whenever assigning an object to a String variable - isn't the 'toString'
method called automatically ?
- object has the toString method - so any object can be assigned to a
String.

In general, calling the toString() method is the best way:

 String myString = myobj.toString();

If the object is actually a String, toString() will return itself.

I have a feeling this is not the answer you're looking for. In that case, please provide
more information, such as the code you wish to use.

   You might be asking for this:

    String string = myobj.toString();

... or you might be asking for this:

    String string = null;
    if (myobj instanceof String)
       string = (String)myobj;

... or you might be asking for something else I'm unable
to imagine -- if so, please explain more fully.

Every class inherits the toString() method from Object. So you can assign a
representation of any object to a String by:

String str = anyObject.toString();

However, unless the class overrides the toString() method to return
something useful all you will get is the class name and a string
representation of the objects hash code - exactly what you would get if you
executed System.out.println(anyObject) which uses toString() implicitly.