在类中绘制，扩展了javafx.scene.canvas.Canvas

Question

我正在尝试扩展java fx Canvas：

package com.kavricious.math;

import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.paint.Color;

import java.util.Random;

public class LPlotCanvas extends Canvas{
  boolean grid = false;
  boolean axes = true;
  boolean ticks = true;
  boolean startDrawn = false;
  double xMin;
  double xMax;
  double yMin;
  double yMax;
  int density;
  short canvasWidth = 800;
  short canvasHeight = 800;
  byte colorIndex = 0;
  private GraphicsContext gc = null;

  public LPlotCanvas(double xmin,double xmax, double ymin, double ymax){
    xMin = xmin;
    xMax = xmax;
    yMin = ymin;
    yMax = ymax;
    density = 100;
    setWidth(canvasWidth);
    setHeight(canvasHeight);
    gc = getGraphicsContext2D();
    drawStart();
  }
  private void drawStart(){
    System.out.println("drawing start");
    if(gc == null)System.out.println("it null");
    gc.setLineWidth(2);
    gc.setStroke(Color.BLACK);
    if(axes){
      if(xMin<0.0&&xMax>0.0){
        drawLine(gc,0,yMin-1,0,yMax+1);
      }
      if(yMin<0.0&&yMax>0.0){
        drawLine(gc,xMin-1,0,xMax+1,0);
      }
    }
    if(grid){
      for(int i =(int)((xMin>0)?Math.floor(xMin):Math.ceil(xMin));i<xMax+1;i++){
        gc.setLineWidth(0.25);
        drawLine(gc,i,yMin,i,yMax);
      }
      for(int i =(int)((yMin>0)?Math.floor(yMin):Math.ceil(yMin));i<yMax+1;i++){
        gc.setLineWidth(0.25);
        drawLine(gc,xMin,i,xMax,i);
      }
    }
    startDrawn = true;
  }
  public void showGrid(boolean b){
    grid = b;
  }
  public void showAxes(boolean b){
    axes = b;
  }
  public void showTicks(boolean b){
    ticks = b;
  }
  public void setCanvasWidth(short width){
    this.canvasWidth = width;
  }
  public void setCanvasHeight(short height){
    this.canvasHeight = height;
  }
  private void plotPoint(double x,double y,Color color){
    if(!startDrawn)drawStart();
    gc.setFill(color);
    gc.fillOval(convertX(x),convertY(y),5,5);
  }
  public void drawPolynomial(LPolynomial function)throws Exception{
    if(!startDrawn)drawStart();
    gc.setStroke(getNextColor(colorIndex));
    double start = xMin;
    double step = (xMax-xMin)/density;
    for(int i=0;i<density;i++){
      drawLine(gc,start+step*i,function.evaluateAt(start+step*i),start+step*(i+1),function.evaluateAt(start+step*(i+1)));
    }
    colorIndex++;
  }
  private void drawLine(GraphicsContext gc,double x1,double y1,double x2, double y2){
    int nx1 = convertX(x1);
    int ny1 = convertY(y1);
    int nx2 = convertX(x2);
    int ny2 = convertY(y2);
    gc.strokeLine(nx1,ny1,nx2,ny2);
    System.out.println("line drawn");
  }
  private int convertX(double in){
    double widthOfPixel = (xMax-xMin)/canvasWidth;//how much space on the real number line is put into one pixel
    return (int) Math.floor((in-xMin)/widthOfPixel);//how much greater than left side DIVIDED by pixel size -> how many pixels away from left side
  }
  private int convertY(double in){
    double heightOfPixel = (yMax-yMin)/canvasHeight;
    return canvasHeight - (int) Math.floor((in-yMin)/heightOfPixel);
  }
  public Color getNextColor(byte i){
    switch(i){
      case 0: return Color.rgb(255,0,0);//red
      case 1: return Color.rgb(0,200,0);//lime
      case 2: return Color.rgb(0,0,235);//blue
      case 3: return Color.rgb(255,255,0);//yellow
      case 4: return Color.rgb(255,50,255);//magenta/fuchsia
      case 5: return Color.rgb(255,165,0);//orange
      case 6: return Color.rgb(0,235,235);
      case 7: return Color.rgb(107,30,119);
      case 8: return Color.rgb(0,100,0);
      default: Random rand = new Random();
              int r = rand.nextInt(256);
              int g = rand.nextInt(256);
              int b = rand.nextInt(256);
              return Color.rgb(r,g,b);
    }
  }
}

然后在另一个应用程序中调用它：

package com.kavricious.math;

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;


import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.paint.Color;
import javafx.scene.shape.ArcType;

import java.util.ArrayList;

public class LPlotter2 extends Application{
  private ArrayList<LPolynomial> functions;
  public static void main(String[] args){
    launch(args);
  }
  @Override
  public void start(Stage ps){
    this.functions = new ArrayList<LPolynomial>();
    LPolynomial[] funcs = new LPolynomial[]{new LPolynomial(2.3,3.4,2.2,1.1),new LPolynomial(0,1,-2.3),new LPolynomial(1,2),new LPolynomial(2,2,2.3,1,0.4)};
    for(LPolynomial f: funcs){
      this.functions.add(f);
    }



    ps.setTitle("LPlotter");
    StackPane layout = new StackPane();
    LPlotCanvas canv = new LPlotCanvas(-10,10,-10,10);



    for(LPolynomial function : functions){
      try{
        canv.drawPolynomial(function);
      }catch(Exception e){System.out.println("an exception wad caught");e.printStackTrace();}
    }




    layout.getChildren().add(canv);

    ps.setScene(new Scene(layout,800,800));

    ps.show();


  }
}

（在上面的代码中，我尝试在初始化LPlotCanvas之后使用for循环在每个位置绘制多项式，并且会出现相同的结果。）

然后我运行它，什么也看不见： 它应该是这样的（没有扩展名创建）。 那有什么不对？