Java Exceptions
SWE 332
Last Modified Spring 2010
Paul Ammann
Rationale for Exceptions
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Preconditions document undefined behavior
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Undefined behavior  Partial specification
In any implementation, something happens
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Should clients rely on undocumented behavior?
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Bottom line:
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Preconditions are usually undesirable
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What happens in next release?
But sometimes unavoidable…
Exception handling:
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Transform preconditions to defined behavior
One of several possible mechanisms…
2
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: …
public double sqrt (double x)
// precondition:
// postcondition: If x < 0 throw IllegalArgumentException
// else …
public double sqrt (double x)
/**
* @param x the square of the intended result
* @return approximate square root of x…
* @throws IllegalArgumentException if x < 0
*/
3
Java Exception Mechanism
Throwable
Error
Exception
…
RuntimeException
Checked Exceptions
…
Unchecked Exceptions
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Exceptions
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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.
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5
Custom Exceptions
public MyFavoriteException extends Exception {
public MyFavoriteException() {
super();
}
public MyFavoriteException(String s) {super(s);}

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can add more
like state at the time of throwing the exception
methods that let the invalid state be printed and other useful
information
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Throwing Exceptions
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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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Catching Exceptions

In a special code block called try-block
try{
}catch(ExceptionType instance){
}
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8
Catching Exceptions

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Can have multiple catch blocks, with
different execution logic for different
exceptions raised
Class hierarchy matters in catching!

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If you catch an exception of type
Exception, all subtypes are also caught!
catching Exception is dangerous! WHY?
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Remember!

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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
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10