Question

以下是代码：

<div class="test">
        <div class="inner">
            <div class="pure-g">
                <div class="pure-u-1-4">
                    <div class="spec__test">123
                    </div>
                </div>
                <div class="pure-u-1-4">
                    <div class="spec__test">123
                    </div>
                </div>
                <div class="pure-u-1-4">
                    <div class="spec__test">123
                    </div>
                </div>
                <div class="pure-u-1-4">
                    <div class="spec__test">123
                    </div>
                </div>
            </div>
        </div>
    </div>

可以预期package scalafx.ensemble.example.charts import javafx.scene.chart.PieChart.Data import scala.util.Random import scalafx.application.JFXApp import scalafx.scene.Scene import scalafx.Includes._ import scalafx.collections.ObservableBuffer import scalafx.scene.chart.PieChart import scalafx.scene.input.MouseEvent object BasicPie extends JFXApp { // data level 1 val pieChartDataBuffer: ObservableBuffer[Data] = ObservableBuffer( PieChart.Data("A", 20), PieChart.Data("B", 30), PieChart.Data("C", 10), PieChart.Data("D", 40) ) pieChartDataBuffer.foreach( (data: Data) => data.node().onMouseClicked = { (_: MouseEvent) => pieChart.data = pieChartDataSubBuffer(data.name()) } ) val pieChart = new PieChart { data = pieChartDataBuffer title = "DrillDown Pie Chart" } // data level 2 val pieChartDataSubBuffer: Map[String, ObservableBuffer[Data]] = _ Array("A", "B", "C", "D").foreach( (letter: String) => { val subData: ObservableBuffer[Data] = _ (1 to 10).foreach( i => { val subName = letter + "-" + i subData.add(PieChart.Data(subName, Random.nextInt(100))) } ) pieChartDataSubBuffer.updated(letter, subData) } ) pieChartDataSubBuffer.foreach( (nameDataBufferTuple) => { nameDataBufferTuple._2.foreach( (data: Data) => { data.node().onMouseClicked = { (_: MouseEvent) => pieChart.data = pieChartDataBuffer } } ) } ) // set listener for one data node def drillDownData = (pieData: Data, pie: PieChart) => { pieData.node().onMouseClicked = (_: MouseEvent) => pie.data = pieChartDataSubBuffer(pieData.name()) } stage = new JFXApp.PrimaryStage { title = "Drilldown Pie Chart Example" scene = new Scene { root = pieChart } } }会使用subData进行初始化，但我会收到错误：null。为什么呢？

Answer 1

您无法创建未初始化的val，因为之后您永远无法为其分配任何有用的内容。

说你明确说明：

val subData: ObservableBuffer[Data] = null

subData = ObservableBuffer.empty -> error: reassignment to val!
subData.add(data) -> NullPointerException

您可以将其初始化为var

var subData: ObservableBuffer[Data] = _

或（特别是在可变集合的情况下）为空缓冲区

val subData: ObservableBuffer[Data] = ObservableBuffer.empty // comes to mind

您的代码会建议您执行后者。

Answer 2

仅供记录，以下是编译的修改版本：

package scalafx.ensemble.example.charts

import javafx.scene.chart.PieChart.Data

import scala.util.Random
import scalafx.application.JFXApp
import scalafx.scene.Scene
import scalafx.Includes._
import scalafx.collections.ObservableBuffer
import scalafx.scene.chart.PieChart
import scalafx.scene.input.MouseEvent
import scala.collection.mutable.{HashMap => MMap}

object BasicPie extends JFXApp {

  stage = new JFXApp.PrimaryStage {
    title = "Drilldown Pie Chart Example"
    scene = new Scene {
      root = {
        val pieChartDataBuffer = ObservableBuffer(
          PieChart.Data("A", 20),
          PieChart.Data("B", 30),
          PieChart.Data("C", 10),
          PieChart.Data("D", 40)
        )
        val pieChart = new PieChart {
          data = pieChartDataBuffer
          title = "DrillDown Pie Chart"
        }

        def subPieChartData: MMap[String, ObservableBuffer[Data]] = {
          val subDataMap: MMap[String, ObservableBuffer[Data]] = MMap.empty
          for(letter <- Array("A", "B", "C", "D")) {
            val dataBuffer: ObservableBuffer[Data] = ObservableBuffer.empty
            for(i <- 1 to 10)  {
              val subLabel = letter + i
              dataBuffer.add(PieChart.Data(subLabel, Random.nextInt(100)))
            }
            subDataMap.update(letter, dataBuffer)
          }
          subDataMap
        }

        def drillDownData = (pie: PieChart, pieData: PieChart.Data) => {
          val labelPrefix = pieData.name()
          val subData = subPieChartData(pieData.name())
          pieData.node().onMouseClicked = (_: MouseEvent) => pie.data = subData
          subData.foreach(
            (data) => climbUpData(pie, data)
          )
        }

        def climbUpData = (pie: PieChart, pieData: PieChart.Data) => {
          val node = pieData.node()
          if(node != null) {
            node.onMouseClicked = (_: MouseEvent) => pie.data = pieChartDataBuffer
          }
        }

        pieChartDataBuffer.foreach((data: Data) => drillDownData(pieChart, data))

        pieChart
      }
    }
  }

}

使用默认值初始化时的未绑定占位符参数

2 个答案: