Question

I have a Term class to define a polynomial:

public class Term
{
    final private int coef;
    final private int expo;

    private static Term zero, unit;

    static
    {
        try
        {
            zero = new Term(0, 0); // the number zero
            unit = new Term(1, 0); // the number one
        }
        catch (Exception e)
        {
            // constructor will not throw an exception here anyway
        }
    }

    /**
     * 
     * @param c
     *            The coefficient of the new term
     * @param e
     *            The exponent of the new term (must be non-negative)
     * @throws NegativeExponent
     */
    public Term(int c, int e) throws NegativeExponent
    {
        if (e < 0)
            throw new NegativeExponent();
        coef = c;
        expo = (coef == 0) ? 1 : e;
    }

    final public static Term Zero = zero;
    final public static Term Unit = unit;

    public boolean isConstant()
    {
        boolean isConst = false;
        if (this.expo == 0)
        {
            isConst = true;
        }
        return isConst;
    }
}

And I have the JUnit test as follows:

 /*
 *      const1      isConstant(zero)        =>      true    (0,0)
 *      const2      isConstant(unit)        =>      true    (1,0)
 *      const3      isConstant(0,5)         =>      true
 *      const4      isConstant(5,1)         =>      false
 */

 @Test 
 public void const1() throws TError { assertTrue(Term.Zero.isConstant()); }

 @Test 
 public void const2() throws TError { assertTrue(Term.Unit.isConstant()); }

 @Test 
 public void const3() throws TError { assertTrue(new Term(0,5).isConstant()); }

 @Test 
 public void const4() throws TError { assertFalse(new Term(5,1).isConstant()); }

Tests 2 and 4 pass as they should, but Tests 1 and 3 come up as failures and I cannot work out why, “Zero” is defining the polynomial as (0,0) and the other is defining it as (0,5). So by my thinking, the 1st one should give a green tick and the 3rd test should give a red cross as it has 5 as the exponent.

Any ideas?

Answer 1

Please review your constructor:

public Term(int c, int e) throws NegativeExponent{
    if (e < 0) throw new NegativeExponent();
    coef = c;
    expo = (coef == 0) ? 1 : e;
}

When coef == 0, then exp is assigned with 1.

This makes zero as (0,1) not (0,0). this is the reason for test outcome as below:

    const1 -> false as expo = 1-->failure as expecting true
    const2 -> true as expo = 0 -->success as expecting true
    const3 -> false as expo = 5.-->failure as expecting true
    const4 -> false as expo = 1.-->success as expecting false

EDIT: To reverse fix your code to make the test cases pass, I think you may update your constructor as below:

public Term(int c, int e) throws NegativeExponent{
    if (e < 0) throw new NegativeExponent();
    coef = c;
    expo = (c == 0 && e != 0) ? 0 : e;
}

Here, I have updated the constructor to set expo as 0, if coef is 0 as any expo for coef 0 is immaterial.