如何找到实际上包含Java中元素的类

Question

Say I have two classes

class parentClass{
   String myElement;
}
class childClass extends parentClass{
   String notMyElemtent;
}

Now say there is an object of class childClass. Is there a way programatically tell that myElement in that object belongs to parentClass originally??

Answer 1

You can do it with reflection. Use obj.getClass().getField("myElement") to get Field object, representing your field. Now you can use getDeclaringClass() method of Member interface to get class or interface actually declaring this member. So do something like this

childClass obj = new childClass();
Field field = obj.getClass().getField("myElement");
if (field.getDeclaringClass().equals(parentClass.class)) {
    // do whatever you need
}

Answer 2

Is there a way to tell that myElement in that object belongs to parentClass originally?

Yes, you can use reflection to examine the fields of the super class:

• Use Class.getSuperClass() to get the super class
• Then, use
  • Class.getDeclaredFields() on the returned class from the above call to get a list of all fields declared by the super class
  • Class.getDeclaredField(String fieldName) to directly check the existence of a given field

In code, this could look like:

public static boolean belongsToParent(Object o, String fieldName) {
   Class<?> sc = o.getClass().getSuperclass();
   boolean result = true;
   try {
      sc.getDeclaredField(fieldName);
   } catch (NoSuchFieldException e) {
      result = false;
   }
   return result;
}

public static void main(String[] args) {
   childClass cc = new childClass();

   System.out.println("myElement belongs to parentClass: " + 
                      belongsToParent(cc,  "myElement"));
   System.out.println("notMyElemtent belongs to parentClass: " + 
                      belongsToParent(cc,  "notMyElemtent"));
}

Output:

myElement belongs to parentClass: true
notMyElemtent belongs to parentClass: false

Answer 3

Well, use getDeclaredField(name) of a class, and if its not there, try looking at its super class and so on. Works for multiple levels of inheritance:

Class<?> clazz = childClass.class;
do {
    try {
        Field f = clazz.getDeclaredField(fieldName);
        //there it is! print the name of the super class that holds the field
        System.out.println(clazz.getName());
    } catch (NoSuchFieldException e) {
        clazz = clazz.getSuperclass();
    }
} while (clazz != null);

Answer 4

import java.lang.reflect.Field;

public class Test4 {

    public static void main(String[] args){
        Child child = new Child();
        System.out.println(getDeclaringClass(child.getClass(), "value"));

    }

    public static String getDeclaringClass(Class<?> clazz, String name) {
        try {
            Field field = clazz.getDeclaredField(name);
        } catch (NoSuchFieldException e) {
            if(clazz.getSuperclass() != null){
                return getDeclaringClass(clazz.getSuperclass(), name);              
            }else{
                return null;
            }
        }

        return clazz.getName();
    }
}

class Parent {
    String value = "something";

}

class Child extends Parent {

}