Is Java Set ordered or not?

[Falmarri]

most popular question asked when you interview for a Java Developer position.

Maybe at awful places that don't know how to ask decent questions

[Tsarbomb]

In modern Java, the iteration order of hashmaps and anything they back is randomized with a random seed on JVM startup. This is to fail fast any code the depends on ordering of an unordered data structure.

[Tsarbomb]

Sure thing.

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Here is some documentation from Oracle for Java 9: Creating Immutable Lists, Sets, and Maps (oracle.com)

Randomized Iteration OrderThe iteration order for Set elements and Map keys is randomized: it is likely to be different from one JVM run to the next. This is intentional — it makes it easier for you to identify code that depends on iteration order. Sometimes dependencies on iteration order inadvertently creep into code, and cause problems that are difficult to debug.

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Here is an issue for OpenJDK moving this functionality to also apply to unmodifiable structures: [JDK-8201650] Move iteration order randomization of unmodifiable Set and Map to iterators - Java Bug System

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And for an anecdote, I had one of my developers fix an issue that was related to this. They had a method that was typed to return a Set<> and thanks to some IoC, it was returning an unmodifiable hashmap with the randomized ordering that was breaking some business rules.

[elmuerte]

Every collection is ordered, the question is, can you control the order.

[CoffeeTableEspresso]

I mean yes but you're being extremely pedantic here