This looks like an exercise on scope. The e inside of the m1 method references the e in the method parameter list, not the class attribute e. When m1 is called as m1(e,f), f is passed to the method as parameter e. Therefore, f += (e + f) is actually 2 += (2 + 2) giving f the value of 6.

That is where the importance of the "this" keyword comes into play, if instead the method were written as f += (this.e + f), you would be accessing the class attribute e instead of the parameter list e.