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.

 

public int findLast (int[] x, int y) { //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 (int i=x.length-1; i > 0; i--) { if (x[i] == y) { return i; } } return -1; } // test: x=[2, 3, 5]; y = 2 // Expected = 0

 

public static int lastZero (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 (int i = 0; i < x.length; i++) { if (x[i] == 0) { return i; } } return -1; }

// test: x=[0, 1, 0] // Expected = 2

 

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.

（1）fault：由i>0知，i不可能为0，所以不可能得到期望值0

       不执行故障测试用例  ：x=[]; y=2  期望值=-1       执行故障不导致错误用例  ： x=[3,2,5]; y=2 期望值=1

       导致错误但不是失败结果的用例： x=[1,3,5]; y=2 期望值=-1

 ( 2 )fault: 因为数组第一个即为0，所以到第一个时已经返回了0，不可能得到期望2

       不执行故障测试用例 ：x=[]  期望值=-1

       执行故障不导致错误用例  ： x=[1,2,3] 期望值=-1

       导致错误但不是失败结果的用例： x=[2,3,0,4] 期望值=2