How do I find out what type each object is in a ArrayList Object

Question

I have a ArrayList made up of different elements imported from a db  made up of strings  numbers  doubles and ints  Is there a way to use a reflection type technique to find out what each type of data each element holds     FYI  The reason that there is so many types of data is that this is a piece of java code being written to be implemented with different DB s

User · Answer

Just call  getClass   on each Object in a loop   Unfortunately  Java doesn t have map

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instead of using object getClass   getName   you can use object getClass   getSimpleName    because it returns a simple class name without a package name included   for instance   Object   intArray     1      for  Object object   intArray         System out println object getClass   getName         System out println object getClass   getSimpleName         gives   java lang Integer Integer

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You almost never want you use something like   Object o       if  o getClass   equals Foo class                 because you aren t accounting for possible subclasses   You really want to use Class isAssignableFrom   Object o       if  Foo class isAssignableFrom o

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import java util ArrayList           author potter        public class storeAny                    param args             public static void main String   args               TODO Auto-generated method stub          ArrayList lt Object gt  anyTy new ArrayList lt Object gt             anyTy add new Integer 1            anyTy add new String  Jesus             anyTy add new Double 12 88            anyTy add new Double 12 89            anyTy add new Double 12 84            anyTy add new Double 12 82             for  Object o   anyTy                if o instanceof String                   System out println o toString                   else if o instanceof Integer                    System out println o toString                      else if o instanceof Double                    System out println o toString

User · Answer

If you expect the data to be numeric in some form  and all you are interested in doing is converting the result to a numeric value  I would suggest   for  Object o list      Double parseDouble o toString

User · Answer

Instanceof works if you don t depend on specific classes  but also keep in mind that you can have nulls in the list  so obj getClass   will fail  but instanceof always returns false on null

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You say  this is a piece of java code being written   from which I infer that there is still a chance that you could design it a different way     Having an ArrayList is like having a collection of stuff   Rather than force the instanceof or getClass every time you take an object from the list  why not design the system so that you get the type of the object when you retrieve it from the DB  and store it into a collection of the appropriate type of object   Or  you could use one of the many data access libraries that exist to do this for you

User · Answer

In Java just use the instanceof operator  This will also take care of subclasses   ArrayList lt Object gt  listOfObjects   new ArrayList lt Object gt     for Object obj  listOfObjects      if obj instanceof String       else if obj instanceof Integer       etc

User · Answer

You can use the getClass   method  or you can use instanceof  For example  for  Object obj   list      if  obj instanceof String                   or  for  Object obj   list     if  obj getClass   equals String class                   Note that instanceof will match subclasses  For instance  of C is a subclass of A  then the following will be true   C c   new C    assert c instanceof A    However  the following will be false   C c   new C    assert  c getClass   equals A class

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Since Java 8           mixedArrayList forEach  o  -                String type   o getClass   getSimpleName                switch  type                    case  String                          treat as a String                     break                  case  Integer                          treat as an int                     break                  case  Double                          treat as a double                     break                                      default                         whatever

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for  Object object   list        System out println object getClass   getName

User · Answer

If you expect the data to be numeric in some form  and all you are interested in doing is converting the result to a numeric value  I would suggest   for  Object o list      Double parseDouble o toString

User · Answer

In C  Fixed with recommendation from Mike  ArrayList list           List lt object gt  list        foreach  object o in list        if  o is int            HandleInt  int o             else if  o is string            HandleString  string o                     In Java   ArrayList lt Object gt  list        for  Object o   list        if  o instanceof Integer             handleInt  Integer o  intValue               else if  o instanceof String             handleString  String o

[java] How do I find out what type each object is in a ArrayList<Object>?

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