fault error failure的深层理解

Below Below are are two faulty programs. Each includes a test case that results in faulty programs. Each includes a test case that results in faulty programs. 

要求：

Identify the fault.

identify a test case that does not execute the fault.

identify a test case that executes the fault, but does not result in an error state.

identify a test case that results in an error, but not a failure.

第一个例子：

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

 

fault: i<0

　　　it should be if(int i = x.length-1; i>=0; i--)

test: x = []

test: x = [2,3,5]  y = 5

　　  Expected = 3

test: x = [2,3,5]  y = 4

　　  Expected = -1

 

第二个例子：

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

 

fault: int i = 0; i<x.length; i++

　　　it should be if(int i = x.length-1; i>=0; i--)

test: x = []

test: x = [1,1,1]

　　  Expected = -1

test: x = [1,0,1]

　　  Expected = 2