我不明白lastIndexOf方法在Java中的工作方式 [英] I don't understand how lastIndexOf method works in java

问题描述

我必须编写一个名为LastIndexOf的方法，该方法接受一个整数值作为参数，并返回该值最后一次出现的列表中的索引；如果找不到该值，则返回-1.这是我的代码，但不返回任何内容.在我看来，它总是会返回-1，但我在输出中看不到它，因为它不打印方法返回的内容.

I have to write a method called LastIndexOf that accepts an integer value as a parameter and that returns the index in the list of the last occurrence of the value, or -1 if the value is not found. This is the code I have but it doesn't return anything. To me it looks that it always is going to return -1 but I can't see it on the output because it doesn't print what the method returns.

这些是列表存储的值.

列表-> [2、5、7、24、5、9、13、2]

list -> [2, 5, 7, 24, 5, 9, 13, 2]

    public class LastIndexOf {

    public static void main(String[] args) {

    System.out.println("index of 5 = " + list.lastIndexOf(5)); // should to return index of 5= 4
    System.out.println("index of 100 = " + list.lastIndexOf(100)); // should return index of 100 = -1

    }

    public static int lastIndexOf (int element) {
    int index = 0;
    ListNode current = list;
    while (current != null) {
       if (current.data == element) {
           return index;
       }
       index ++;
       current = current.next;
    }
    return -1;
    }
}

这是我得到的输出:

index of 5 = 
index of 100 = 

推荐答案

此代码段返回正确的值.

This snippet returns correct values.

public class Test
{
    public static java.util.List<Integer> list = Arrays.asList(2, 5, 7, 24, 5, 9, 13, 2);

    public static void main(String[] args)
    {

        System.out.println("index of 5 = " + list.lastIndexOf(5));
        System.out.println("index of 100 = " + list.lastIndexOf(100));

        System.out.println(lastIndexOf(5));
        System.out.println(lastIndexOf(100));
    }


    public static int lastIndexOf (int element) {
    int index = 0;
    int found = -1;
    List<Integer> current = list;
    while (index < current.size()) {
       if (current.get(index) == element) {
           found = index;
       }
       index ++;
    }
    return found;
    }
}

我不知道ListNode是干什么的，因为实际上并不需要它.

I didn't know what ListNode was for, because it was actually not needed.

我想鼓励您了解openjdk中ArrayList <>的实现方式:

I would like to encourage you to see how ArrayList<> implementation in openjdk looks like: ArrayList.java#ArrayList.lastIndexOf in OpenJDK

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