Image to binary value - Pixel Value

I need to convert image to a hexadecimal value. When I try to get a color code of specific pixel on uiimage I got the wrong value. In Java I'm using bitmap.getpixel() method to reach pixel color and I'm getting correct value which is 255 but on swift same image pixel returning 0. Do you have any comment on this? or Is there a convenient way to convert image to a hexadecimal?

Java Code Reference

private func createPrintableObject (img:UIImage) -> String{
        let sb = StringBuilder()
        let height:Int = Int(img.size.height)
        let width:Int = Int(img.size.width)
        var index = 0
        var rgb:Int,red:Int,green:Int,blue:Int
        var rgbColor:UIColor
        var auxBinaryChar: [Character] = ["0","0","0","0","0","0","0","0"]
        widthBytes = width / 8
        if(width % 8 > 0){
            widthBytes = ((Int)(width/8))+1
        }else{
            widthBytes = width / 8
        }
        self.total = widthBytes * height
        let pixelData = img.cgImage!.dataProvider!.data
        let data: UnsafePointer<UInt8> = CFDataGetBytePtr(pixelData)

        for h in 0...height-1 {
            for w in 0...width-1 {
                let value = Int(data[((width * w) + h) * 4])
                //value returns in Java "-1" but In Swift returning "0"
                // JAVA -> rgb = bitmapImage.getPixel(w, h); returning "-1"


                red = (value >> 16) & 0x000000FF
                green = (value >> 8) & 0x000000FF
                blue = (value) & 0x000000FF

                let totalColor = red + green + blue
                var auxChar = "1"
                if(totalColor > self.blackLimit){
                    auxChar = "0"
                }
                auxBinaryChar[index] = auxChar[0]
                index+=1
                if(index == 8 || w == (width-1)){
                    sb.append(self.fourByteBinary(binaryStr: String(auxBinaryChar)))
                    auxBinaryChar = ["0","0","0","0","0","0","0","0"]
                    index = 0
                }
            }
            sb.append("\n")
        }

        return sb.toString()

    }



 private func fourByteBinary(binaryStr:String) -> String{
        var decimal:Int = Int(binaryStr,radix: 2) ?? 0
        if(decimal > 15){
            return String(decimal, radix: 16, uppercase: true)
        }else{
           decimal = Int(String(decimal),radix:16) ?? 0
            return "0" + String(decimal, radix: 16, uppercase: true)
        }
    }