Items 57-65
Last modified Fall 2012
Paul Ammann

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Don’t do this!
try{ int i=0; while (true) { range[i++].climb(); }
}catch(IndexOutOfBoundsException e){}
Note that code may hide unrelated exception!
Implications for API design
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A well designed API must not force its clients to use exception handling for ordinary flow control
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Provide state testing method instead of forcing client to catch exception.

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Do this instead for (Mountain m: range) { m.climb(); }
Note:
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(implicit) “State Testing” method: i < range.length
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No possibility of unintentionally hiding
IndexOutOfBoundsException from climb()
Bottom line: Don’t use exceptions for ordinary flow control

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How about?
try{
Iterator i = collection.iterator(); while (true) {
Foo foo = (Foo) i.next(); ...
}
}catch(NoSuchElementException e){}
 versus: for (Iterator i = collection.iterator(); i.hasNext();){
Foo foo = (Foo) i.next(); ...
}

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Two basic options
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State testing method
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Doesn’t work in concurrent environment
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State may change between state test and actual call
May be undesirable if state testing method is as much computation as actual call
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Then, it’s better to have a distinguished return value
Distinguished return value
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May not be an unused special value
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Example: “null” for Object return types
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Iterator next() method can legitimately return “null”
Faulty use of distinguished return value model harder to detect

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Unchecked exceptions indicate programming errors
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Precondition violations
Recovery is impossible
Checked exceptions indicate recoverable conditions
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Force the caller to handle the exception

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Use unchecked exceptions if
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 client has nothing to do
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OutOfMemoryException client knows better
 doesn’t need try-catch block; if-then block would suffice.
Use checked exceptions if client has some reasonable action
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IOException  Calling code is correct, but exception can still be raised!

try{ obj.action(args)
}catch(SomeCheckedException e){ throw new Error (“Assertion Error”);
} // should never happen
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What is the point of making a client do this?
Conditions for using checked exceptions:
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Exceptional condition cannot be prevented by proper use of the API
Programmer can take some useful action

Standard Transformation: if (obj.actionPermitted(args)) { obj.action(args);
} else {
// Handle exceptional condition
}
Or even simply (where appropriate): obj.action(args); // Client allows call to fail

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IllegalArgumentException
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- Inappropriate parameter; several special cases
NullPointerException (param null where
 prohibited)
IndexOutOfBoundsException (index param out of range)
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ClassCastException

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IllegalStateException
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Object state is inappropriate for method invocation
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Object may not be initialized before calling accessing its state
Special subtype: ConcurrentModificationException
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Concurrent modification detected where not allowed
UnsupportedOperationException
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Object does not support the method
Substitution principal

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Reasons to use standard exceptions:
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Using your API is easier
Reading your programs is easier
Performance advantage (relatively minor)
No additional namespace to manage
Note that standard Java exception list is more than the 6 mentioned in Bloch

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Propagated exceptions may make no sense
Higher layers should translate lower level exceptions
// Bad example : public int min (int[] a) { int minVal = a[0]; // Bad: Throws IndexOutOfBoundsException for (int val: a) {if (val < minVal) minVal = val;} return minVal;
}

Exception Chaining Transformation: try {
// Use low level abstraction to satisfy contract
} catch (LowerLevelException cause) { throw new HigherLevelException (cause);
}
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Achieves 2 very different goals
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Gives client exception at expected level of abstraction
Also preserves low level detail for failure analysis

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Checked Exceptions:
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Declare (required by compiler)
And Document (with @throws tag in JavaDoc)
Unchecked Exceptions:
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Document (with @throws tag in JavaDoc)
Don’t shortcut with superclass exceptions
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Example: Don’t catch “Exception” instead of
NullPointerException
Ok to document common exceptions at class level
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Example: “All methods in class throw NullPointerException if a null object reference is passed in any parameter”

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Uncaught exceptions result in printing of stack trace, including “detail” messages
These messages should contain values of all parameters that “contributed to the exception”
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In other words, include relevant state information
Example: IndexOutOfBoundsException includes upper bound, lower bound, and offending index

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Failure atomicity:
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A failed method invocation should leave the object in the state that it was in prior to the invocation
Ways to achieve this effect:
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Design immutable objects
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Check parameters for validity before performing the operation
Order the computation – parts that fail come before modification
Write recovery code – cause the object to roll back its state
Perform the operation on a temporary copy of the object

public int addMax(List<Integer> list, Integer x) throws …
//pre: true
//post: if x is not max w.r.t. list, throw IAE
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// else append x to list
Don’t throw exception in between modifications to state variables
Procedure should have an atomic effect throw exception in the beginning if you are not sure you can pull it off

public Object pop() throws … {
//Requires: this != EmptyStack
//Modifies: this
//Effects: pops this and returns top
Object result = elements[--size]; elements[size] = null; return result;
} // Note: Client can corrupt state – oops!

public Object pop() throws … {
//Requires:
//Modifies: this
//Effects: If this empty throw ISE
// else pop this and returns top if (size == 0) { throw new ISE(…);
Object result = elements[--size]; elements[size] = null; return result;
} // Note atomic execution, normal or exception

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Programmers should never leave a catch block empty
Its easy to ignore exceptions, but don’t!
try{
...
}catch(Exception e){
} // empty catch block wont raise complaints
// from compiler! Don’t do this!

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If its empty, its highly suspect
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The purpose of exception is to force programmers handle exceptional conditions
At least have a comment which explains why its ok to ignore this exception