HW02- Answer the questions about each program includes a test case that results in failure.

Below are two faulty programs. Each includes a test case that results in failure. Answer the following questions about each program:

1、Identify the fault.

2、If possible, identify a test case that does not execute the fault. (Reachability)

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

4、If possible identify a test case that results in an error, but not a failure.

 

First：

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

Answer：

1、Fault：The loop condition should be i>=0 not i>0

2、x=null  y=1  Expected = NullPointerException  Actual = NullPointerException

3、x=[1,2,3]  y=3  Expected = 2  Actual = 2

4、x=[1,2,3]  y=4  Expected = -1  Actual = -1

 

Second：

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

Answer：

1、Fault：The loop condition should be for (int i=x.length-1; i > 0; i--) not for (int i = 0; i < x.length; i++)

2、x=null  Expected = NullPointerException  Actual = NullPointerException

3、x=[1,2,0]  Expected = 2  Actual = 2

4、x=[1,0,2]  Expected = 1  Actual = 1