1. CS-1020 and Exception Handling
5/15/2019
Assertions
and Exception Handling
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick
Dr. Mark L. Hornick

2. Assertions can be used to help debug your programs
The syntax for the assert statement is
assert( <boolean expression> );
where <boolean expression> represents the condition that must be true if the code is working correctly.
If the expression is false, an AssertionError (a subclass of Error) is thrown.
You can’t (or shouldn’t) catch AssertionError’s
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick

3. Assert Example public double fromDollar(double dollar){
assert( exchangeRate > 0.0 ); // throw if <0
return (dollar * exchangeRate); }
public double toDollar(double foreignMoney){
assert( exchangeRate > 0.0 );// throw if < 0
return (foreignMoney / exchangeRate);
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick

4. The assert() statement may also take the form:
assert(<boolean expr>): <expression>;
where <expression> represents the value passed as an argument to the constructor of the AssertionError class.
The value serves as the detailed message of a thrown error.
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick

5. Assertions Example public double fromDollar(double dollar){
assert exchangeRate > 0.0:
“Exchange rate = “ + exchangeRate +
“.\nIt must be a positive value.”;
return (dollar * exchangeRate); }
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick

6. Assertions are normally disabled, and are usually only enabled for program testing
To run the program with assertions enabled, add the “-ea” argument to the VM in the Run Dialog
If the –ea option is not provided, the program is executed without checking assertions.
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick

7. Do not use the assertion feature to ensure the validity of an argument during normal execution
Use assertions only to detect internal programming errors while debugging
Use exceptions during normal execution to notify client programmers of the misuse of classes.
Portions adapted with permission from the textbook author.
CS-1020
Dr. Mark L. Hornick