.drawItem();
            
            System.out.println(String.format("%s will occupy seat %s.",
            person, seat)); 
            
            boolean isEmpty = myStringBox.isEmpty();
            System.out.println("The box is empty: " + isEmpty);
            
            System.out.print("Enter 'd' to make draw. ('q' to stop)");
            code = scan.nextLine();
                if (code.equals("q")){
                    System.out.println("you quit");
                    break;
                }
        } 
    } 
    } System.out.println("You pressed 'q'");
} 
}

绘图类

import java.util.*;
@SuppressWarnings("unchecked")

public class Draw<T>
{
boolean isEmpty = true;

Random random = new Random();

List<T> items;

public Draw(List<T> items) {
    this.items = items;
}

public Draw(T...items) {
    this(new ArrayList<>(Arrays.asList(items)));
}

public T drawItem() {
    if (items.size() > 0) {
        int index = random.nextInt(items.size());
        return items.remove(index);
    } else return null;
}

public boolean isEmpty() {
    if (items.size() <= 0) {
        isEmpty = true;
    } return isEmpty = false;
}

public String toString() {
    return items.toString();
}
}

标签： java

Answer 1

您的方法将 false 分配给isEmpty，因此始终将其返回为false. 将其更改为更简单的内容，如下所示：

public boolean isEmpty() {
    return items.size() == 0;
}

或者正如@chrylis-cautiouslyoptimistic- 所建议的那样（感谢您的提示）：

public boolean isEmpty() {
    return items.isEmpty();
}

---

关于你的第二个问题，你需要添加一个 if 条件来检查myIntegerBox在尝试绘制项目之前是否也是空的：

public class DrawTester {
    public static void main ( String[] args)
    {
        String code;

        Draw<String> myStringBox = new Draw<>();
        Draw<Integer> myIntegerBox = new Draw<>(1, 2, 3, 4, 5);

//scanner to draw when user requests
        Scanner scan = new Scanner(System.in);

        System.out.print("Let's draw some names. Press 'd' to draw. ('q' to stop)");
        code = scan.nextLine();

        if (!code.equals("q"))
        {
            if (code.equals("d"))
            {
                myStringBox = new Draw<>("Happy", "Lucky", "Freddy", "Muddy", "Fuddy");
                myIntegerBox = new Draw<>(1, 2, 3, 4, 5);
                for (int index = 0; index < 10 ; index++) {
                    String person = myStringBox.drawItem();

                    if (!myIntegerBox.isEmpty()) {
                        int seat = myIntegerBox.drawItem();

                        System.out.println(String.format("%s will occupy seat %s.",
                                person, seat));

                        boolean isEmpty = myStringBox.isEmpty();
                        System.out.println("The box is empty: " + isEmpty);

                        System.out.print("Enter 'd' to make draw. ('q' to stop)");
                        code = scan.nextLine();
                        if (code.equals("q")) {
                            System.out.println("you quit");
                            break;
                        }
                    } else {
                        System.out.println("The box is empty");
                    }
                }
            }
        } System.out.println("You pressed 'q'");
    }
}

不过，我认为代码中还有一些需要改进的地方。例如，为什么要使用带有条件的 for 循环index < 10？为什么是10？