What does the Java assert keyword do, and when should it be used?

Question

What are some real life examples to understand the key role of assertions?

Answer 1

Assertions are disabled by default. To enable them we must run the program with -ea options (granularity can be varied). For example, java -ea AssertionsDemo.

There are two formats for using assertions:

1. Simple: eg. assert 1==2; // This will raise an AssertionError.
2. Better: assert 1==2: "no way.. 1 is not equal to 2"; This will raise an AssertionError with the message given displayed too and is thus better. Although the actual syntax is assert expr1:expr2 where expr2 can be any expression returning a value, I have used it more often just to print a message.