Better Strategies for Null Handling in Java
Stephan Schmidt
Team manager PMI-3
Berlin, 05.08.2008
2
Most problematic errors in Java
� 2 runtime problems in Java
ClassCastException
„Solved“ with Generics
NullPointerException (NPE)
Solution?
3
Problems with NPEs
– RunTime Exception
– Point of NPE easy to find
=> But not clear where the NULL value comes
from
4
Handling of NULL Values
– Easy to forget
– No support from type system
– No tracking of NULL values
• Can a reference be NULL ?
Check before
if (map.containsKey("Hello")) {
String name = map.get(“hallo”);
} else { … }
Check after
String name = map.get("Hello");
if (name != null) {
...
} else { … }
5
Null Handling in Groovy
�Safe Navigation Operator ?.
user, address can be NULL
will simply return NULL instead of throwing an exception
def user = users[“hello”]
def streetname = user?.address?.street
6
Null types in Nice language
Nice language NULL types
- ?String name => possibly NULL
- String name => not NULL
String name = null;
=> Compiler error
7
NULL Handling with Annotations
�@NotNull, @Nullable in Java
IDEA and others, JSR 308
Automatic checks for NULL
IDEA tells you when NPEs will occure
@NotNull
public String get(@NotNull String name) { … }
Everything not null and @Optional for NULL better solution
8
Scala Option Class
map.get("Hello") match {
case Some(name) => // do something with name
case None => // do nothing
}
Option can have a value or not (think container with 0 or 1 elements).
Subclasses are Some and None
Must deal with None (NULL) value, cannot ignore
Called Maybe (Just, Nothing) in Haskell
9
Option in Java
Option<String> option = map.get(„hello“);
if (option instanceof Some) {
String name = ((Some) option).value();
….
} else {
// option is none, there is no „hello“
}
�Explicit handling of „NULL“ value necessary
�Or:
�option.isSome() and option.value() without cast
10
For Trick for Option with Iterable
public class Option<T> implements Iterable<T> { … }
for (String name: getName(“hello”)) {
// do something with name
}
Sometimes the none case needs no handling
For and Iterable<T> can be used
For automatically unwraps Option, does nothing in None case
None returns EMPTY list, Some one element list with option value
11
Convenience methods
Option<String> name = none();
Option<String> name = option(dontKnow);
Option<String> name = some(„stephan“);
12
How does this method handle NULL values?
�API makes the intention clear
� public Option<String> getName() {…}
� public String getName() { …}
� public void setName(Option<String> name) { … }
� public void setName(String name) { … }
13
Easy default values with orElse()
�Easy handling of default values
�Very little code compared to Check Before or
Check After for default handling in Java
String name = map.get(„hello“).orElse(„stephan“);
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Questions?
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