What does the "assert" keyword do?
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
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 >= 0 && d <= s.length();
, as posted in your question, is equivalent to
if (!(d >= 0 && d <= 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 anassert
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 tofalse
. 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.
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