java How can I do dynamic casting of a variable from one type to another

Question

I would like to do dynamic casting for a Java variable  the casting type is stored in a different variable   This is the regular casting    String a    String  5    This is what I want    String theType    String    String a    theType  5    Is this possible  and if so how  Thanks   Update  I m trying to populate a class with a HashMap that I received   This is the constructor   public ConnectParams HashMap lt String Object gt  obj         for  Map Entry lt String  Object gt  entry   obj entrySet              try               Field f    this getClass   getField entry getKey                                 f set this  entry getValue         lt   CASTING PROBLEM              catch  NoSuchFieldException ex                log error  did not find field      entry getKey                     catch  IllegalAccessException ex                log error ex getMessage                                   The problem here is that some of the class  variables are of type Double  and if the number 3 is received it sees it as Integer and I have type problem

User · Accepted Answer

Regarding your update  the only way to solve this in Java is to write code that covers all cases with lots of if and else and instanceof expressions  What you attempt to do  looks as if are used to program with dynamic languages  In static languages  what you attempt to do is almost impossible and one would probably choose a totally different approach for what you attempt to do  Static languages are just not as flexible as dynamic ones         Good examples of Java best practice are the answer by BalusC  ie ObjectConverter  and the answer by Andreas D  ie Adapter  below      That does not make sense  in   String a    theType  5    the type of a is statically bound to be String so it does not make any sense to have a dynamic cast to this static type    PS  The first line of your example could be written as Class lt String gt  stringClass   String class  but still  you cannot use stringClass to cast variables

User · Answer

Don t do this  Just have a properly parameterized constructor instead  The set and types of the connection parameters are fixed anyway  so there is no point in doing this all dynamically

User · Answer

Just thought I would post something that I found quite useful and could be possible for someone who experiences similar needs   The following method was a method I wrote for my JavaFX application to avoid having to cast and also avoid writing if object x instance of object b statements every time the controller was returned   public  lt U gt  Optional lt U gt  getController Class lt U gt  castKlazz       try           return Optional of fxmlLoader  lt U gt getController          catch  Exception e           e printStackTrace              return Optional empty        The method declaration for obtaining the controller was  public  lt T gt  T getController     By using type U passed into my method via the class object  it could be forwarded to the method get controller to tell it what type of object to return  An optional object is returned in case the wrong class is supplied and an exception occurs in which case an empty optional will be returned which we can check for   This is what the final call to the method looked like  if present of the optional object returned takes a Consumer  getController LoadController class  ifPresent controller- gt controller onNotifyComplete

User · Answer

It works and there s even a common pattern for your approach  the Adapter pattern  But of course   1  it does not work for casting java primitives to objects and  2  the class has to be adaptable  usually by implementing a custom interface    With this pattern you could do something like   Wolf bigBadWolf   new Wolf    Sheep sheep    Sheep  bigBadWolf getAdapter Sheep class     and the getAdapter method in Wolf class   public Object getAdapter Class clazz      if  clazz equals Sheep class            return a Sheep implementation     return getWolfDressedAsSheep this          if  clazz equals String class            return a String     return this getName           return null     not adaptable     For you special idea - that is impossible  You can t use a String value for casting

User · Answer

You can do this using the Class cast   method  which dynamically casts the supplied parameter to the type of the class instance you have  To get the class instance of a particular field  you use the getType   method on the field in question  I ve given an example below  but note that it omits all error handling and shouldn t be used unmodified   public class Test        public String var1      public Integer var2     public class Main        public static void main String   args  throws Exception           Map lt String  Object gt  map   new HashMap lt String  Object gt             map put  var1    test            map put  var2   1            Test t   new Test             for  Map Entry lt String  Object gt  entry   map entrySet                  Field f   Test class getField entry getKey                  f set t  f getType   cast entry getValue                         System out println t var1           System out println t var2

User · Answer

For what it is worth  most scripting languages  like Perl  and non-static compile-time languages  like Pick  support automatic run-time dynamic String to  relatively arbitrary  object conversions  This CAN be accomplished in Java as well without losing type-safety and the good stuff statically-typed languages provide WITHOUT the nasty side-effects of some of the other languages that do evil things with dynamic casting  A Perl example that does some questionable math   print     foo    99       prints  100  print     foo    a0       prints  a1    In Java  this is better accomplished  IMHO  by using a method I call  cross-casting   With cross-casting  reflection is used in a lazy-loaded cache of constructors and methods that are dynamically discovered via the following static method    Object fromString  String value  Class targetClass    Unfortunately  no built-in Java methods such as Class cast   will do this for String to BigDecimal or String to Integer or any other conversion where there is no supporting class hierarchy  For my part  the point is to provide a fully dynamic way to achieve this - for which I don t think the prior reference is the right approach - having to code every conversion  Simply put  the implementation is just to cast-from-string if it is legal possible   So the solution is simple reflection looking for public Members of either   STRING CLASS ARRAY    new Class    String class     a  Member member   targetClass getMethod method getName   STRING CLASS ARRAY   b  Member member   targetClass getConstructor STRING CLASS ARRAY    You will find that all of the primitives  Integer  Long  etc  and all of the basics  BigInteger  BigDecimal  etc  and even java regex Pattern are all covered via this approach  I have used this with significant success on production projects where there are a huge amount of arbitrary String value inputs where some more strict checking was needed  In this approach  if there is no method or when the method is invoked an exception is thrown  because it is an illegal value such as a non-numeric input to a BigDecimal or illegal RegEx for a Pattern   that provides the checking specific to the target class inherent logic   There are some downsides to this   1  You need to understand reflection well  this is a little complicated and not for novices   2  Some of the Java classes and indeed 3rd-party libraries are  surprise  not coded properly  That is  there are methods that take a single string argument as input and return an instance of the target class but it isn t what you think    Consider the Integer class   static Integer getInteger String nm        Determines the integer value of the system property with the specified name    The above method really has nothing to do with Integers as objects wrapping primitives ints  Reflection will find this as a possible candidate for creating an Integer from a String incorrectly versus the decode  valueof and constructor Members - which are all suitable for most arbitrary String conversions where you really don t have control over your input data but just want to know if it is possible an Integer   To remedy the above  looking for methods that throw Exceptions is a good start because invalid input values that create instances of such objects should throw an Exception  Unfortunately  implementations vary as to whether the Exceptions are declared as checked or not  Integer valueOf String  throws a checked NumberFormatException for example  but Pattern compile   exceptions are not found during reflection lookups  Again  not a failing of this dynamic  cross-casting  approach I think so much as a very non-standard implementation for exception declarations in object creation methods   If anyone would like more details on how the above was implemented  let me know but I think this solution is much more flexible extensible and with less code without losing the good parts of type-safety  Of course it is always best to  know thy data  but as many of us find  we are sometimes only recipients of unmanaged content and have to do the best we can to use it properly   Cheers

User · Answer

Yes it is possible using Reflection    Object something    something   String theType    java lang String   Class lt   gt  theClass   Class forName theType   Object obj   theClass cast something     but that doesn t make much sense since the resulting object must be saved in a variable of Object type  If you need the variable be of a given class  you can just cast to that class   If you want to obtain a given class  Number for example   Object something   new Integer 123   String theType    java lang Number   Class lt   extends Number gt  theClass   Class forName theType  asSubclass Number class   Number obj   theClass cast something     but there is still no point doing it so  you could just cast to Number   Casting of an object does NOT change anything  it is just the way the compiler treats it  The only reason to do something like that is to check if the object is an instance of the given class or of any subclass of it  but that would be better done using instanceof or Class isInstance     Update  according your last update the real problem is that you have an Integer in your HashMap that should be assigned to a Double  What you can do in this case  is check the type of the field and use the xxxValue   methods of Number      Field f    this getClass   getField entry getKey     Object value   entry getValue    if  Integer class isAssignableFrom f getType           value   Integer valueOf   Number  entry getValue    intValue       else if  Double class isAssignableFrom f getType           value   Double valueOf   Number  entry getValue    doubleValue          other cases as needed  Long  Float       f set this  value          not sure if I like the idea of having the wrong type in the Map

User · Answer

I recently felt like I had to do this too  but then found another way which possibly makes my code look neater  and uses better OOP   I have many sibling classes that each implement a certain method doSomething    In order to access that method  I would have to have an instance of that class first  but I created a superclass for all my sibling classes and now I can access the method from the superclass   Below I show two ways alternative ways to  dynamic casting       Method 1  mFragment   getFragmentManager   findFragmentByTag MyHelper getName mUnitNum    switch  mUnitNum    case 0        MyFragment0  mFragment  sortNames sortOptionNum       break  case 1        MyFragment1  mFragment  sortNames sortOptionNum       break  case 2        MyFragment2  mFragment  sortNames sortOptionNum       break      and my currently used method      Method 2  mSuperFragment    MySuperFragment  getFragmentManager   findFragmentByTag MyHelper getName mUnitNum    mSuperFragment sortNames sortOptionNum

User · Answer

You can write a simple castMethod like the one below   private  lt T gt  T castObject Class lt T gt  clazz  Object object      return  T  object      In your method you should be using it like  public ConnectParams HashMap lt String Object gt  object     for  Map Entry lt String  Object gt  entry   object entrySet          try           Field f    this getClass   getField entry getKey                             f set this  castObject entry getValue   getClass    entry getValue         lt   CASTING PROBLEM          catch  NoSuchFieldException ex            log error  did not find field      entry getKey                 catch  IllegalAccessException ex                log error ex getMessage

User · Answer

So  this is an old post  however I think I can contribute something to it   You can always do something like this   package com dyna test     import java io File    import java lang reflect Constructor     public class DynamicClass        SuppressWarnings  unchecked       public Object castDynamicClass String className  String value         Class lt   gt  dynamicClass         try                   We get the actual  class object associated with the specified name         dynamicClass   Class forName className               We get the constructor that received only       a String as a parameter  since the value to be used is a String  but we could easily change this to be  dynamic  as well  getting the Constructor signature from the same datasource we get the values from             Constructor lt   gt  cons              Constructor lt   gt   dynamicClass getConstructor new Class lt   gt    String class               We generate our object  without knowing until runtime   what type it will be  and we place it in an Object as   any Java object extends the Object class             Object object    Object  cons newInstance new Object   value             return object                catch  Exception e                  e printStackTrace                  return null             public static void main String   args               DynamicClass dynaClass   new DynamicClass                We specify the type of class that should be used to represent   the value  3 0   in this case a Double  Both these parameters   you can get from a file  or a network stream for example           System out println dynaClass castDynamicClass  java lang Double    3 0                We specify a different value and type  and it will work as   expected  printing 3 0 in the above case and the test path in the one below  as the Double toString   and   File toString   would do           System out println dynaClass castDynamicClass  java io File    C   testpath               Of course  this is not really dynamic casting  as in other languages  Python for example   because java is a statically typed lang  However  this can solve some fringe cases where you actually need to load some data in different ways  depending on some identifier  Also  the part where you get a constructor with a String parameter could be probably made more flexible  by having that parameter passed from the same data source  I e  from a file  you get the constructor signature you want to use  and the list of values to be used  that way you pair up  say  the first parameter is a String  with the first object  casting it as a String  next object is an Integer  etc  but somehwere along the execution of your program  you get now a File object first  then a Double  etc   In this way  you can account for those cases  and make a somewhat  dynamic  casting on-the-fly   Hope this helps anyone as this keeps turning up in Google searches

User · Answer

You ll need to write sort of ObjectConverter for this  This is doable if you have both the object which you want to convert and you know the target class to which you d like to convert to  In this particular case you can get the target class by Field getDeclaringClass     You can find here an example of such an ObjectConverter  It should give you the base idea  If you want more conversion possibilities  just add more methods to it with the desired argument and return type

User · Answer

Your problem is not the lack of  dynamic casting   Casting Integer to Double isn t possible at all  You seem to want to give Java an object of one type  a field of a possibly incompatible type  and have it somehow automatically figure out how to convert between the types   This kind of thing is anathema to a strongly typed language like Java  and IMO for very good reasons   What are you actually trying to do  All that use of reflection looks pretty fishy

User · Answer

Try this for Dynamic Casting  It will work          String something    1234       String theType    java lang Integer       Class lt   gt  theClass   Class forName theType       Constructor lt   gt  cons   theClass getConstructor String class       Object ob    cons newInstance something       System out println ob equals 1234

[java] java: How can I do dynamic casting of a variable from one type to another?

Examples related to java

Examples related to casting

Examples related to dynamic-cast