Lab 2 : Experimenting with Exception Handling

Completed lab is due by: tomorrow but see DELIVERABLES below

Getting Ready: Before going any further you should:

1. Make a directory on your N: drive for this lab. Call it ExceptionHandlingLab
2. Copy this document to your N: drive for this lab. All answers should be placed into the document.
3. You are to use jGRASP for this lab.

Part 1: This part of the lab considers a simple example of exception handling.

1.

a. Create a file named Example1.java that contains the following code along with your header information:

public class Example1
{
public static void main(String[] args)
{
int denominator, numerator, ratio;

numerator = 5;
denominator = 2;

ratio = numerator / denominator;
System.out.println(“Ratio is: “ +

ratio);

}
}

b. Compile and execute the application Example1.

c. What was output by the application when you executed it?

Ratio is: 2

2.

a. Change the value of denominator to 0.

b. Re-compile and re-execute Example1.

c. What "error" was generated by the application when you executed it?

ÏÏÏÏ
ÏÏ«Ï ----jGRASP exec: java Example1a
ÏϧÏ
ÏϧÏjava.lang.ArithmeticException: / by zero
ÏϧÏ_at Example1a.main(Example1a.java:20)
ÏϧÏException in thread "main"
ÏÏ§Ï ----jGRASP wedge2: exit code for process is 1.
ÏÏ©Ï ----jGRASP: operation complete.
¼¼ÏÏ

d. Why was this "error" generated at run-time (rather than at compile-time)?

because the compiler did not know the value of the denominator when the program was compiled so it didn’t know that an illegal operation would occur

3.

a. Put only the statement that generated the exception inside of the try portion of a try-catch block (and leave the catch block empty. (Hint: You should be able to determine what exception to catch from the error message generated during the previous step.)

b. Re-compile Example1.

c. What error is generated and why?

Ï «Ï ----jGRASP exec: javac -g A:\Example1b.java
ÏϧÏ
ϼ§ÏExample1b.java:27: variable ratio might not have been initialized
ÏÏ§Ï System.out.println("Ratio is: " + ratio);
ÏÏ§Ï ^
ÏϧÏ1 error
ÏϧÏ
ÏÏ§Ï ----jGRASP wedge2: exit code for process is 1.
ÏÏ©Ï ----jGRASP: operation complete.

4.

a. Move the "output statement" into the try block (as well).

b. Add the statement System.out.println("Divide by 0"); to the catch block.

c. Re-compile and re-execute Example1.

d. What output was generated?

Ï «Ï ----jGRASP exec: java Example1c
ÏϧÏ
ÏÏ§Ï Divide by zero
ÏϧÏ
ÏÏ©Ï ----jGRASP: operation complete.

5.

a. Add a call to the printStackTrace() method of the ArithmeticException to the catch block. (Hint: use javadoc to see how to use this).

b. Re-compile and re-execute Example1.

c. What output was generated?

Ï«Ï ----jGRASP exec: java Example1d
ÏϧÏ
ÏϧÏjava.lang.ArithmeticException: / by zero
ÏϧÏ_at Example1d.main(Example1d.java:24)
ÏÏ§Ï Divide by zero
ÏϧÏ
ÏÏ©Ï ----jGRASP: operation complete.

d. Did the application execute properly or not?

depends on what you mean by properly. saw StackTrace before println

6.

a. Change the program to input the two values to evaluate from the keyboard.. Use System.in as your source.

b. Try running this program using both examples, 5 and 2 and 5 and 0 as operands for the division. Did anything change?

No – but I had to add import java.util.Scanner at the top

c. Create a text file input.txt. Include one line with the values 5 and 2 separated by a space. Use this file as input to the program (change the Scanner source from System.in to the file (see either listing 5.11 on page 240 of our text OR one of the ReadFromFile.java programs in yesterday’s lecture notes). Read the values from the file. Were there any differences from keyboard processing to file processing? What did you need to do to compile this program.

I had to add import java.io.* and I had to add throws FileNotFoundException

Part II: This part of the lab considers an example of exception handling within and outside of block statements.

1. Create a file named Example2.java that contains the following:

public class Example2
{
public static void main(String[] args)
{
int i, ratio;
int[] numbers = {100,10,0,5,2,8,0,30};

try
{
for (i=0; i < numbers.length-1; i++)
{
ratio = numbers[i] / numbers[i+1];
System.out.println
(numbers[i]+"/"+numbers[i+1]+"="+ratio);
}
}

catch (ArithmeticException ae)
{

System.out.println
("Couldn't calculate "+ numbers[i]+"/"+numbers[i+1]);
}
}
}

2. Compile Example2.
3. What error was generated?

«Ï ----jGRASP exec: javac A:\Example2.java
ÏϧÏ
ϼ§ÏExample2.java:29: variable i might not have been initialized
ÏÏ§Ï ("Couldn't calculate "+ numbers[i]+"/"+numbers[i+1]);
ÏÏ§Ï ^
ÏϧÏ1 error
ÏϧÏ
ÏÏ§Ï ----jGRASP wedge2: exit code for process is 1.
ÏÏ©Ï ----jGRASP: operation complete.

1. Initialize i to 0 inside of the try block (but before the for loop).
2. Compile Example2.
3. What error was generated?

«Ï ----jGRASP exec: javac A:\Example2a.java
ÏϧÏ
ϼ§ÏExample2a.java:31: variable i might not have been initialized
ÏÏ§Ï ("Couldn't calculate "+ numbers[i]+"/"+numbers[i+1]);
ÏÏ§Ï ^
ÏϧÏ1 error
ÏϧÏ
ÏÏ§Ï ----jGRASP wedge2: exit code for process is 1.
ÏÏ©Ï ----jGRASP: operation complete.

4. It is not possible for i to be used before it is initialized. Why is this error generated anyway? (Hint: Think about block statements.)

Because i only has a value inside the try block… Need to give it a value inside the catch block or before the try block. ASSIGNMENTS WHICH TAKE PLACE IN A BLOCK ARE ONLY KNOWN WITHIN THAT BLOCK.

1. Move the initialization of i before the try block.
2. Compile and execute Example2.
3. What output is generated?

Ï «Ï ----jGRASP exec: java Example2b
ÏϧÏ
ÏϧÏ100/10=10
ÏϧÏCouldn't calculate 10/0ÏϧÏ
ÏÏ©Ï ----jGRASP: operation complete.

4. Why aren't all of the divisions even attempted?

Because the for block is inside the try block. Once something has failed in the try block , the block won’t be returned to. CONTROL TRANSFERRED TO THE CATCH BLOCK AND LEFT THE FOR LOOP BECAUSE THE TRY/ CATCH BLOCK WASN’T INSIDE THE FOR LOOP SO THE FOR LOOP ENDED WHEN THE TRY/CATCH OCCURRED

4. Fix Example2 so that it executes properly. (Hint: Move the try-catch block inside of the for block.)

Part III: This part of the lab considers an inappropriate use of exception handling and how to "fix" it.

1. Create a file named Example3.java that contains the following:

public class Example3
{
public static void main(String[] args)
{
int i;
int[] data = {50, 320, 97, 12, 2000};
try
{
for (i=0; i < 10; i++)
{
System.out.println(data[i]);
}
}
catch (ArrayIndexOutOfBoundsException aioobe)
{

System.out.println("Done");
}
}
}

2. Compile and execute Example3 and verify that it outputs all of the values followed by the word "Done".
3. Modify Example3 so that it loops "properly" and does not need to use a try-catch block. (Note: The output should not change.)

DELIVERABLES

1. Turn in a hardcopy of this document with your answers at the end of the lab. IF you do not complete the lab today, turn in a printout of what you have today, and bring the completed lab to class tomorrow.
2. Turn in a disk with the three programs with the given files names in the root directory of the disk.
3. Be sure that your programs have the required heading and other documentation including the names of others with whom you have worked.