Question

I’ve got the following Java code but List.indexOf() seems to do pretty much the same thing (including returning -1 if not found. Is there any way of passing indexOf() an Object that expresses the idea that the object is not 0?

/**
 * Find the first non-zero element in a List of Integers
 * @param row List of Integers
 * @return -1 if all zeros in row
 * otherwise position of first non zero element
 */
public static int leading(List<Integer> row) {

 for (int i = 0; i < row.size(); i++) {
   if (row.get(i)!= 0) {
      return i;
   }
 }

 return -1;
}

Re: Thorbjørn Ravn Andersen: If I passed in null into IndexOf() it will always return -1 because my list always contains Integers. I want to do something like row.indexOf(Integer a where !a.equals(0)). Not sure if possible

Answer 1

List.indexOf "solution"

List.indexOf(Object o) is defined as follows:

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that

(o==null ? get(i)==null : o.equals(get(i)))

is true, or -1 if there is no such index.

It is tempting to try to give a "meta" element object that is not really in the List, and may not even be of the same type as the actual elements of the List, and yet is equals to some desired element based on a predicate. This should work, since indexOf is defined in terms of the given Object o‘s equals method against an element in the list (and not the other way around), but is a really "hacky" way of achieving what you want.

Here’s a proof of concept:

// PROOF OF CONCEPT ONLY! DO NOT IMITATE!
// abusing indexOf(Object) to find index of a negative integer in List

List<Integer> nums = Arrays.asList(3,4,5,-6,7);
Object equalsNegativeInteger = new Object() {
    @Override public boolean equals(Object o) {
        return (o instanceof Integer) && ((Integer) o) < 0;
    }
};
System.out.println(nums.indexOf(equalsNegativeInteger));
// prints 3

The "meta" element object is equals to any negative Integer, and yet no Integer can ever be equals to it. This asymmetry is a gross violation of the equals contract, but it "works" nonetheless.

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Guava solution

A much better solution that conveys the intent is using Guava’s higher-order functions. Here’s one from com.google.commons.collect.Iterables:

<T> int indexOf(Iterable<T> iterable, Predicate<? super T> predicate)

Returns the index in iterable of the first element that satisfies the provided predicate, or -1 if the Iterable has no such elements. More formally, returns the lowest index i such that:

 predicate.apply(Iterables.get(iterable, i))

is true, or -1 if there is no such index.

Snippet

Here’s a snippet to illustrate the expressive power of Guava’s higher-order functions:

import com.google.common.collect.*;
import com.google.common.base.*;
import java.util.*;

public class IterablesPredicateExample {
    public static void main(String[] args) {
        List<Integer> nums = Arrays.asList(1,2,-3,4,-5,6,-7,-8);
        Predicate<Integer> isNegative = new Predicate<Integer>() {
            @Override public boolean apply(Integer n) {
                return n < 0;
            }
        };          
        // Get index of first negative number
        System.out.println(Iterables.indexOf(nums, isNegative));
        // 2

        // Find that number
        System.out.println(Iterables.find(nums, isNegative));
        // -3

        // Find all negative numbers
        System.out.println(Iterables.filter(nums, isNegative));
        // [-3, -5, -7, -8]

        // Are all numbers negative?
        System.out.println(Iterables.all(nums, isNegative));
        // false

        // Find all non-negative numbers
        System.out.println(Iterables.filter(nums, Predicates.not(isNegative)));
        // [1, 2, 4, 6]
    }
}

---

Summary

• List.indexOf(Object) can be abused to find elements that satisfy a given predicate, but this is a violation the equals contract
• Using Guava’s Predicate and higher-order functions like indexOf, find, filter, all, any etc allows you to express these operations in a much more powerfully expressive ways