1. SWE 332 Last Modified Spring 2010 Paul Ammann
Java Exceptions
SWE 332
Last Modified Spring 2010
Paul Ammann

2. Rationale for Exceptions
Preconditions document undefined behavior
Undefined behavior  Partial specification
In any implementation, something happens
Should clients rely on undocumented behavior?
What happens in next release?
Bottom line:
Preconditions are usually undesirable
But sometimes unavoidable…
Exception handling:
Transform preconditions to defined behavior
One of several possible mechanisms…

3. Transforming a Precondition to an Exceptional Postcondition
Key: Postcondition defines behavior for inputs excluded by precondition
public double sqrt (double x)
// precondition: x >= 0
// postcondition: …
// precondition:
// postcondition: If x < 0 throw IllegalArgumentException
// else …
/**
x the square of the intended result
approximate square root of x…
IllegalArgumentException if x < 0 */

4. Java Exception Mechanism
Checked Exceptions
Unchecked Exceptions
Ammann 2008

5. Exceptions
Unchecked: client does not need to explicitly write special code
Checked: client has to pay attention to these. Must write special code blocks
Many different types of exceptions are provided.
You can provide your own.
Ammann 2008

6. Custom Exceptions public MyFavoriteException extends Exception {
super(); }
public MyFavoriteException(String s) {super(s);}
can add more
like state at the time of throwing the exception
methods that let the invalid state be printed and other useful information
Ammann 2008

7. Throwing Exceptions Explicit throws
throw new NullPointerException(“”);
throw new NPE(“class, method”);
System throws
String [ ] a = {“1”,”2”,”3”,”4”,”5”};
print(a[-1]);
System throws IndexOutOfBoundsException
Passing through (goes up the call chain)
keeps going till it finds a handler
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8. Catching Exceptions In a special code block called try-block try{
}catch(ExceptionType instance){ }
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9. Catching Exceptions
Can have multiple catch blocks, with different execution logic for different exceptions raised
Class hierarchy matters in catching!
If you catch an exception of type Exception, all subtypes are also caught!
catching Exception is dangerous! WHY?
Ammann 2008

10. Remember! try-catch block should be as short as possible.
catch the right type of exception: to be sure of what’s the right thing to do
Ammann 2008