Software testing is a huge topic. We're going to cover a small piece called unit testing, which is the testing used to ensure that individual pieces of code (at the level of a function or method or class) are working properly.

There are many guidelines for defining unit tests for your code (example), but here are a few of the most important:

• Unit tests should be automated. You don't want to have to put any work into running them.
• Unit tests should be fast. You want to be able to run the tests that pertain to the code you're working on just about every time you make a change.
• Unit tests should be comprehensive. The hard part is understanding what comprehensive means. Sometimes the code helps: you should strive to test every branch of code. But, depending on the domain, it can be hard to determine if you've caught every possible case. For example, with some numeric functions, you clearly can't test with every possible number as input, so you need to examine boundary cases, extreme cases, and expected cases.

Unit testing is important enough that there are lots of tools to make it easier. JUnit is the most popular for Java, and most IDEs make it easy to add JUnit tests with just a click. We'll just set up tests by hand, but when you know more about how to define classes, you're better off letting the tools do as much work as possible.

An additional benefit of unit testing is that it helps you think about how to design your program: if part of your program is hard to test, it's often also hard to integrate with other parts of your program. So keeping testing in mind as you go is a good idea. In fact, it's the basis of Test Driven Development, a popular way of approaching program design (see also here).

Here's an example we'll use in class:

/** Geometry-related utility methods.
 */
public class Geometry {

	public static double abs(double x) {
		if (x >= 0)
		    return x;
		else 
			return -x;	
	}

    /** Determine if a point is contained in a circle with given
     *  radius and center.
     *
     * @param x x-coordinate of point
     * @param y y-coordinate of point
     * @param centerX x-coordinate of center of circle
     * @param centerY y-coordinate of center of circle
     * @param radius radius of circle
     * @return true if point is contained in circle
     */
    public static boolean contains(double x, double y,
                                   double centerX, double centerY,
                                   double radius) {
        // Compute squared distance from center
        double dx = x - centerX;
        double dy = y - centerY;
        double dist2 = dx * dx + dy * dy;
        return dist2 <= radius * radius;
    }
                                   

    /** Compute area of triangle with given corners.
     * 
     * @param aX x-coordinate of corner 1
     * @param aY y-coordinate of corner 1
     * @param bX y-coordinate of corner 2
     * @param bY x-coordinate of corner 2
     * @param cX x-coordinate of corner 3
     * @param cY y-coordinate of corner 3
     * @return area of triangle with given corner coordinates
     */
    public static double area(double aX, double aY, double bX, double bY,
                              double cX, double cY) {
        double uX = bX - aX;
        double uY = bY - aY;
        double vX = bX - cX;
        double vY = bY - cY;
        return Math.abs(0.5 * cross(uX, uY, vX, vY));
    }

    /** Return true if line segments, each defined by a start and end point,
     *  meet at any point.
     * 
     * @param aX starting x-coordinate of segment 1
     * @param aY starting y-coordinate of segment 1
     * @param bX ending x-coordinate of segment 1
     * @param bY ending y-coordinate of segment 1
     * @param cX starting x-coordinate of segment 2
     * @param cY starting y-coordinate of segment 2
     * @param dX ending x-coordinate of segment 2
     * @param dY ending y-coordinate of segment 2
     * @return true if line segments meet
     */
    public static boolean intersects(double aX, double aY,
                                     double bX, double bY,
                                     double cX, double cY,
                                     double dX, double dY) {
        
        // Some details just in case you'd like to know the math here:
        // In vector notation, we'll parametrize segment 1 and segment 2
        // respectively as:
        // s1 + u*p1, 0<=p1<=1
        // s2 + v*p2, 0<=p2<=1
        // We'll then solve for values of p0 and p1 where segments would
        // intersect, and determine if they lie in the valid range.
        // This intersection occurs if s1 + u*p1 = s2 + v*p2 for some p1,p2, or
        // s2 - s1 = u*p1 - v*p2.  Let s2 - s1 = t.  If we use the cross product
        // on both sides, we can get (with "x" denoting cross product):
        // t x u = p2*(u x v), and
        // t x v = p1*(u x v).
        // The simplification happens because the cross product of any vector
        // with itself (or a parallel vector) is 0.  Hence p2 = (t x u)/(u x v),
        // and p1 = (t x v)/(u x v) at the intersection.  If those values are
        // both in the range [0, 1], the segments intersect.
        
        // directions of each line segment
        double uX = bX - aX;
        double uY = bY - aY;
        double vX = dX - cX;
        double vY = dY - cY;

        // difference between line segment starting points
        double tX = cX - aX;
        double tY = cY - aY;
        
        // cross products
        double uvCross = cross(uX, uY, vX, vY);
        double tuCross = cross(tX, tY, uX, uY);
        double tvCross = cross(tX, tY, vX, vY);

        // solve for where lines intersect.  Can you spot a bug?
        double p1 = tuCross / uvCross;
        double p2 = tvCross / uvCross;

        return p1 >= 0 && p1 <= 1 && p2 >= 0 && p2 <= 1;
        
    }
    
    // 2-D cross product 
    public static double cross(double aX, double aY, double bX, double bY) {
        return aX * bY - aY * bX;
    }
    
	// Assert two numbers are within 1e-10, with given error message.
	public static void checkClose(double x, double y, String msg) {
		assert Math.abs(x - y) < 1e-10: msg + x + ", " + y;
	}
    
    public static void main(String[] args) {
        // Insert basic unit testing routines
        
		// Tests for abs()
		assert abs(0) == 0: "abs(0) must be 0";
		assert abs(1) == 1;
		assert abs(-1) == 1;
		
		// Tests for area()
		assert area(1, 1, 1, 1, 1, 1) == 0: "3 point degenerate case";
		assert area(1, 1, 1, 1, 2, 2) == 0: "2 point degenerate case";
                assert area(1, 1, 2, 2, 3, 3) == 0: "Colinear case";
		assert area(0, 0, 2, 0, 0, 2) == 2: "Right triangle case";
		checkClose(area(0, 0, 0.2, 0, 0, 0.2), .02, "Small right triangle");
		checkClose(area(0, 0, 2, 0, 1, 2), 2, "Non-right triangle");
		
		// Tests for intersects()
		
    }
    
}

When we write our unit tests, we'll use the assert statement, which allows our program to exit with an error message (as long as we don't manually handle the error) as soon as one of our checks fails. However, you have to remember that assert statements are not run by default. So you must enable them in some way when running the tests we'll create. If you run from the command line, you just specify an extra -ea flag for the Java interpreter: java -ea MyTests. If you're using NetBeans, you'll have to do the following, as shown here:

• Edit your project properties.
• Under the Run section, create a new configuration with a name that indicates it's for testing.
• Add -ea to the VM Options for the new configuration.

If you're using JUnit or another framework, you don't have to worry about that stuff.

In case you want to use JUnit, here's a short example:

import org.junit.Test;
import static org.junit.Assert.*;

/** Tests for the Geometry class static methods. */
public class GeometryTest {
    @Test
    public void testAbs() {
        assertEquals("abs(0) should be 0", Geometry.abs(0), 0.0, 0.0);
        assertEquals("abs(1) should be 1", Geometry.abs(1), 1.0, 0.0);
        assertEquals("abs(-1) should be 1", Geometry.abs(-1), 1.0, 0.0);
    }
    
    @Test
    public void testArea() {
        assertEquals("3 point degenerate case",
                Geometry.area(1, 1, 1, 1, 1, 1), 0.0, 0.0);
        assertEquals("2 point degenerate case",
                Geometry.area(1, 1, 1, 1, 2, 2), 0.0, 0.0);
    }
}