Iteration order of HashSet

java algorithm collections hashset
eljenso picture eljenso · Apr 24, 2010 · Viewed 22.7k times · Source

If every object added to a java.util.HashSet implements Object.equals() and Object.hashCode() in a deterministic fashion, is the iteration order over the HashSet guaranteed to be identical for every identical set of elements added, irrespective of the order in which they were added?

Bonus question: what if the insertion order is identical as well?

(Assuming Sun JDK6 with same HashSet initialization.)

Edit: My original question was not clear. It is not about the general contract of HashSet, but what Sun's implementation of HashSet in JDK6 offers as guarantees concerning determinism. Is it inherently non-deterministic? What influences the order used by its Iterator?

Answer

Michael Borgwardt picture Michael Borgwardt · Apr 24, 2010

Absolutely not.

The insertion order directly influences the iteration order whenever you have a bucket collision:

When two elements end up in the same bucket, the first one that was inserted will also be the first one returned during iteration, at least if the implementation of collision handling and iteration is straightforward (and the one in Sun's java.util.HashMap is)

Related questions

Which Java Collection should I use?

In this question How can I efficiently select a Standard Library container in C++11? is a handy flow chart to use when choosing C++ collections. I thought that this was a useful resource for people who are not sure which …

Read More
best way to pick a random subset from a collection?

I have a set of objects in a Vector from which I'd like to select a random subset (e.g. 100 items coming back; pick 5 randomly). In my first (very hasty) pass I did an extremely simple and perhaps overly clever …

Read More
Finding the max/min value in an array of primitives using Java

It's trivial to write a function to determine the min/max value in an array, such as: /** * * @param chars * @return the max value in the array of chars */ private static int maxValue(char[] chars) { int max = chars[0]; for (int ktr = 0; …

Read More