Assertion in Java

In Java, assertions are a way to test assumptions about your program during runtime. They are primarily used for debugging purposes and help catch programming errors early. An assertion evaluates a boolean expression, and if the expression evaluates to false, the JVM throws an AssertionError.

Syntax

assert condition;

assert condition;

or

assert condition : expression;

assert condition : expression;

• condition – a boolean expression that you expect to be true.
• expression – an optional message or object that is displayed if the assertion fails.

Example

public class AssertionExample {
    public static void main(String[] args) {
        int age = 15;

        // Basic assertion
        assert age > 18 : "Age must be greater than 18";

        System.out.println("Age is valid: " + age);
    }
}

public class AssertionExample {
    public static void main(String[] args) {
        int age = 15;

        // Basic assertion
        assert age > 18 : "Age must be greater than 18";

        System.out.println("Age is valid: " + age);
    }
}

Explanation:

• The assertion assert age > 18 : "Age must be greater than 18"; checks if age is greater than 18.
• Since age is 15, the assertion fails and throws an AssertionError with the message: "Age must be greater than 18".
• If the condition were true, the program would continue normally.

How to Enable Assertions

By default, assertions are disabled in Java. To enable them, use the -ea flag when running your program:

java -ea AssertionExample

java -ea AssertionExample

Key Points

1. Assertions are used for testing assumptions, not for handling runtime errors like exceptions.
2. They are disabled by default in production, so they shouldn’t replace normal error handling.
3. Assertions can include a message for better debugging.