What is Assertion in java ?

Assertions are generally part of Unit Testing.
You are going to test something which are are you expecting.
Like if you are expecting null then you can assert

assert i==null : "throw this message"

Assertions are bydefault disabled so adding assertions will not affect your run time. but when you need it.

just enable is by sending  jvm parameter -ea.

example :  java -ea <className>

Good one but very less used. everyone is using IDE with debugger.

Assertion are used to verify the correctness of an invariant in the code. They should never be triggered in production code, and are indicative of a bug or misuse of a code path.