Éviter NullPointer Exception en Java

92

assert <condition> 

assert <condition> : <object> 

public interface Action {   void doSomething(); }  public interface Parser {   Action findAction(String userInput); } 

public class MyParser implements Parser {   private static Action DO_NOTHING = new Action() {     public void doSomething() { /* do nothing */ }   };    public Action findAction(String userInput) {     // ...     if ( /* we can't find any actions */ ) {       return DO_NOTHING;     }   } } 

Parser parser = ParserFactory.getParser(); if (parser == null) {   // now what?   // this would be an example of where null isn't (or shouldn't be) a valid response } Action action = parser.findAction(someInput); if (action == null) {   // do nothing } else {   action.doSomething(); } 

ParserFactory.getParser().findAction(someInput).doSomething(); 

try {     ParserFactory.getParser().findAction(someInput).doSomething(); } catch(ActionNotFoundException anfe) {     userConsole.err(anfe.getMessage()); } 

public Action findAction(final String userInput) {     /* Code to return requested Action if found */     return new Action() {         public void doSomething() {             userConsole.err("Action not found: " + userInput);         }     } } 

89

@NotNull public static String helloWorld() {     return "Hello World"; } 

@Nullable public static String helloWorld() {     return "Hello World"; } 

public static void main(String[] args) {     String result = helloWorld();     if(result != null) {         System.out.println(result);     } } 

void someMethod(@NotNull someParameter) { } 

someMethod(null); 

@Nullable iWantToDestroyEverything() { return null; } 

iWantToDestroyEverything().something(); 

75

public void method(Object object) {   if (object == null) {     throw new IllegalArgumentException("...");   } 

public String getFirst3Chars(String text) {   return text.subString(0, 3); } 

if (object == null) {   // something } else {   // something else } 

68

public String getPostcode(Person person) {     return person?.getAddress()?.getPostcode();   }