Java assert Keyword

The assert keyword in Java is used for debugging purposes. It is primarily used to test assumptions in the code by throwing an AssertionError if an expression evaluates to false. Assertions are typically used during development and testing but are disabled by default at runtime.

To enable assertions, you need to use the -ea (enable assertions) JVM option when running the program.

Syntax:

Single Expression

If the condition evaluates to false, an AssertionError is thrown.

Expression with an Error Message

If the condition is false, an AssertionError is thrown with a message.

Program 1: Basic Assertion

Output:

Number is: 5

After enabling assertions

Output:

Exception in thread "main" java.lang.AssertionError: Number is not greater than 10

Explanation

The program d demonstrates the use of the assert keyword in Java to validate assumptions during runtime; the program initializes num as 5 and uses an assertion assert num > 10 : "Number is not greater than 10", which checks if num is greater than 10; since the condition is false, an AssertionError with the message "Number is not greater than 10" is thrown when executed with assertions enabled (-ea), otherwise, assertions are ignored, and the program prints "Number is: 5".

Program 2: Using Assertions in a Method

Output:

16
9

After enabling assertions

Output:

16
Exception in thread "main" java.lang.AssertionError: Number should be non-negative
	at AssertionExample2.square(AssertionExample2.java:3)
	at AssertionExample2.main(AssertionExample2.java:7)

Explanation

The program demonstrates the use of the assert keyword in Java to validate method inputs by ensuring that the given number is non-negative before calculating its square. When assertions are disabled (default mode), the program executes normally and prints 16 and 9, even though square(-3) violates the assertion condition, however, when assertions are enabled using -ea, the second method call fails, throwing an AssertionError with the message "Number should be non-negative", terminating execution.

Program 3: Assertions in Loops

Output:

Even number: 1
Even number: 2
Even number: 3
Even number: 4
Even number: 5

After enabling assertions

Output:

Exception in thread "main" java.lang.AssertionError: Odd number encountered: 1

Explanation

The program demonstrates the use of the assert keyword in Java to validate conditions during execution; in this program, an assertion checks whether each number in the loop is even (i % 2 == 0), and if the condition fails (i.e., i is odd), an AssertionError with the message "Odd number encountered: " is thrown; when run without enabling assertions (-ea not used), the assertion is ignored, and all numbers (1 to 5) are printed, but when assertions are enabled (java -ea AssertionExample3), the program terminates at i = 1 with an AssertionError.

Important Points to Remember

1. Assertions should not be used for regular error handling or user input validation. They are meant for development and debugging, not for runtime robustness.
2. By default, assertions are disabled in the Java runtime, so we need to enable them explicitly.