java - ListView : getHeight() on a null object reference

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

我有一个子类 ListView我有 onDraw()我尝试动态获取高度的方法。但我得到了 NullPointerException .

错误:

Fatal Exception: java.lang.NullPointerException
       at in.justickets.android.view.ExpandedListView.onDraw(ExpandedListView.java:25)
       at android.view.View.draw(View.java:14493)
       at android.widget.AbsListView.draw(AbsListView.java:3869)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3077)
       at android.view.View.getDisplayList(View.java:13328)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.ViewGroup.dispatchGetDisplayList(ViewGroup.java:3077)
       at android.view.View.getDisplayList(View.java:13328)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.widget.AbsListView.dispatchDraw(AbsListView.java:2467)
       at android.view.View.draw(View.java:14496)
       at android.widget.AbsListView.draw(AbsListView.java:3869)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.getDisplayList(View.java:13385)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.draw(View.java:14496)
       at android.support.v4.view.ViewPager.draw(ViewPager.java:2336)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2947)
       at android.view.View.draw(View.java:14496)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.getDisplayList(View.java:13385)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.draw(View.java:14496)
       at android.widget.FrameLayout.draw(FrameLayout.java:472)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.getDisplayList(View.java:13385)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.getDisplayList(View.java:13385)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.getDisplayList(View.java:13385)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.View.draw(View.java:14210)
       at android.view.ViewGroup.drawChild(ViewGroup.java:3103)
       at android.view.ViewGroup.dispatchDraw(ViewGroup.java:2940)
       at android.view.View.draw(View.java:14496)
       at android.widget.FrameLayout.draw(FrameLayout.java:472)
       at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:2326)
       at android.view.View.getDisplayList(View.java:13390)
       at android.view.View.getDisplayList(View.java:13432)
       at android.view.HardwareRenderer$GlRenderer.buildDisplayList(HardwareRenderer.java:1577)
       at android.view.HardwareRenderer$GlRenderer.draw(HardwareRenderer.java:1451)
       at android.view.ViewRootImpl.draw(ViewRootImpl.java:2420)
       at android.view.ViewRootImpl.performDraw(ViewRootImpl.java:2292)
       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1922)
       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1039)
       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5648)
       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
       at android.view.Choreographer.doCallbacks(Choreographer.java:574)
       at android.view.Choreographer.doFrame(Choreographer.java:544)
       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
       at android.os.Handler.handleCallback(Handler.java:733)
       at android.os.Handler.dispatchMessage(Handler.java:95)
       at android.os.Looper.loop(Looper.java:136)
       at android.app.ActivityThread.main(ActivityThread.java:5111)
       at java.lang.reflect.Method.invokeNative(Method.java)
       at java.lang.reflect.Method.invoke(Method.java:515)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:806)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:622)
       at dalvik.system.NativeStart.main(NativeStart.java)

代码:

public class SubListView extends ListView {

    private int count = 0;

    public SubListView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if (getCount() != count) {
            count = getCount();
            android.view.ViewGroup.LayoutParams params = getLayoutParams();
            if(params!=null) {
                params.height = getCount() * (count > 0 ? getChildAt(0).getHeight() : 0);
                setLayoutParams(params);

            }
        }

        super.onDraw(canvas);
    }

}

Answer 1

问题是调用 getHeight() 时，您的 View 尚未绘制。

您应该在此之前调用 super.onDraw():

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    if (getCount() != count) {
        count = getCount();
        android.view.ViewGroup.LayoutParams params = getLayoutParams();
        if(params!=null) {
            params.height = getCount() * (count > 0 ? getChildAt(0).getHeight() : 0);
            setLayoutParams(params);
        }
    }
}