Java Question, how to get the value of a method from an unknown object

Question

I have lots of object defined in the system, perhaps 1000 objects, and some of them have this method:

public Date getDate();

Is there anyway I can do something like this:

Object o = getFromSomeWhere.....;

Method m = o.getMethod("getDate");
Date date = (Date) m.getValue();

Answer 1

If there is an interface requiring a getDate() method you can check using the following code:

if (o instance of GetDateInterface){
    GetDateInterface foo = (GetDateInterface) o;
    Date d = foo.getDate();
}

Answer 2

If you can make them all implement an interface, that would certainly be the best option. However, reflection will also work, and your code was nearly there:

Object o = getFromSomeWhere.....;
Method m = o.getClass().getMethod("getDate");
Date date = (Date) m.invoke(o);

(There's a bunch of exceptions you'll need to handle, admittedly...)

For a full example:

import java.lang.reflect.*;
import java.util.*;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        Object o = new Test();
        Method m = o.getClass().getMethod("getDate");
        Date date = (Date) m.invoke(o);
        System.out.println(date);
    }

    public Date getDate()
    {
        return new Date();
    }
}

Answer 3

To complete the other answer(s), I'd also identify the classes with an interface. This is how I'd do it

import java.util.Date;

interface Dated {
  public Date getDate();
  public void setDate(Date date);
}

class FooObject implements Dated {
  private Date date;
  public void setDate(Date date) { this.date = date; }
  public Date getDate() { return date; }
  // other code...
}

public static void main (String[] args) {
  Object o = getRandomObject(); // implemented elsewhere
  if (o instanceof Dated) {
    Date date = ((Dated)o).getDate();
    doStuffWith(date);
  }
}

Answer 4

Try this:

Object aDate = new Date();
Method aMethod = aDate.getClass().getMethod("getDate", (Class<Void>[]) null);
Object aResult = aMethod.invoke(aDate, (Void) null);

You should add try-catch to determine if there really is a method getDate before invoking.

Answer 5

If you don't mind the extra dependency, BeanUtils can do this for you.