Java reflection how to get field value from an object not knowing its class

Question

Say  I have a method that returns a custom List with some objects  They are returned as Object to me  I need to get value of a certain field from these objects  but I don t know the objects  class    Is there a way to do this via Reflecion or somehow else

User · Answer

If you know what class the field is on you can access it using reflection  This example  it s in Groovy but the method calls are identical  gets a Field object for the class Foo and gets its value for the object b  It shows that you don t have to care about the exact concrete class of the object  what matters is that you know the class the field is on and that that class is either the concrete class or a superclass of the object   groovy 000 gt  class Foo   def stuff    asdf       gt  true groovy 000 gt  class Bar extends Foo        gt  true groovy 000 gt  b   new Bar       gt  Bar 1f2be27 groovy 000 gt  f   Foo class getDeclaredField  stuff       gt  private java lang Object Foo stuff groovy 000 gt  f getClass       gt  class java lang reflect Field groovy 000 gt  f setAccessible true      gt  null groovy 000 gt  f get b      gt  asdf

User · Answer

Assuming a simple case  where your field is public    List list     from your method for Object x   list        Class lt   gt  clazz   x getClass        Field field   clazz getField  fieldName      Note  this can throw an exception if the field doesn t exist      Object fieldValue   field get x       But this is pretty ugly  and I left out all of the try-catches  and makes a number of assumptions  public field  reflection available  nice security manager    If you can change your method to return a List lt Foo gt   this becomes very easy because the iterator then can give you type information   List lt Foo gt  list    From your method for Foo foo list        Object fieldValue   foo fieldName      Or if you re consuming a Java 1 4 interface where generics aren t available  but you know the type of the objects that should be in the list     List list  for Object x  list       if  x instanceof Foo          Object fieldValue     Foo x  fieldName           No reflection needed

User · Answer

There is one more way  i got the same situation in my project  i solved this way  List lt Object   gt  list   HQL list     In above hibernate query language i know at which place what are my objects so what i did is    for Object   obj   list   String val   String valueOf obj 1    int code  Integer parseint String valueof obj 0         this way you can get the mixed objects with ease  but you should know in advance at which place what value you are getting or you can just check by printing the values to know  sorry for the bad english I hope this help

User · Answer

I strongly recommend using Java generics to specify what type of object is in that List  ie  List lt Car gt   If you have Cars and Trucks you can use a common superclass interface like this List lt Vehicle gt    However  you can use Spring s ReflectionUtils to make fields accessible  even if they are private like the below runnable example   List lt Object gt  list   new ArrayList lt Object gt      list add  some value    list add 3    for Object obj   list        Class lt   gt  clazz   obj getClass         Field field   org springframework util ReflectionUtils findField clazz   value        org springframework util ReflectionUtils makeAccessible field        System out println  value     field get obj        Running this has an output of      value  C 1b67f74   value 3

User · Answer

public abstract class Refl           Use  Refl  lt TargetClass gt get myObject  x y 0  z           public static lt T gt  T get Object obj  String fieldPath            return  T  getValue obj  fieldPath             public static Object getValue Object obj  String fieldPath            String   fieldNames   fieldPath split                         String success               Object res   obj          for  String fieldName   fieldNames                if  fieldName isEmpty    continue              int index   toIndex fieldName               if  index  gt   0                    try                       res     Object   res  index                     catch  ClassCastException cce                        throw new RuntimeException  cannot cast   res getClass     object   res   to array  path   success  cce                     catch  IndexOutOfBoundsException iobe                        throw new RuntimeException  bad index   index    array size     Object   res  length    object   res     path   success  iobe                                   else                   Field field   getField res getClass    fieldName                   field setAccessible true                   try                       res   field get res                     catch  Exception ee                        throw new RuntimeException  cannot get value of    fieldName    from   res getClass     object   res     path   success  ee                                               success    fieldName                          return res             public static Field getField Class lt   gt  clazz  String fieldName            Class lt   gt  tmpClass   clazz          do               try                   Field f   tmpClass getDeclaredField fieldName                   return f                catch  NoSuchFieldException e                    tmpClass   tmpClass getSuperclass                            while  tmpClass    null            throw new RuntimeException  Field      fieldName      not found in class     clazz              private static int toIndex String s            int res   -1          if  s    null  amp  amp  s length    gt  0  amp  amp  Character isDigit s charAt 0                  try                   res   Integer parseInt s                   if  res  lt  0                        res   -1                                  catch  Throwable t                    res   -1                                  return res            It supports fetching fields and array items  e g    System out println    Refl getValue b  x q 0  z y       there is no difference between dots and braces  they are just delimiters  and empty field names are ignored   System out println    Refl getValue b  x q 0  z y value      System out println    Refl getValue b  x q 1 y z value     System out println    Refl getValue b  x q 1 y z value

[java] Java reflection: how to get field value from an object, not knowing its class

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