"assert" Statements

Java Tutorials - Herong's Tutorial Examples

∟"assert" Statements

This section describes 'assert' statement, which allows you to make an assertion in the execution flow. 'assert' statements are executed only when -ea' JVM option is specified.

What Is "assert" Statement? - An "assert" statement allows you to make an assertion in the execution flow. "assert" statements are executed only when "-ea" JVM option is specified. When executed an "assert" statement will throw an exception if the given assertion condition fails.

Here is the syntax for an "assert" statement. "exception_message" is optional.

assert assertion_condition
assert assertion_condition : exception_message

Here is a sample program that shows you how to use "assert" statements:

/* AssertStatementTest.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
class AssertStatementTest {
   public static void main(String[] arg) {
      java.io.PrintStream out = System.out;

      int hour = Integer.parseInt(arg[0]);
      out.println("Hour specified: "+hour);
      greeting(hour);
   }

   private static void greeting(int hour) {

      // Assertion on hour must >= 0
      assert hour >= 0 : "Hour = "+hour+". It be must be >= 0.";

      // Assertion on hour must < 24
      assert hour < 24 : "Hour = "+hour+". It be must be < 24.";

      // Assuming hour value is between 0 and 23
      if (hour < 6)
         System.out.println("Why are you not sleeping?");
      else if (hour < 12)
         System.out.println("Good morning!");
      else if (hour < 18)
         System.out.println("Good afternoon!");
      else if (hour < 22)
         System.out.println("Good evening!");
      else
         System.out.println("Good night!");
   }
}

If you compile and run the above program with different hour values, you will see:

herong> java AssertStatementTest.java 8
Hour specified: 8
Good morning!

herong> java AssertStatementTest.java 18
Hour specified: 18
Good evening!

herong> java AssertStatementTest.java 28
Hour specified: 28
Good night!

herong> java AssertStatementTest.java -8
Hour specified: -8
Why are you not sleeping?

As you can see, "assert" statements were not executed by default. the greeting() method prints unwanted greeting messages, when invoked as greeting(28) and greeting(-8).