PMD: Avoid instantiating new objects inside loops

Question

I've got an issue with the PMD rule Avoid instantiating new objects inside loops. Here is some example code:

import java.awt.Dimension;

public class PMDDemo {
    public static void main(final String[] args) {
        final Dimension[] arr = new Dimension[10];
        for (int i = 0; i < arr.length; i++) {
            arr[i] = new Dimension(i, i); // rule violation here
        }
    }
}

PMD gives me the above mentioned rule violation at the marked spot in the code. How am I supposed to create n instances of a class without creating them within a loop?

I know that some of PMD's rules are controversial (like the onlyOneExit rule). But up to now I at least understood the idea behind them. I don't understand the reasoning behind this rule. Can someone help me with that?

It makes no sense in this scenario (or in most scenarios I can think of actually)... — assylias
– assylias, Commented Jun 27, 2013 at 10:19
What I presume this rule is trying to encorage is object pooling for short lived objects and as I understand it that isn't a good idea these days either: programmers.stackexchange.com/questions/149563/… — Richard Tingle
– Richard Tingle, Commented Jun 27, 2013 at 10:25
@assylias Can you think of a scenario where it makes sense? If not, I am disabling the rule. — brimborium
– brimborium, Commented Jul 1, 2013 at 14:03

Community · Accepted Answer · 2017-05-23 11:54:06Z

28

For your specific use case it makes no sense as you keep the reference to the new Object after the loop. So there is no real alternative to your solution.

More generally speaking, creating short lived objects in Java is cheap* (apart from the hidden cost that the GC will run more often). In particular, the allocation is almost free and the time of GC mostly depends on the quantity of reachable objects - dead objects do not increase GC time for typical GC algorithms.

The JIT can also perform various optimisations if it detects that unnecessary objects are created.

Obviously, creating useless is not a recommended practice, but trying to reuse objects is often counterproductive.

As a practical example, you can have a look at this post which shows that creating a new set within a loop is cheaper than creating one before the loop and clearing it at each iteration.

^{* Thanks @RichardTingle for the link}

edited May 23, 2017 at 11:54

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answered Jul 3, 2013 at 21:36

assylias

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Comments

brimborium · Accepted Answer · 2016-12-05 16:33:54Z

-1

for (int i = 0; i < arr.length; i++) {
  arr[i] = new Dimension(i, i); // rule violation here
}

The Above Pmd can be resolved by

 for (int i = 0; i < arr.length; i++) {
   arr[i] = createNewDimension(i,i); // rule violation here
 }

 private static Dimension createNewDimension(i,i) {
   return new Dimension(i, i);
 }

we should not directly use new operator inside a loop just move this inside a private method.

edited Dec 5, 2016 at 16:33

brimborium

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answered Dec 1, 2016 at 21:23

Vishnu Ranganathan

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3 Comments

brimborium Over a year ago

Thanks for the input. Would you actually prefer the resolved version (with the creation wrapper)? I think this makes it less readable. But maybe I am missing something. +1 for actually resolving the violation though.

Linus Fernandes Over a year ago

Simply wrapping the creation of the object in a method does not actually prevent instantiation within a loop. It's simply a subversion of the rule's implementation.

Harsh Raj Over a year ago

This actually helps to resolve pmd violation when creating an object inside the loop cannot be avoided at all.

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