Java The Complete Reference, Seventh Edition

As explained, you can specify the message displayed when an assertion fails. For example,

if you substitute

assert n > 0 : "n is negative!";

for the assertion in the preceding program, then the following output will be generated:

n is 3

n is 2

n is 1

Exception in thread "main" java.lang.AssertionError: n is

negative!

at AssertDemo.main(AssertDemo.java:17)

One important point to understand about assertions is that you must not rely on them

to perform any action actually required by the program. The reason is that normally, released

code will be run with assertions disabled. For example, consider this variation of the preceding

program:

// A poor way to use assert!!!

class AssertDemo {

// get a random number generator

static int val = 3;

// Return an integer.

static int getnum() {

return val--;

}

public static void main(String args[])

{

int n = 0;

for(int i=0; i < 10; i++) {

assert (n = getnum()) > 0; // This is not a good idea!

System.out.println("n is " + n);

}

}

}

In this version of the program, the call togetnum( )is moved inside theassertstatement.

Although this works fine if assertions are enabled, it will cause a malfunction when assertions

are disabled, because the call togetnum( )will never be executed! In fact,nmust now be

initialized, because the compiler will recognize that it might not be assigned a value by the

assertstatement.

Assertions are a good addition to Java because they streamline the type of error checking

that is common during development. For example, prior toassert, if you wantedto verifythat

nwas positive in the preceding program, you had to use a sequence of code similar to this:

308 Part I: The Java Language