How an object will call toString method implicitly?

You're not explicitly calling toString(), but implicitly you are:

See:

System.out.println(foo); // foo is a non primitive variable

System is a class, with a static field out, of type PrintStream. So you're calling the println(Object) method of a PrintStream.

It is implemented like this:

   public void println(Object x) {
       String s = String.valueOf(x);
       synchronized (this) {
           print(s);
           newLine();
       }
   }

As we see, it's calling the String.valueOf(Object) method.
This is implemented as follows:

   public static String valueOf(Object obj) {
       return (obj == null) ? "null" : obj.toString();
   }

And here you see, that toString() is called.

Every object in Java IS-A(n) Object as well. Hence, if a toString() implementation has not been provided by a class the default Object.toString() gets invoked automatically.

Object.toString()'s default implementation simply prints the object's class name followed by the object's hash code which isn't very helpful. So, one should usually override toString() to provide a more meaningful String representation of an object's runtime state.

even I am not writing the toString() method so what are the implementation,how it is calling toString() internally?

toString() is one of the few methods (like equals(), hashCode() etc.) that gets called implicitly under certain programmatic situations like (just naming a few)

• printing an object using println()
• printing a Collection of objects (toString() is invoked on all the elements)
• concatenation with a String (like strObj = "My obj as string is " + myObj;)

Everything inherits from Object, so the toString on Object will be called if you have not defined one.