What does the assert keyword do

Question

What does assert do  For example in the function   private static int charAt String s  int d        assert d  gt   0  amp  amp  d  lt   s length        if  d    s length    return -1      return s charAt d

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It ensures that the expression returns true   Otherwise  it throws a java lang AssertionError   http   java sun com docs books jls third edition html statements html 14 10

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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  The purpose of the detail message is to capture and communicate the details of the assertion failure  The message should allow you to diagnose and ultimately fix the error that led the assertion to fail  Note that the detail message is not a user-level error message  so it is generally unnecessary to make these messages understandable in isolation  or to internationalize them  The detail message is meant to be interpreted in the context of a full stack trace  in conjunction with the source code containing the failed assertion   JavaDoc

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Assertions are generally used primarily as a means of checking the program s expected behavior  It should lead to a crash in most cases  since the programmer s assumptions about the state of the program are false  This is where the debugging aspect of assertions come in  They create a checkpoint that we simply can t ignore if we would like to have correct behavior    In your case it does data validation on the incoming parameters  though it does not prevent clients from misusing the function in the future  Especially if they are not   and should not  be included in release builds

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If the condition isn t satisfied  an AssertionError will be thrown   Assertions have to be enabled  though  otherwise the assert expression does nothing  See   http   java sun com j2se 1 5 0 docs guide language assert html enable-disable

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Assert does throw an AssertionError if you run your app with assertions turned on   int a   42  assert a  gt   0  amp  amp  d  lt   10    If you run this with  say  java -ea -jar peiska jar  It shall throw an java lang AssertionError

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Although I have read a lot documentation about this one  I m still confusing on how  when  and where to use it    Make it very simple to understand   When you have a similar situation like this       String strA   null      String strB   null      if  2  gt  1           strA    Hello World              strB   strA toLowerCase       You might receive warning  displaying yellow line on strB   strA toLowerCase      that strA might produce a NULL value to strB  Although you know that strB is absolutely won t be null in the end  just in case  you use assert to  1  Disable the warning   2  Throw Exception error IF worst thing happens  when you run your application    Sometime  when you compile your code  you don t get your result and it s a bug  But the application won t crash  and you spend a very hard time to find where is causing this bug   So  if you put assert  like this       assert strA    null    Adding here     strB   strA  toLowerCase      you tell the compiler that strA is absolutely not a null value  it can  peacefully  turn off the warning  IF it is NULL  worst case happens   it will stop the application and throw a bug to you to locate it

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If you launch your program with -enableassertions  or -ea for short  then this statement  assert cond    is equivalent to  if   cond      throw new AssertionError      If you launch your program without this option  the assert statement will have no effect   For example  assert d  gt   0  amp  amp  d  lt   s length     as posted in your question  is equivalent to  if    d  gt   0  amp  amp  d  lt   s length         throw new AssertionError       If you launched with -enableassertions that is      Formally  the Java Language Specification  14 10  The assert Statement says the following      14 10  The assert Statement   An assertion is an assert statement containing a boolean expression  An assertion is either enabled or disabled  If the assertion is enabled  execution of the assertion causes evaluation of the boolean expression and an error is reported if the expression evaluates to false  If the assertion is disabled  execution of the assertion has no effect whatsoever    Where  enabled or disabled  is controlled with the -ea switch and  An error is reported  means that an AssertionError is thrown     And finally  a lesser known feature of assert   You can append    Error message  like this   assert d    null    d is null     to specify what the error message of the thrown AssertionError should be     This post has been rewritten as an article here

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assert is a debugging tool that will cause the program to throw an AssertionFailed exception if the condition is not true   In this case  the program will throw an exception if either of the two conditions following it evaluate to false   Generally speaking  assert should not be used in production code

[java] What does the "assert" keyword do?

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