CODEWITHSUNDEEP
java
chrisc17
chrisc17

Reputation: 23

Java indexOf method as a short cut to manual search

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

Upvotes: 2

Views: 2787

Answers (4)

polygenelubricants
polygenelubricants

Reputation: 383756

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.

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

Upvotes: 6

meriton
meriton

Reputation: 70564

No, java doesn't support closures.

The standard workaround is using an anonymous inner class, but that requires enough boilerplate code to render the solution just a complex as implementing that loop.

Upvotes: 0

coobird
coobird

Reputation: 160974

The List.indexOf(Object) method will return the first index at which the specified Object is found.

The code that is given in the question appears to have the following requirements:

  • Return the first index at which 0 is not found in the list.
  • If no such element exists in the list, return -1.

Unfortunately, there is no way to express the above using the indexOf method.

Therefore, the code you've presented seems to be an acceptable implementation for the requirements.

Upvotes: 1

Thorbj&#248;rn Ravn Andersen
Thorbj&#248;rn Ravn Andersen

Reputation: 75376

I do not believe such a method exists in the standard runtime library.

Question is whether you need this so much that it pays off to keep track of the not-zero values, or you can do with a brute force search. I would do with the latter, and keep track of the difference between System.currentMillis() and if it goes over a limit decided by you, log it. Then you don't have to run a profiler to locate a known possible bottleneck.

Also note you do autoboxing in your sample. That overhead may be unneccesary.

Upvotes: -1

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