Better Strategies for Null Handling in Java

Better Strategies for Null Handling in JavaStephan SchmidtTeam manager PMI-3Berlin, 05.08.2008Most problematic errors in Java2 runtime problems in JavaClassCastException„Solved“ with GenericsNullPointerException (NPE)Solution?2Problems with NPEs– RunTime Exception– Point of NPE easy to find=> But not clear where the NULL value comesfrom3Handling of NULL ValuesCheck beforeCheck afterif (map.containsKey("Hello")) {String name = map.get("Hello");String name = map.get(“hallo”);if (name != null) {} else { … }...} else { … }–Easy to forget–No support from type system–No tracking of NULL values•Can a reference be NULL ?4Null Handling in Groovydef user = users[“hello”]def streetname = user?.address?.streetSafe Navigation Operator ?.user, address can be NULLwill simply return NULL instead of throwing an exception5Null types in Nice languageNice language NULL types- ?String name => possibly NULL- String name => not NULLString name = null;=> Compiler error6NULL Handling with Annotations@NotNull, @Nullable in JavaIDEA and others, JSR 308Automatic checks for NULLIDEA tells you when NPEs will occure@NotNullpublic String get(@NotNull String name) { … } Everything not null and @Optional for NULL better solution7Scala Option ClassOption can have a value or not (think container with 0 or 1 elements). Subclasses are Some and NoneMust deal with None (NULL) value, cannot ignoreCalled Maybe (Just, Nothing) in Haskellmap.get("Hello") match {case Some(name) => // do something with namecase None => // do nothing }8Option in JavaOption<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 necessaryOr:option.isSome() and option.value() without cast9For Trick for Option with IterableSometimes the none case needs no handlingFor and Iterable<T> can be usedFor automatically unwraps Option, does nothing in None caseNone returns EMPTY list, Some one element list with option valuepublic class Option<T> implements Iterable<T> { … }for (String name: getName(“hello”)) {// do something with name}10