Exceptions

SWE 619: Software ConstructionLast Modified Spring 2010

Saket Kaushik, Paul Ammann

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Inputs to Procedures Sqrt procedure

public float sqrt (float x) Square root can only be calculated for non

negative floats. sqrt() does not makes sense for negative floating point numbers.

Procedure definition allows any float value to be passed (positive or negative)

How should a procedure cope with negative floats?

Use Partial Procedures? Usually a bad idea

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Partial Procedures Announces its contract for use

Set of preconditions that must be ensured at the time of invocation

Theoretically sound… Guarantees post conditions subject to

preconditions BUT clients can always call method without satisfying preconditions How does code behave in such a case Not possible to be sure what client gets

back What happens in next release?

Do clients rely on undocumented behavior?

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Handling Undefined Inputs “Normal” processing is not possible Only option: Define behavior to inform

client How to inform client?

One option: Return special valuesfloat sqrt (float x)//Effects: if x>=0.0 returns approx square

root; // if x <0.0 returns 0.0

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Problems with special values

float sqrt (float x)//Effects: if x>=0.0 returns approx square root; // if x <0.0 returns 0.0Consider following calls:float root1 = sqrt(-45.4); float root2 = sqrt(0.0);if(root1-root2) {bla bla…} if special value is a possible return value of the

procedure, it results in ambiguities no leftover values to use

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Problems with special values

public int search(int [ ] a, int x) //Effects: If a not null and not

empty, //returns index of x in a if present; else //returns -1 not found? empty? null?

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Alternate approach: Exceptions Provide an alternate return mechanism No overlap with ‘real’ values

ArrayIndexOutofBoundsException never overlaps with another real value, but -1 or null does

Exceptions force client to treat special values ‘specially’ usually in a separate code block; not in the

normal execution sequence.

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Specifications Signature:public int foobar(int x, int y) throws <List of exceptions>

e.g: float sqrt (float x) throws NegativeNumberException

Liskov: List all Exceptions thrown Bloch: List only checked exceptions We will generally follow Liskov, but

the issue is mostly about style, not substance

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Requires EffectsEffects clause should explain what causes

each exception to be thrown, define behavior for all inputs in require clause

float sqrt (float x)//Requires: x>=0.0//Effects: …

float sqrt (float x)//Requires://Effects: If x < 0.0 throw //NegativeNumberException; …

float sqrt (float x) throws NNE //Effects: If x< 0.0 throw NNE; …

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Java Exception Mechanism

Throwable

Error Exception

RuntimeException…

…

Checked Exceptions

Unchecked Exceptions

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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.

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Custom Exceptionspublic MyFavoriteException extends Exception {

public MyFavoriteException() {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

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Throwing Exceptions Explicit throws

throw new NullPointerException(“”);throw new NPE(“class, method”);

System throwsString [ ] a = {“1”,”2”,”3”,”4”,”5”};print(a[-1]);System throws IndexOOBE

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-blocktry{

}catch(ExceptionType instance){

}

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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?

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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

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Reflection Vs. Masking

Masking: handle the exception in the procedure. client does not expect the exception client may not understand the

exception Reflection: pass it over to the

calling code client expects it

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Reflection Example

public static int min (int[] a) throws NPE,EE { // Effects if a is null throw NPE else if a is

empty throw EE else return min value in a int m; try { m = a[0];} catch (IOOBE e) { throw new EE (“Arrays.min”); } for (int i=0; i < a.length; i++) if (a[i] < m) m = a[i]; return m;}

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Masking Examplepublic static boolean sorted (int[] a) throws NPE

{ // Effects if a is null throw NPE else if a is sorted

in ascending order return true else return false int p; try { p = a[0];} catch (IOOBE e) { return true; } for (int i=0; i < a.length; i++) if (p < a[i]) {p = a[i];} else {return false;} return true;}

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Meyer vs Liskov

Meyer’s approach is clean and easy to understand

software by contract! if preconditions not met, it’s the client’s fault. Client should suffer!

Recognizes 1 exception: Failure exception

Industry practice is to follow Liskov