validation - Assert keyword in Java

Question

Do you use the assert keyword or throw some validation runtime exception? What benefits does it give to you or why do you think it's not worth it to use?

Answer 1

Assert will throw a runtime error (AssertionError) if its condition is false. Asserts give you a streamlined way of documenting, checking, and enforcing correctness criteria for your code. The benefits are a language-level hook for defining and manipulating these correctness conditions. To the extent that you wish to enable or disable them (there are arguments about whether or not this is a good idea) you can do so from the JVM command-line. Some commenters below note that assertions are disabled by default unless running in debug mode; my practice is to add "-ea" (enable assertions) in my wrapper scripts at all times. Even in performance sensitive code, for me the tradeoff weighs in favor of the security/correctness confidence I get from assertions. Assertions at Oracle and API Description for AssertionError

Note the distinction between expected or unexpected failures (exceptions), which may be outside your control, and assertion failures -- assertion failures document programmer assumptions, and indicate an incorrect program rather than an unexpected external condition or expected exceptional condition. If an assertion failure occurs, the interpretation is that the programmer has misunderstood or incorrectly expressed the program, rather than other sources of error or failure.

In practice, I use it to document obvious or non-obvious assumptions I make and invariants which I want to enforce as I produce (particularly private/internal) code, making it clear to myself and others why these assumptions are made, where they are made, and whether or not they are validated. Much better than comments to the same effect. This is a (small) step toward Design by Contract.

Effective Java item #38 "Check Parameters for Validity" (Google Books, Amazon.com) provides a useful presentation of the distinction between parameter checking and appropriate use of assertions.

Related on SO: (Enabling assertions in netbeans), (Assertions vs. Exceptions), (Near duplicate, asking for examples), (Badly named, but very similar content)