What is the purpose of the assert keyword?

assert is not the same as methods like assertTrue() from testing libraries. It is a reserved keyword and a unary operator in Java.

This operator does not return anything but takes a boolean expression as its argument. If the expression evaluates to false, the assertion is considered failed, and an AssertionError is thrown. It's like a shorthand for a combination of if and throw with a fixed exception type.

In Java versions prior to 4, assert was not a keyword. To maintain backward compatibility, assertion checks are disabled by default—program logic should never rely on assert!

Assertions can be enabled using the -ea or -enableassertions flags with the java command. Specific classes and packages where assertions are enabled can also be specified. There's an opposite flag,-da (-disableassertions), and these flags can be used in combination.

Assertions are mainly used for additional checks of object state invariants and as safeguards in code that should never be executed. An AssertionError usually indicates a programmer error.

Additionally, the assert operator has a syntax for passing a detailMessage parameter to the AssertionError constructor:

assert 2*2==5 : "two times two is not five!";