super keyword without extends to the super class

There is not need to add super() because it is by default added.

It will call Object class's default constructor because in JAVA every class extends Object by default.

Every class that doesn't explicitly extend another class implicitly extends java.lang.Object. So super() simply calls the no-arg constructor of Object.

Note that this explicit call is unnecessary since the compiler would add it for you. You only need to add a super() call in a constructor when you want to invoke a superclass constructor with arguments.