Assertions in Java

An assertion allows us to document our view of the code's behavior

An assertion is a predicate expressing that an expression is true at a specific time. Example:

b = a / 2;
assert (a >= 0 && b <= a) || (a < 0 && b > a);

Assertions allow people reading our code to understand its operation.

The compiler can create code to verify assertions at runtime.

Often assertions are enabled during development and disabled in production execution. This can improve the program's performance.

With assertions we can define

preconditions: These must be true before some code is executed.
postconditions: These are true after some code is executed.

In Java an assertion is written with the keyword assert, followed by a Boolean value that should be true.

An optional string that will be printed if the assertion fails (the condition is false)

If an executed assertion's expression is false an AssertionError is raised.

When we use assertions, we enable them during execution using the following command.

java -ea ClassName

Example

/*
* Run With java -ea FindMax
*/

class FindMax {

        /** Return the maximum number in non-empty array v */
        public static int findMax(int v[]) {
                int max = Integer.MIN_VALUE;

                // Precondition: v[] is not empty
                assert v.length > 0 : "v[] is empty";
                // Precondition: max <= v[i] for every i
                for (int i = 0; i < v.length; i++)
                        assert max <= v[i] : "Found value < MIN_VALUE";

                // Locate the real maximum value
                for (int i = 0; i < v.length; i++)
                        if (v[i] > max)
                                max = v[i];

                // Postcondition: max >= v[i] for every i
                for (int i = 0; i < v.length; i++)
                        assert max >= v[i] : "Found value > MIN_VALUE";
                return max;
        }

        // Test harness
        public static void main(String argv[]) {
                int t[] = new int[5];

                t[0] = 4;
                t[1] = -4;
                t[2] = 145;
                t[3] = 0;
                t[4] = Integer.MIN_VALUE;
                System.out.println("Max value is " + findMax(t));
        }
}