Invoke a method of a private object defined in a class (Java reflection)

Question

I have a class:

public class MyClass {

  private final AnotherClass myField = new AnotherClass() {
       @Override
       public long getSize() { 
            ...
       }
}

I have already got the Class object of MyClass :

Class<MyClass> myClazz = LOAD_CLASS("MyClass");

how to use Java reflection to invoke the getSize() method of myField defined in MyClass ?

Answer 1

You will have to use the Field#setAccessible(boolean b) method in order to get access on the private field.

You can do :

MyClass obj = new MyClass();
try {
    Field field = obj.getClass().getDeclaredField("myField");
    field.setAccessible(true);
    AnotherClass privateField = (AnotherClass) field.get(obj);
    long size = privateField.getSize(); //invoke the getSize() method
    field.setAccessible(false);
} catch (NoSuchFieldException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

Answer 2

You need to use getDeclaredMethod on the myField object:

Field field = obj.getClass().getDeclaredField("myField");
field.setAccessible(true);

Object privateField = field.get(obj);

Method getSizeMethod = privateField.getClass().getDeclaredMethod("getSize");

Long result = (Long)getSizeMethod.invoke(privateField);