In Java, assertions are a way to test assumptions about the state of your code. An assertion is a statement that evaluates a condition and throws an exception if the condition is false.

Assertions are typically used to check for programming errors, such as null values or out-of-range indices, that should never occur during normal program execution. By using assertions, you can catch these errors early in the development process and correct them before the code is released to production.

In Java, you can use the assert keyword to specify an assertion. Here is an example:

int x = 10;
assert x > 0 : "x must be greater than zero";

In this example, we declare a variable x and assert that it is greater than zero. If x is not greater than zero, an AssertionError is thrown with the message "x must be greater than zero".

Assertions are disabled by default in Java, so you need to enable them using the -ea (or -enableassertions) option when running the JVM. For example:

java -ea MyApp

In this example, we enable assertions when running the MyApp class.

You can also specify an assertion message by including a string after the condition. The message is displayed when the assertion fails and is useful for providing additional information about the cause of the failure.

Assertions can also be used with boolean expressions, as in the following example:

assert isPalindrome("racecar");

In this example, we call a method isPalindrome() that returns a boolean value. If the value is false, an AssertionError is thrown.

Java When to use Assertions

Assertions are typically used during development and testing to catch programming errors that should never occur during normal program execution. Here are some situations in which you might want to use assertions:

1. Check for programming errors: Assertions can be used to catch programming errors, such as null values or out-of-range indices, that should never occur during normal program execution.

2. Debugging: Assertions can be used to help debug code by providing additional information about the cause of a failure.

3. Documentation: Assertions can be used to document assumptions about the behavior of a method or class.

It's important to note that assertions should not be used for normal error handling or input validation. Instead, they should be used to catch programming errors that should never occur during normal program execution.

When using assertions, you should also keep in mind that they are disabled by default in Java, so you need to enable them using the -ea (or -enableassertions) option when running the JVM. You can also specify an assertion message by including a string after the condition. The message is displayed when the assertion fails and is useful for providing additional information about the cause of the failure.

Assertions should not be used for normal error handling or input validation. Instead, they should be used to catch programming errors that should never occur during normal program execution.