Java assert Keyword
Example
Use assert to test the truth value of expressions:
public class Main {
public static void main(String[] args) {
// Enable assertions
ClassLoader loader = ClassLoader.getSystemClassLoader();
loader.setDefaultAssertionStatus(true);
// Run the assert example
AssertExample example = new AssertExample();
example.run();
}
}
class AssertExample {
public void run() {
int a = 12;
try {
assert a == 12; // Assertion without a fail message
assert a == 12 : "a is not 12";
assert a == 15 : "a is not 15";
} catch (AssertionError e) {
System.out.println(e.getMessage());
}
}
}
Definition and Usage
The assert keyword evaluates a boolean expression and throws an AssertionError exception if the expression evaluates to false. When the exception is thrown we say that the assertion failed.
An optional expression can be added which will be used as the exception message if the assertion fails.
Assertions are disabled by default. assert statements are ignored unless assertions are enabled.
The purpose of assertions is to clearly mark where a program is doing something unintended when debugging and testing a program.
