What is an AssertionError In which case should I throw it from my own code

Question

In Item 2 of the  Effective Java  2nd edition  book  there is this snippet of code  in which the author wants to forbid the empty initialization of an object   class Example       private Example             throw new AssertionError              The type of exception thrown  is what confuses me here   I don t understand if the AssertionError is thrown just because of an absence of more suited errors or because it should be this way   As I understand  this error is thrown by the framework when an assert statement fails  Also  in the javadoc it s just written      An AssertionError is  Thrown to indicate that an assertion has failed    But I don t see any assertion  true-false statement  being violated here  Of course the  You shall not instantiate an item of this class  statement has been violated  but if this is the logic behind that  then we should all throw AssertionErrors everywhere  and that is obviously not what happens   FWIW  I d have just thrown a  new IllegalStateException  Must not instantiate an element of this class     Is there something wrong with that  In which case should I throw an AssertionError in my own code   Sorry if it s just a subtle doubt but I use this pattern a lot in my code and I want to make sure I m doing the right thing

User · Answer

The meaning of an AssertionError is that something happened that the developer thought was impossible to happen   So if an AssertionError is ever thrown  it is a clear sign of a programming error

User · Answer

An assertion Error is thrown when say  You have written a code that should not execute at all costs because according to you logic it should not happen  BUT if it happens then throw AssertionError  And you don t catch it   In such a case you throw an Assertion error   new IllegalStateException  Must not instantiate an element of this class       Is an Exception not error    Note  Assertion Error comes under java lang Error And Errors not meant to be caught

User · Answer

AssertionError is an Unchecked Exception which rises explicitly by programmer or by API Developer to indicate that assert statement fails    assert x gt 10     Output   AssertionError   If x is not greater than 10 then you will get runtime exception saying AssertionError

User · Answer

I m really late to party here  but most of the answers seem to be about the whys and whens of using assertions in general  rather than using AssertionError in particular   assert and throw new AssertionError   are very similar and serve the same conceptual purpose  but there are differences    throw new AssertionError   will throw the exception regardless of whether assertions are enabled for the jvm  i e   through the -ea switch   The compiler knows that throw new AssertionError   will exit the block  so using it will let you avoid certain compiler errors that assert will not    For example                 boolean b   true          final int n          if   b                 n   5            else               throw new AssertionError                      System out println  n       n                        boolean b   true          final int n          if   b                 n   5            else               assert false                    System out println  n       n           The first block  above  compiles just fine   The second block does not compile  because the compiler cannot guarantee that n has been initialized by the time the code tries to print it out

User · Answer

Of course the  You shall not instantiate an item of this class  statement has been violated  but if this is the logic behind that  then we should all throw AssertionErrors everywhere  and that is obviously not what happens    The code isn t saying the user shouldn t call the zero-args constructor  The assertion is there to say that as far as the programmer is aware  he she has made it impossible to call the zero-args constructor  in this case by making it private and not calling it from within Example s code   And so if a call occurs  that assertion has been violated  and so AssertionError is appropriate

[java] What is an AssertionError? In which case should I throw it from my own code?

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