Blinded by the lights

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

Examples:

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
   ...
}

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