Assertions are special features added in java 1.4 onwards by default assertions will be disabled u will have enable assertion when u compile the java file
The assertion statement has two forms. The first, simpler form is: assert Expression1 ; where Expression1 is a boolean expression. When the system runs the assertion, it evaluates Expression1 and if it is false throws an AssertionError with no detail message. The second form of the assertion statement is: assert Expression1 : Expression2 ; where: �Expression1 is a boolean expression. �Expression2 is an expression that has a value. (It cannot be an invocation of a method that is declared void.) Use this version of the assert statement to provide a detail message for the AssertionError. The system passes the value of Expression2 to the appropriate AssertionError constructor, which uses the string representation of the value as the error's detail message.
hope this helps!! usually this helps a lot while debugging ur code in big projects!!