java - 安卓/Java : failure passing Canvas as a class constructor parameter

Question

在 GraphicsView 类的 onDraw 方法中，我创建了 Plotter 类的实例，但构造函数未设置类内 Canvas 字段

我们从这里开始

protected void onDraw(Canvas canvas) {
    Plotter plotter = new Plotter(1, canvas);
}

..然后转到 Plotter 创建它的实例:

Plotter (int dotsPerPx, Canvas canvas_) {
    this.dotsPerPx = dotsPerPx;
    this.canvas = canvas_;
}

int dotsPerPx;  //This one gets set to 1, as supposed
Canvas canvas;  //And this remains null, though canvas_==openGLCanvas{...}
int screenWidth = canvas.getWidth();  //NullPointerException!

出了什么问题？

Answer 1

Canvas 尚未实例化。使用这个:

Plotter (int dotsPerPx, Canvas canvas_) {
    this.dotsPerPx = dotsPerPx;
    this.canvas = canvas_;
    screenWidth = canvas.getWidth();
}

int dotsPerPx;  //This one sets to 1, as supposed
Canvas canvas;  //And this remains null, though canvas_==openGLCanvas...
int screenWidth;

基本上，构造函数和方法之外的所有代码(字段创建、初始化 block )都会在调用构造函数之前执行。因此， int screenWidth = canvas.getWidth(); 尝试调用 null 字段上的方法(默认情况下，除非您指定其他内容)，因为尚未调用构造函数。