When you have this(i), you are calling the constructor in the same class with the integer argument. That's why you get two 10 in the output. The first 10 is generated by the over2(int i), the second is self explanetary.
However I want to call you attention when overriding constructor in the subclass. If you comment out the this(i) line, you will get a compiling error. That is because that
Java invokes super() implicitly if you do not do so explicitly in the first
line of the subclass's overidden constructor. That's why you see super(i) in the first constructor. However there is one exception. If the first line of a constructor C1, uses the this() syntax to invoke another constructor C2, of the class, Java relies on C2 to invoke the superclass Constructor,
and does not insert a call to super() into C1.Reference. Therefore, it's always recommended to explicitly have your default(no argument)constructor ready in the superclass.
Hope this helps.
Tom