数组索引打印错误值

Question

I've the below Java code.

import java.util.Arrays;

public class Cook {
    public static void main(String[] args) {
        int num[] = { 3, 1, 5, 2, 4 };
        getMaxValue(num);
    }

    public static void getMaxValue(int[] num) {
        int maxValue = num[0];
        int getMaxIndex = 0;
        for (int i = 1; i < num.length; i++) {
            if (num[i] > maxValue) {
                maxValue = num[i];
            }
        }
        getMaxIndex = Arrays.asList(num).indexOf(maxValue);
        System.out.println(getMaxIndex + " and " +maxValue);
    }   
}

In the above code I'm trying to retrieve the maximum value in the array and also its index, but here the output that I'm getting is

-1 and 5

The max value is returned fine, but not sure of what's wrong with the index. This should actually print 2 , but it is printing -1 , please let me know where am i going wrong and how can I fix this.

Thankd

Answer 1

You should update the max index in the loop :

    int maxValue = num[0];
    int getMaxIndex = 0;
    for (int i = 1; i < num.length; i++) {
        if (num[i] > maxValue) {
            maxValue = num[i];
            getMaxIndex = i;
        }
    }

The reason Arrays.asList(num).indexOf(maxValue); returns -1 is that an array of primitives is converted by Arrays.asList to a List of a single element (the array itself), and that List doesn't contain maxValue (it only contains the original array).

Answer 2

Need to update index while iterating, getMaxIndex = i;

public static void getMaxValue(int[] num) {
        int maxValue = num[0];
        int getMaxIndex = 0;
        for (int i = 1; i < num.length; i++) {
            if (num[i] > maxValue) {
                maxValue = num[i];
                getMaxIndex = i;
            }
        }
        System.out.println(getMaxIndex + " and " + maxValue);
    }

output

2 and 5

Below is something @Eran is referring to.

It is converted to List of size 1 , containing a single element (the array itself).

As per Javadoc, indexOf

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.

So it searches for maxValue inside List and not inside array stored in 0th index of List .

Answer 3

Everyone gives good hints, but no one explains in detail why it doesn't work.

Arrays.asList() is defined with the signature public static <T> List<T> asList(T... a) which takes a variable number of objects or just an array of objects.

However, int is a primitive type and not an object type. So Arrays.asList(num) is not interpreted as "take this array", but as "take this object as one object". The result is thus a List<int[]> , where the given number (of course) can't be found.

So it is better to keep the index while searching for the maximum, as the other answers already suggest.

Answer 4

above answers are correct but you can also do

import java.util.Arrays;

public class Cook {

    public static void main(String[] args) {
        Integer num[] = { 3, 1, 5, 2, 4 };
        getMaxValue(num);
    }

    public static void getMaxValue(Integer[] num) {
        int maxValue = Arrays.asList(num).get(0);
        int getMaxIndex = 0;
        for (int i = 1; i < num.length; i++) {
            if (Arrays.asList(num).get(i) > maxValue) {
                maxValue = Arrays.asList(num).get(i);
            }
        }
        getMaxIndex = Arrays.asList(num).indexOf(maxValue);
        System.out.println(getMaxIndex + " and " +maxValue);
    }
}