为什么我得到这个NullPointerException？ [英] Why am I getting this NullPointerException?

问题描述

我不知道。在使用新的Canvas（）初始化Canvas之后，引用它会导致NullPointerException。文档告诉我这可能发生时，当画布不是启用，但我不知道它是什么意思被启用。我试图调试它包括while（！cvs.isEnabled（））;但程序刚刚挂起。

I can't figure this out. After initializing a Canvas with new Canvas(), referencing it results in a NullPointerException. The doc tells me this can occur when the Canvas is not "enabled", but I don't know what it means to be enabled. I tried to debug it by including while(! cvs.isEnabled()); but the program just hung. What are the conditions that could result in a Canvas not being enabled, and how do I fix them?

Exception in thread "main" java.lang.NullPointerException
    at matt.io.ConsoleCanvas.<init>(ConsoleCanvas.java:72)
    at matt.io.ConsoleCanvas.<init>(ConsoleCanvas.java:51)
    at matt.io.ConsoleCanvas.main(ConsoleCanvas.java:32)

public class ConsoleCanvas extends JFrame
{
    private static final Font DEFAULT_FONT = new Font("Monospaced", Font.PLAIN, 12);
    public static void main(String[] args)
    {
        ConsoleCanvas me = new ConsoleCanvas(); //ConsoleCanvas.java:32
        //Program has crashed by this point, so rest of main removed to be concise
    }

    protected JTextField in;
    private Canvas cvs;
    private int row;
    private int col;

    public ConsoleCanvas()
    {
        this("Console Pane", 80, 10); //ConsoleCanvas.java:51
    }

    public ConsoleCanvas(String title, int rows, int cols)
    {
        in = new JTextField();
        in.setEditable(true);
        in.setFont(DEFAULT_FONT);
        in.setColumns(cols);

        cvs = new Canvas();
        cvs.setSize(in.getWidth(), in.getHeight() * rows);
        cvs.setFont(DEFAULT_FONT);
        row = 0;
        col = cvs.getGraphics().getFontMetrics().getHeight(); //ConsoleCanvas.java:72
        //Program crashes at this line, so I'll leave out the rest for brevity again
        //I've isolated the null to the Graphics returned by cvs.getGraphics()
    }
}

推荐答案

我没有使用swing相当一段时间，但尝试在调用get graphics之前将画布添加到Panel中（面板也应该具有某种形式的布局）。
您可以在这里找到一个有趣的教程： http://docs.oracle.com/ javase / tutorial / uiswing /
为了出现在屏幕上，每个GUI组件都必须是包含层次结构的一部分，包含层次结构是一个以顶层容器为根的组件树我会给你一个。所以我认为实例化canvas类是不够的。图形对象只有在将其添加到顶级容器后才会分配给它。

I didn't work with swing for quite some time, but try adding the canvas to a Panel before calling get graphics (the panel should also have some form of layout). You can find an interesting tutorial here : http://docs.oracle.com/javase/tutorial/uiswing/ "To appear onscreen, every GUI component must be part of a containment hierarchy. A containment hierarchy is a tree of components that has a top-level container as its root. We'll show you one in a bit." So I think that instantiating the canvas class is not enough. The graphics object is assigned to your canvas object only after you add it to a top-level container.

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