Since Java 1.4, assertions have been like detectives in the Java world, helping developers expose hidden issues in their code and fix them.

1. How to Use the Assert Detective

Using the assert statement is like asking a detective to check something:

• Short Version: assert expression1;

  or

• Full Version: assert expression1 : expression2;

  • expression1 is a simple check (true or false).
  • expression2 is an optional additional message if something goes wrong.

How it works:

• If assertions are turned on, the detective checks the statement; otherwise, it does nothing.
• If the check (expression1) is false, the detective raises an alarm (AssertionError), optionally showing the extra message from expression2.
• If the check is true, the program continues as usual.

2. Turning On Assertions

By default, assertions are asleep. To wake them up, use -ea at the command line:

java -ea MyProgram

3. Detective Stories: Examples

Let's see the detective in action with simple examples:

public class DetectiveExample {
    public static void main(String[] args) {
        int number = Integer.parseInt(args[0]);

        assert number <= 10; // Stops if the number is greater than 10

        System.out.println("All clear!");
    }
}

If you run this with java -ea DetectiveExample 15, it shouts:

Exception in thread "main" java.lang.AssertionError
    at DetectiveExample.main(DetectiveExample.java:6)

But if you run it with java -ea DetectiveExample 8, it says, "All clear!"

Here's another example with an optional message:

public class DetectiveMessage {
    public static void main(String[] args) {
        int argCount = args.length;

        assert argCount == 5 : "There must be exactly 5 arguments";

        System.out.println("Everything looks good!");
    }
}