Java keyword: assert

The assert keyword is used in assert statement which is a feature of the Java programming language since Java 1.4. Assertion enables developers to test assumptions in their programs as a way to defect and fix bugs.

Syntax of assert statement

 Syntax of an assert statement is as follow (short version):

assert expression1;

or (full version):

assert expression1 : expression2;

Where:

• expression1 must be a boolean expression.
• expression2 must return a value (must not return void).

The assert statement is working as follows:

• • If assertion is enabled, then the assert statement will be evaluated. Otherwise, it does not get executed.
  • If expression1 is evaluated to false, an AssertionError error is thrown which causes the program stops immediately. And depending on existence of expression2:

• • • If expression2 does not exist, then the AssertionError is thrown with no detail error message.
    • If expression2 does exist, then a String representation of expression2’s return value is used as detail error message.

• • If expression1 is evaluate to true, then the program continues normally.

Enable assertion

By default, assertion is disabled at runtime. To enable assertion, specify the switch –enableassertions or -ea at command line of java program. For example, to enable assertion for the program called CarManager:

java –enableassertions CarManager

or this for short:

java –ea CarManager

Assertion can be enabled or disable specifically for named classes or packages.

Assertion examples

 The following simple program illustrates the short version of assert statement:

AssertionExample {

   


 public static void main(String[] args) {
 


      

 // get a number in the first argument




       

 int number = Integer.parseInt(args[0]);

 



        assert number <= 10; // stops if number > 10


 



        System.out.println("Pass");


    }




}

 When running the program above with this command:

java -ea AssertionExample 15

A java.lang.AssertionError error will be thrown:

Exception in thread "main" java.lang.AssertionError

       at AssertionExample.main(AssertionExample.java:6)

But the program will continue and print out “Pass” if we pass a number less than 10, in this command:

java -ea AssertionExample 8

And the following example is using the full version of assert statement:

public class AssertionExample2 {


    public static void main(String[] args) {
 


        int argCount = args.length;
 


        assert argCount == 5 : "The number of arguments must be 5";
 


        System.out.println("OK");
 


    }


}

 When running the program above with this command:

java -ea AssertionExample2 1 2 3 4

it will throw this error:

Exception in thread "main" java.lang.AssertionError: The number of arguments must be 5

       at AssertionExample2.main(AssertionExample2.java:6)

Generally, assertion is enabled during development time to defect and fix bugs, and is disabled at deployment or production to increase performance.