What does the Java assert keyword do, and when should it be used?

Question

What are some real life examples to understand the key role of assertions?

Answer 1

Assertions are checks which may get switched off. They're rarely used. Why?

• They must not be used for checking public method arguments as you have no control over them.
• They should not be used for simple checks like result != null as such checks are very fast and there's hardly anything to save.

So, what's left? Expensive checks for conditions really expected to be true. A good example would be the invariants of a data structure like RB-tree. Actually, in ConcurrentHashMap of JDK8, there are a few such meaningful asserts for the TreeNodes.

• You really don't want to switch them on in production as they could easily dominate the run time.
• You may want to switch them on or off during tests.
• You definitely want to switch them on when working on the code.

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Sometimes, the check is not really expensive, but at the same time, you're pretty sure, it'll pass. In my code, there's e.g.,

assert Sets.newHashSet(userIds).size() == userIds.size();

where I'm pretty sure that the list I just created has unique elements, but I wanted to document and double check it.