Better Strategies for Null Handling in Java

Better Strategies for Null Handling in Java Stephan Schmidt Team manager PMI-3 Berlin, 05.08.2008

Most problematic errors in Java 2 runtime problems in Java ClassCastException „Solved“ with Generics NullPointerException (NPE) Solution? 2

Problems with NPEs RunTime Exception – Point of NPE easy to find – => But not clear where the NULL value comes from 3

Handling of NULL Values Check after Check before String name = map.get(quot;Helloquot;); if (map.containsKey(quot;Helloquot;)) { if (name != null) { String name = map.get(“hallo”); ... } else { … } } else { … } Easy to forget – No support from type system – No tracking of NULL values – Can a reference be NULL ? • 4

Null Handling in Groovy def user = users[“hello”] def streetname = user?.address?.street Safe Navigation Operator ?. user, address can be NULL will simply return NULL instead of throwing an exception 5

Null types in Nice language Nice language NULL types - ?String name => possibly NULL - String name => not NULL String name = null; => Compiler error 6

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 7

Scala Option Class 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 map.get(quot;Helloquot;) match { case Some(name) => // do something with name case None => // do nothing } 8

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 9

For Trick for Option with Iterable 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 public class Option<T> implements Iterable<T> { … } for (String name: getName(“hello”)) { // do something with name } 10

Convenience methods Option<String> name = none(); Option<String> name = option(dontKnow); Option<String> name = some(„stephan“); 11

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) { … } 12

Easy default values with orElse() String name = map.get(„hello“).orElse(„stephan“); Easy handling of default values Very little code compared to Check Before or Check After for default handling in Java 13

Questions? www.ImmobilienScout24.de

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Better Strategies for Null Handling in Java

by Stephan Schmidt

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