Get index of an arraylist using property of an contained object in java

I'm having an list of Object type. In that I have one String property idNum. Now I want to get the index of the object in the list by passing the idNum.

List<Object1> objList=new ArrayList<Object1>();

I don't know how to give objList.indexOf(// Don't know how to give here);

Is it possible to do this without iterating the list. I want to use indexOf() method only.