What does assert do?

Question

What does assert do? for example in the function?

private static int charAt(String s, int d) { 

    assert d >= 0 && d <= s.length();

    if (d == s.length()) return -1;

    return s.charAt(d);
}

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

Assert does throw an AssertionError if you run your app with assertions turned on.

int a = 42;
assert a >= 0 && d <= 10;

If you run this with, say: java -ea -jar peiska.jar

It shall throw an java.lang.AssertionError

Answer 6

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.