Java The Complete Reference, Seventh Edition

Chapter 13: I/O, Applets, and Other Topics 307

Here is an example that usesassert. It verifies that the return value ofgetnum( )is positive.

// Demonstrate assert.
class AssertDemo {
static int val = 3;

// Return an integer.

static int getnum() {

return val--;

}

public static void main(String args[])

{

int n;

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

n = getnum();

assert n > 0; // will fail when n is 0

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

To enable assertion checking at run time, you must specify the-eaoption. For example,
to enable assertions forAssertDemo, execute it using this line:

java -ea AssertDemo

After compiling and running as just described, the program creates the following output:

n is 3

n is 2

n is 1

Exception in thread "main" java.lang.AssertionError

at AssertDemo.main(AssertDemo.java:17)

Inmain( ), repeated calls are made to the methodgetnum( ), which returns an integer value.
The return value ofgetnum( )is assigned tonand then tested using thisassertstatement:

assert n > 0; // will fail when n is 0

This statement will fail whennequals 0, which it will after the fourth call. When this happens,
an exception is thrown.