软件测试基础（英文版）P16第三题

Below are four faulty programs. Each includes a test case that results in failure. Answer the following questions (in the next slide) about each program.

Questions
Identify the fault.
If possible, identify a test case that does not execute the fault. (Reachability)
If possible, identify a test case that executes the fault, but does not result in an error state.
If possible identify a test case that results in an error, but not a failure.

public intfindLast(int[] x, inty) {
//Effects: If x==null throw
NullPointerException
// else return the index of the last element
// in x that equals y.
// If no such element exists, return -1
for (inti=x.length-1; i> 0; i--)
{
if (x[i] == y)
{
return i;
}
}
return -1;
}
// test: x=[2, 3, 5]; y = 2
// Expected = 0

(a)Fault: i > 0, 导致x[0]无法被遍历，正确应该为i >= 0。

(b)Test: x = null; y = 2

(c)Test: x = [2,3,5]; y = 5.   Excepted = 2

(d)Test: x = [2,3,5]; y = 2.   Excepted = -1

public static intlastZero(int[] x) {
//Effects: if x==null throw
NullPointerException
// else return the index of the LAST 0 in x.
// Return -1 if 0 does not occur in x
for (inti= 0; i< x.length; i++)
{
if (x[i] == 0)
{
return i;
}
} return -1;
}
// test: x=[0, 1, 0]
// Expected = 2

(a)Fault: 遍历顺序错误，正确结果应该为从后往前遍历，即 for(int i = x.length - 1; i >= 0; i --)

(b)Test: x = null

(c)Test: x = [2,1,0]   Excepted = 2

(d)Test: x = [0,1,0]   Excepted = 2, but result = 0