Java LanguageAsserting


  • assert expression1;
  • assert expression1 : expression2;


expression1The assertion statement throws an AssertionError if this expression evaluates to false.
expression2Optional. When used, AssertionErrors thrown by the assert statement have this message.


By default, assertions are disabled at runtime.

To enable assertions, you must run java with -ea flag.

java -ea com.example.AssertionExample

Assertions are statements that will throw an error if their expression evaluates to false. Assertions should only be used to test code; they should never be used in production.

Checking arithmetic with assert

a = 1 - Math.abs(1 - a % 2);

// This will throw an error if my arithmetic above is wrong.
assert a >= 0 && a <= 1 : "Calculated value of " + a + " is outside of expected bounds";

return a;