CSC/ECE 517 Fall 2010/ch6 6b yc

Support for Assertions in Various O-O Programming Languages

Problem Statement

"Compare the support for assertions in various o-o programming languages. How well is it integrated with the language (instead of being supplied by libraries)? How many kinds of assertions are supported? How are assertions used in the various flavors of xUnit testing frameworks?"

Introduction

Definition

"An imperfect solution is better than none. - B. Meyer "[1]

Assertions are formal constraints on software systems which are inserted as annotations in the source program text. They had their origin in program verification [2]. Program correctness is usually defined in relation to a specification and assertions can encode the semantic properties of a specification. Using assertions to show program correctness is in general a non-trivial task and therefore it is hardly followed in practice. However, many key properties of a program can still be encoded in a simple assertion language. In such a scenario, if a program executes without any assertion violation, it can give some confidence about the program’s correctness. In a sense, assertions test a program without using any test data.

Example

Following is a simple example of an assertion in Java.

int total = NumberOfUsers();
if (total % 2 == 0) {
    // total is even
} else {
    // total is odd
    assert(total % 2 == 1);
}

In Java, % is the remainder operator (or modulus) — if its first operand is negative, the result can also be negative. Here, we have assumed that total is non-negative, so that the remainder of a division with 2 will always be 0 or 1. The assertion makes this assumption explicit — if NumberOfUsers does return a negative value, the program may have a bug.

Different Types of Assertions

Assertions are meant to encode the key properties of O-O programs. They can be classified into the following types.

• Preconditions: This defines what must be true when a method is invoked.
• Postconditions: This defines what must be true after a method completes successfully.
• Class invariants: This defines what must be true about each instance of a class.

Support for Assertions in Various O-O Programming Languages

Assertions in O-O programming languages are either evaluated at compile time or at run time, depending on the concrete programming language [3]. In this section, we will investigate the support for assertions in various O-O programming languages.

Java

Each assertion in Java contains a boolean expression that we believe will be true when the assertion executes. If it is not true, the system will throw an error. By verifying that the boolean expression is indeed true, the assertion confirms our assumptions about the behavior of the program.

The assertion statement in Java has two forms [4]. The first, simpler form is:

assert Expression1 ;

where Expression1 is a boolean expression. When the system runs the assertion, it evaluates Expression1 and if it is false throws an AssertionError with no detail message.

The second form of the assertion statement is:

assert Expression1 : Expression2 ;

where:

• Expression1 is a boolean expression.
• Expression2 is an expression that has a value.

Use this version of the assert statement to provide a detail message for the AssertionError. The system passes the value of Expression2 to the appropriate AssertionError constructor, which uses the string representation of the value as the error's detail message.

Here is an example of using assertion in Java.

import java.util.*;
import java.util.Scanner;
  
public class AssertionExample
{
	public static void main( String args[] )
	{
		Scanner scanner = new Scanner( System.in );
		System.out.print( "Enter a number between 0 and 20: " );
		int value = scanner.nextInt();
		assert value >= 0 && value <= 20 : "Invalid number: " + value;
		System.out.printf( "You have entered %d\n", value );
	}
}

In the above example, When the user enters the number scanner.nextInt() method reads the number from the command line. The assert statement determines whether the entered number is within the valid range. If the user entered a number which is out of range then the error occurs.

References

• [1] B. Meyer, Object Oriented Software Construction, Prentice Hall, 1997.
• [2] C. A. R. Hoare, An Axiomatic Basis for Computer Programming, Communications of the ACM, Vol. 12, No. 10, pp. 576–580,583, October 1969.
• [3] M. Satpathy, N. T. Siebel and D. Rodriguez, Assertions in Object Oriented Software Maintenance: Analysis and a Case Study, Proceedings of the 20th IEEE International Conference on Software Maintenance, 2004.
• [4] Programming With Assertions