CS1110 Lab 12 Exception Handling
1"
This lab concerns exception handling, the topic of the lecture on 7 April. This material will be covered on the fi-
nal. Download file Lab12.java from the course web page. Put it in a new directory, open the file in DrJava, and
compile.
Task 1. Get familiar with function getKeyboard. You will be using this function later. Read its specification and
try it out by typing this into the Interactions pane.
c= Lab12."ge tKeyboard();
c.readLine( )
When the box appears in the Interactions pane, type something into it and hit the return/enter key.
Task 2. Get familiar with checked exceptions. Study function readKeyboardL ine; make sure you understand
the specification and the function body.
Note the throws-clause throws IOExc eption in the function header. IO stands for input-output, and an
IOException is thrown whenever some sort of input-output error happens.
Delete the clause throws IO Exception from the header of function readKeyboard Line and try to
compile. You get a syntax-error message, right? Read it. Then put the throws-clause back in the function header and
compile.
As explained on pp. 323-325 of the text, Java expects that an exception that may be thrown in a method body ei-
ther (1) be caught in that method body or (2) be mentioned in the throws clause of the method header, so that the
user has a syntactic indication that the exception may be thrown.
All exceptions that may be thrown must be checked except for instances of Runtime Exception (and its sub-
classes).
Do not actively be concerned about these throws clause. Instead, work as follows. Whenever you try to compile
but Java says that a throws clause is needed, put it in.
Task 3. Complete function readKeyboardInt. From
the spec of this function, you know that it is supposed to
keep prompting the user for an integer, giving them a
message when they type something different from an
integer. For this purpose, use an infinite loop whose
body is a try statement:
try {
…
} catch (decl. of an exception variable) {
…
}
where the following describes the try- and catch-blocks:
1. The try-block reads a line from the keyboard and returns the value of a suitable call on function Inte-
ger.parseIn t. To help you out, the spec for Integer.pa rseInt is given to the right.
2. The catch-block catches any exception thrown by Integer.parseInt and prints (using System.out.
println) a suitable message for the user. Note that the declaration of an exception variable is simply a declaration
of the form “type-or-class variable”. In this case, the type-or-class is the class of the exception thrown by Inte-
ger.parseIn t.
Task 4. Using exception handling for preconditions to aid in robust programming. Throughout this course, we
have used the term precondition for a true-false statement that should be true when the method is called and it is the
caller’s duty to ensure that it is true. Take a look at function exp in class Lab12. It has the precondition c ≥ 0. In
this case, if the user calls exp with c < 0, infinite recursion results.
From the API specification for Integer.parseInt
(and then edited)
public static int parseInt
(String s)
throws NumberFormatExceptio n
Parse s as a signed decimal integer. The chars in s
must be decimal digits, except that the first charac-
ter may be an ASCII minus sign '-' ('\u002D') to
indicate a negative value. Return the resulting inte-
ger value. Throw a NumberFormatExcepti on
if s does not contain a parsable int.
