Extreme Java When vanilla Java is not enough

18Sep/090

Copying html attributes on Lift bindings

Since Java and Scala are complementary, I will post Scala-related stuff here, too (without creating a new blog).

Today's tip refers to Lift (Scala's JavaEE/Rails/Grails/Django). Suppose your webdesigner gives you this XHTML:

<input type="text" class="x" style="width: 100px;"/>

When you bind this using Lift, one of possible solutions is surround it with a tag:

<mybind:myfield><input type="text" class="x" style="width: 100px;"/></mybind:myfield>

Please notice I put it without spaces or newlines between "mybind:myfield" and "input" - this will be important later.

I prefer this way because this XHTML can be opened in other applications, like Firefox and Dreamweaver, making life easier for the webdesigner.

Then, if I bind it using old mama's recipe:

bind("mybind", form, "myfield" -> myfield.toForm)

Lift will remove my "input" tag and replace it - destroying "class" and "style". To solve this, I merge "toForm" tag with original attributes:

def merge(form : => Box[NodeSeq])(input : NodeSeq) : NodeSeq = {
  var in = input.first
  var attrs = form.open_!.first.attributes
 
  new Elem(in.prefix, in.label,
      in.attributes append attrs,
      in.scope, Group(in.child))
}
bind("mybind", form,
    FuncBindParam("myfield", merge(myfield.toForm)))

Function "merge" takes the form and returns a function that receives the original XHTML and translates it into XHTML with Lift's attributes ("id", "name", "lift:gc", etc). I guess I can improve it somehow, but works great. And, since I'm keeping only the first child of source XHTML, you need to keep "bind" and "input" together (as I said before).

I know I can use a map function to ignore whitespaces, but I'll leave it as an later exercise (for you and for me, too).

16Sep/090

Mandelbrot with Scala and Java

I'm testing if Scala can be used to increase productivity here. Pretty nice language: as powerful as Ruby and JVM-compatible. To improve my skills, I decided to implement Mandelbrot on Scala, using Swing's BufferedImage to show the image. Very simple, indeed. The code is:

import java.awt.image.BufferedImage
import javax.swing.ImageIcon
import javax.swing.JFrame
import javax.swing.JLabel
object Mandelbrot extends Application {
  case class Complex(r : Double, i : Double) {
    def +(b : Complex) = Complex(r + b.r, i + b.i)
    def *(b : Complex) = Complex(r * b.r - i * b.i, r * b.i + i * b.r)
    def insideM = (r * r + i * i) < (2 * 2)
  }
 
  implicit def start = Complex(0, 0)
 
  def pc(z : Complex, c : Complex) : Complex = z * z + c
 
  def iter(qtd : Int, c : Complex)(implicit z : Complex) : Int = {
    if (qtd == 0) 0
    else if (!z.insideM) qtd
    else iter(qtd - 1, c)(pc(z, c))
  }
 
  val scale = 1.0
  def pixToComplex(x : Double, y : Double) =
    Complex(((x / 640.0) * 3.0 - 2.0) / scale,
        ((y / 480.0) * 2.0 - 1.0) / scale)
 
  def raster(f : (Int, Int, Int) => Unit) = {
    for (y <- 0 until 480; x <- 0 until 640) {
      f(x, y, iter(1024, pixToComplex(x, y)))
    }
  }
 
  def qtdToColor(c : Int) = List(c / 4, c / 4, c / 4).toArray
 
  val frame = new JFrame("Mandelbrot")
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
 
  val lbl = new JLabel
  frame.add(lbl)
  frame.setSize(640, 480)
  frame.setVisible(true)
 
  val img = new BufferedImage(640, 480, BufferedImage.TYPE_3BYTE_BGR)
  raster((x, y, c) => img.getRaster.setPixel(x, y, qtdToColor(c)))
  lbl.setIcon(new ImageIcon(img))
}

Since I'm new to Scala, I guess it's not the best solution. Nevertheless, I have an "Yes-we-can" feeling, because this code have some Scala features, like higher-order functions and (possibly) tail recursion. And, of course, I felt productive implementing Mandelbrot's recursive algorithm (I got a lot of errors misunderstanding the algorithm, BTW).

16Sep/090

That's extreme!

And, talking about being extreme, I found a mix of bytecode programming with ASCII art...

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