What are assertions in Java and when should they be used?

Demystifying Assertions

Assertions: Your Code's Private Detective

Just like detectives who are always on the lookout for something suspicious, assertions in Java verify conditions that you assume to hold true in your code. They primarily check for the correctness of method input/output contracts, ensuring that methods behave as expected.

Case-catcher in Enum Switch

Consider an enum in a switch statement — you can utilize assertions as a safety backup to declare that all cases are handled. If a case gets missed, a non-complacent assert false in the default case would raise a red flag.

Example:

switch(dayOfTheWeek){
    case MONDAY:
    // code
    default :
    // Someone's forgotten a day of the week
    assert false : "Looks like we have an eight-day week!";
}
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Striking a Balance: Development and Production

In the development phase, assertions act as a referee ensuring your code is playing by the rules, highlighting any foul play (bugs) on the field. While it's tempting to keep them running even in production for troubleshooting, remember they are designed to ensure the correctness of software internals and not to handle run-time errors related to environment or user-input.

The Safety Guard of Code Evolution

Java 1.4 welcomed the introduction of assertions, highlighting their importance in the evolution of Java. Inspired by the Design-by-Contract principles, assertions ensure the code follows the expected behavior, reducing the chances of bugs sneaking into production by acting as an extra layer over testing.

Adding Security Layers with Assertions

Assertions provide an extra layer of security in a less-than-perfect world. They offer a self-checking mechanism within your code, making them integral for preventing failure in critical systems.

Using Assertions Effectively: An Action Plan

The key to utilizing assertions effectively lies in understanding their proper implementation:

• Invariant checking: Use assertions to verify invariants, particularly in non-public areas of your code where you control environmental factors.
• For public methods parameters: Rely more on explicit parameter validation via if-throw, and use assertions sparingly, perhaps in the context of private methods.
• Clarity in communication: Assertions can tell future developers expectations of certain conditions that must hold true, making the code more robust and easier to understand.
• Cornerstone of Design choices: Base your assertions usage on the Design-by-Contract concept—with assertions, each method is obliged to fulfill its contract provided that it's fed with proper inputs.