Assertions in Java are a useful tool for developers to test assumptions about their program’s behavior during runtime. When an assertion fails, it throws an AssertionError, which helps in identifying bugs early in the development process.
Enabling Assertions
By default, assertions are disabled. To enable assertions, use the -ea or -enableassertions option when running your Java program.
or
java -enableassertions:arguments
true, the program executes normally.But if the condition evaluates to false while assertions are enabled, JVM throws an AssertionError, and the program stops immediately.
Assertion Syntax
expression evaluates to false, an AssertionError is thrown.Example of Assertions
Ex:
1 2 3 4 5 6 7 8 9 10 11 | public class AssertionExample { public static void main(String[] args) { int x = 1; assert x > 0 : "x must be greater than 0"; System.out.println("x is: " + x); x = -1; assert x > 0 : "x must be greater than 0"; System.out.println("x is: " + x); } } |
When running the above code with -ea, the program will throw an AssertionError at the second assertion when x is -1.
Ex:
1 2 3 4 5 6 7 | class Main { public static void main(String args[]) { String[] weekends = {"Friday", "Saturday", "Sunday"}; assert weekends.length == 2; System.out.println("There are " + weekends.length + " weekends in a week"); } } |
Output:
1 | There are 3 weekends in a week |
We get the above output because this program has no compilation errors and by default, assertions are disabled.
After enabling assertions, we get the following output:
1 | Exception in thread "main" java.lang.AssertionError |
When to Use Assertions
- Checking Preconditions: Ensure that the input conditions to a method are valid.
- Checking Postconditions: Ensure that the conditions after executing a method are correct.
- Checking Invariants: Ensure that the state of an object is valid throughout its lifecycle.
- Checking Unreachable Code: Ensure that certain code paths are never executed.
