将可变 java 类转换为不可变 scala 类

Question

我有一个 class 功能类似这样的

class OutputFile(name: String, index: Int = 0){
  val localFile = File.createTempFile(name, ".gz")
  localFile.deleteOnExit()
  val localFileOut =new FileOutputStream(localFile)
  private[this] var bytesWritted: Int = 0
  def write(line: Bytes): OutputFile = {
    if(bytesWritten > SOMESIZE) flush //this function uploads the file to a different location and closes the localfile and localFileOut
    try{
      writeLine(line) //this writes the line to the localfile
      this
    }catch{
      //If the file had been closed by flush create a new object
      case IOException => 
        val outFile = new OutputFile(name, index+1)
        outfile.write(line)
        outfile
    }
    //Other functions like flush and writeLine
  }

但是现在我不能将此对象与不可变对象一起使用。来自 java 背景的我很难将此 class 转换为不可变样式。在 java 代码中，我们可以只为输出流使用一个全局变量，并在需要时更改它。

是否有更好的方法来实现这种情况。

Answer 1

为了使 OutputFile 的工作流程不可变，每次调用都必须 return 一个新的不可变实例，而不仅仅是交换输出文件的调用：

object OutputFile {
  def apply(name: String): OutputFile = {
    new OutputFile(name, newStream(name), 0)
  }

  private def newStream(name: String): FileOutputStream = {
    val localFile = File.createTempFile(name, ".gz")
    localFile.deleteOnExit()
    new FileOutputStream(localFile)
  }
}

class OutputFile(name: String, stream: FileOutputStream, bytesWritten: Int) {

  val THRESHOLD = 1000000

  def write(line: Bytes): OutputFile = write(line, 1)

  private def write(line: Bytes, attempt: Int): OutputFile = {
    try {
      val written = writeLine(line) //this writes the line to the localfile
      if (written > THRESHOLD) {
        flush
        new OutputFile(name, OutputFile.newStream(name), 0)
      } else new OutputFile(name, stream, written)
    } catch {
      case x: IOException =>
        if (attempt > 3) throw x
        else write(line, attempt + 1)
    }
  }

  // returns bytesWritten + length of line
  private def writeLine(line: Bytes): Int = ???

  // uploads file, closes and deletes it
  private def flush: Unit = ???
}

我添加了一个伴生对象 OutputFile，这样实际的构造函数就是新的不可变实例的构造函数，并抽象出新流的打开。

每次写入调用都会创建一个新的 OutputFile 并跟踪已写入当前文件的字节数。达到 THRESHOLD 后，将刷新文件并 returned 具有新流的新实例。 IOException 不再负责触发新文件（当我们知道我们已经刷新时才完成），而是重试最多 3 次尝试。

最后一句警告：这个 class 本质上仍然是有状态的，因为它确实处理文件 I/O。虽然这试图假装没有状态，但它假设 write 永远不会 在单个实例上被调用两次。

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