Working of AssertEquals(Object, Object) in JUnit

[Stephen Masters]

I'm just curious about the workings of the JUnit AssertEquals method when comparing two of my own classes where I have overridden the equals method and I was trying to work out why my unit test for it kept failing. My example is as follows:

BigDecimalMatrix matrix1 = new BigDecimalMatrix(array);
BigDecimalMatrix matrix2 = new BigDecimalMatrix(array);
 
assertTrue(matrix1.equals(matrix2));
assertEquals(matrix1, matrix2);

I thought that the AssertEquals method should just be using the equals method and the implementation in my classes would be called. However, in the above example, the assertTrue passes, but the AssertEquals fails. Debugging the assertEquals call, it never actually calls into my equals method.

Is there something wrong with my understanding of what's going on here?

Am I likely to have missed something that might catch me out later?

Is it generally better to avoid using AssertEquals for Object?

Thanks for any thoughts you might have on this.

[ July 29, 2007: Message edited by: Stephen Masters ]
[ July 29, 2007: Message edited by: Stephen Masters ]

[Ulf Dittmer]

Welcome to JavaRanch.

Are you overriding the hashCode method? You generally need to do that if you override equals; read the javadocs of both methods for some information.

[Stephen Masters]

Yep ... I have also overridden the hashCode() method, but that's not getting called by assertEquals either!

[Ilja Preuss]

What version of JUnit are you using? Can you show us the code for BigDecimalMatrix?

[Stephen Masters]

Looks like it's 4.3.1 - the one that comes bundled with eclipse 3.3.

The whole class is fairly long so I'll just throw in the more relevant bits (I think!). i.e. I won't bother putting in the methods for adding, subtracting, etc. It may be worth noting that Matrix is an abstract class.

public class BigDecimalMatrix extends Matrix {
 
    protected BigDecimal[][] matrix;
 
    /**
     * Two matrices are considered equal if corresponding elements throughout
     * are equal. Therefore both matrices must be of the same order.
     * 
     * @see Matrix
     */
    public boolean equals(Matrix target) {
 
        // If the orders of the matrices are not equal, this allows an immediate
        // response, avoiding checking each element.
        if (!ordersEqual(this, target)) {
            return false;
        }
 
        // Compare each element of this matrix to the corresponding element
        // in the target matrix. If any don't match return false.
        for (int i = 0; i < this.rows; i++) {
            for (int j = 0; j < this.columns; j++) {
                BigDecimal el1 = this.matrix[i][j];
                BigDecimal el2 = ((BigDecimalMatrix) target).matrix[i][j];
                if (!el1.equals(el2)) return false;
            }
        }
        // We haven't found any corresponding elements that did not match.
        return true;
    }
 
    public int hashCode() {
        return this.matrix.hashCode();
    }
 
}

[Stephen Masters]

Sorry guys ... looks like I've worked it out. I just changed it as follows and it's happy now:

// from
public boolean equals(Matrix target) {
// to
public boolean equals(Object obj) {

Obviously, when I was calling .equals(Matrix), it was working fine. However, I'm assuming that assertEquals must be passing Object everywhere. My class didn't have a .equals(Object) method and was therefore falling over. I guess all I need to do now is check whether it's an instance of Matrix before casting and it should be fine.

Thanks for taking a look anyway!

[Ilja Preuss]

With other words, you didn't override the equals method, you overloaded it.

Nice to see that you've found the problem!