I don't understand how lastIndexOf method works in java

Question

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.

these are the values that list stores.

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;
    }
}

This is the output I get:

index of 5 = 
index of 100 = 

Answer 1

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;
    }
}

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

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

Answer 2

This code shouldn't have 2 return statements. Also, you are making a node equal the entire list; when it should equal the head of the list.

Was this code given to you to correct? I ask because it does not appear to have been developed piece-wise; rather it looks written from the top down.

Answer 3

Use this I guess:

    public int lastIndexOf (int value) {
        int index = -1;
        int size = size();
        int count = 0;

        ListNode current = front;

        for (int i = 0; i < size; i++) {
            int indexValue = current.data;
            if (value == current.data) {index = count;}
            count++;
            current = current.next;
         }
         return index;
     }