No Exception while type casting with a null in java

Question

String x    String  null    Why there is no exception in this statement   String x   null  System out println x     It prints null  But  toString   method should throw a null pointer exception

User · Answer

This is by design  You can cast null to any reference type  Otherwise you wouldn t be able to assign it to reference variables

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This language feature is convenient in this situation   public String getName       return  String  memberHashMap get  Name        If memberHashMap get  Name   returns null  you d still want the method above to return null without throwing an exception  No matter what the class is  null is null

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Many answers here already mention      You can cast null to any reference type   and     If the argument is null  then a string equal to  null    I wondered where that is specified and looked it up the Java Specification   The null reference can always be assigned or cast to any reference type    5 2    5 3    5 5     If the reference is null  it is converted to the string  null   four ASCII characters n  u  l  l

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This is very handy when using a method that would otherwise be ambiguous  For example  JDialog has constructors with the following signatures   JDialog Frame  String  boolean  GraphicsConfiguration  JDialog Dialog  String  boolean  GraphicsConfiguration    I need to use this constructor  because I want to set the GraphicsConfiguration  but I have no parent for this dialog  so the first argument should be null  Using  JDialog null  String  boolean  Graphicsconfiguration     is ambiguous  so in this case I can narrow the call by casting null to one of the supported types   JDialog  Frame  null  String  boolean  GraphicsConfiguration

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As others have written  you can cast null to everything   Normally  you wouldn t need that  you can write   String nullString   null    without putting the cast there   But there are occasions where such casts make sense   a  if you want to make sure that a specific method is called  like   void foo String bar           void foo Object bar             then it would make a difference if you type  foo  String  null  vs  foo null    b  if you intend to use your IDE to generate code  for example I am typically writing unit tests like    Test expected NullPointerException class  public testCtorWithNullWhatever         new MyClassUnderTest  Whatever  null       I am doing TDD  this means that the class  MyClassUnderTest  probably doesn t exist yet  By writing down that code  I can then use my IDE to first generate the new class  and to then generate a constructor accepting a  Whatever  argument  out of the box  - the IDE can figure from my test that the constructor should take exactly one argument of type Whatever

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Casting null values is required for following construct where a method is overloaded and if null is passed to these overloaded methods then the compiler does not know how to clear up the ambiguity hence we need to typecast null in these cases   class A     public void foo Long l           do something with l       public void foo String s           do something with s             new A   foo  String null   new A   foo  Long null     Otherwise you couldn t call the method you need

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Print       Print an object  The string produced by the String valueOf Object  method is translated into bytes   ValueOf      if the argument is null  then a string equal to  null   otherwise  the value of obj toString   is returned    It wil simply return a string with value  null  when the object is null

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You can cast null to any reference type   You can also call methods which handle a null as an argument  e g  System out println Object  does  but you cannot reference a null value and call a method on it   BTW There is a tricky situation where it appears you can call static methods on null values   Thread t   null  t yield       Calls static method Thread yield   so this runs fine

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Println Object  uses String valueOf    public static String valueOf Object obj        return  obj    null     null    obj toString        Print String  does null check   public void print String s        if  s    null            s    null             write s

User · Answer

You can cast null to any reference type without getting any exception   The println method does not throw null pointer because it first checks whether the object is null or not  If null then it simply prints the string  null   Otherwise it will call the toString method of that object   Adding more details  Internally print methods call String valueOf object  method on the input object  And in valueOf method  this check helps to avoid null pointer exception   return  obj    null     null    obj toString      For rest of your confusion  calling any method on a null object should throw a null pointer exception  if not a special case

[java] No Exception while type casting with a null in java

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