Assertions in Java

assert Expression1 : Expression2;

Expression1 is asserted to be true; otherwise AssertionError (that shouldn’t be handled) is thrown
Expression2 allows to produce some additional information
Expression2 MUST result in a value – it generates a String message
allow testing during the development and debugging
are disabled at runtime by default
can be enabled using -ea or -enableassertions flags and selectively disabled using -da or -disableassertions flags, e.g:
- java -ea -da:MyClass NotMyClass – enable in general but disable for MyClass
- java -ea -da:net.dobrzanski.util... MyClass – enable in general but disable for all classes from net.dobrzanski.util package and any of its subpackages
- java -ea -dsa MyClass, which is shorter version of java -enableassertions -disablesystemassertions MyClass – enable in general but disable in system classes
DO NOT USE to validate arguments to public methods
DO NOT USE to validate command line arguments
USE to validate arguments to private methods
USE to validate that particular code block will never be reached (use assert false; in that case)
assert can be used as a key word (as of Java 1.4) or as an identifier, but never both at the same time

Shorter version:

private void doSth() {
   ...
   assert (a < b);
   // in there, a is lower than b
   ...
}

Longer version:

private void doSth() {
   ...
   assert (a < b) : "a=" + a + ", b=" + b;
   // in there, a is lower than b
   ...
}