Yes, what is happening is that the variable "x" you're passing to the constructor is hiding the class property "x" (not global variable, but that's not important.) So x=x is just assigning the value of the variable "x" to itself, the class property "x" is not modified.
Think about it, x=x. How would the compiler know that those two x's are different and which is which?
You don't need underscore but you do need to clarify things: use this.x=x in the constructor. Then you're telling the compiler that the first x is a property of the object "this.x" and there's no ambiguity.

Cheers,

F

I find underscores ugly!  
And at the same time, I prefer to avoid the verbose of keyword this. 

In Java you can group variables into these 2 types -> field & local.

Field variables retain its stored value within its class.
While local ones are completely obliterated after they lose scope, that is,
after crossing the curly braces in which they were declared.

One curious and dangerous aspect of it is that if a local variable is declared within a scope,
and it happens to have the same name as a field variable, the local one overshadows the field one.

It means that, from that on, until the end of that scope, when you use that variable name,
it refers to the local one instead.
However, the field one can still be referred to by using keyword this . as its prefix.

As you can notice, the field one doesn't lose its stored value.
So no worries about a local one erasing a field one!  

Now, for aesthetic reasons, instead of _x, use something like xx:  

Constructor( int xx) {
      x = xx;
}

P.S.: Variables declared as method/constructor parameters are local ones as well. For the whole method!

" Now, for aesthetic reasons, instead of _x, use something like xx"
And what if the parameter is called coordX or dimensionOfButton instead?

Actually, code tends to be a lot clearer when naming variables the same as object fields and judicious use of this. I've written 100k+ lines of code and haven't tired of this yet. But I am fed up with aa, bb, xx, rr as variables ;)

Every1 is entitled to their own opinions!  
But for me, I don't see the need of this to refer to variables belonging to current class;
since it's very clear they're either field or local.
And if it's a variable from another class, we have to prefix them w/ that class' name anyways!
Also, I would avoid making local variables have the same name as a field one!!!