java - 抛出 NullPointerException，但对象已创建并正确返回值

Question

正在开发一个程序，该程序具有保存在文件夹中的形状库和说明文件(描述绘图的细节)。尝试开发 draw() 方法时遇到问题(有来自 main 的调用)。 main 首先创建 9 张图，然后将它们全部绘制出来。当 draw() 被调用时，一个 NullPointerException 被抛出:

DrawingPanel dp = new DrawingPanel(canvasI.getWidth(), canvasI.getHeight())

为什么？如果您需要更多详细信息，请告诉我

private ArrayList<Shape> allShapes;
private ArrayList<DrawInstruction> allDrawInstructions;
private CanvasInstruction canvasI;
private DrawInstruction drawI;

public Drawing(ShapeLibrary shapeLib, File oneShape) throws FileNotFoundException{
    try{
        allDrawInstructions = new ArrayList<DrawInstruction>();
        Scanner in = new Scanner(oneShape);
        CanvasInstruction canvasI = CanvasInstruction.readFromFile(in);
        while(in.hasNextLine()){
            DrawInstruction drawInstruction = DrawInstruction.readFromFile(in);
            allDrawInstructions.add(drawI);
        }
    } catch(FileNotFoundException e){
        throw new FileNotFoundException("File doesn't exist");
    }
}


public void draw(){
    DrawingPanel dp = new DrawingPanel(canvasI.getWidth(), 
    canvasI.getHeight());
}

Answer 1

CanvasInstruction canvasI = CanvasInstruction.readFromFile(in); 行的局部变量隐藏在构造函数中private CanvasInstruction canvasI 字段。更改代码如下:-

public Drawing(ShapeLibrary shapeLib, File oneShape) throws FileNotFoundException {
    //...
    Scanner in = new Scanner(oneShape);
    this.canvasI = CanvasInstruction.readFromFile(in);
    //...
}