Android的ImageView的NullPointerException异常

问题描述

我有两个图像，红灯和绿灯。我有一个自定义的ListView，我想显示红灯时，一个列表项是无效的，而绿灯时，它是积极的。当pressed列表项被激活。

I have two images, a red light and a green light. I have a custom ListView that I would like to display a red light when a list item is inactive, and a green light when it is active. A list item is activated when it is pressed.

下面是我的code

row.xml

<ImageView
    android:id="@+id/iconLight"
    android:src="@drawable/light_off"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"/>

main.java

main.java

ImageView iconLight = (ImageView)findViewById(R.id.iconLight);
iconLight.setImageResource(R.drawable.light_on);

我得到执行设置的图像资源就行了NullPointerException异常。所以，我做了一个小测试，我删除了线路设置的XML文件中的SRC，只是试图将其设置在主类。还是一个NPE。所以，我想在不改变资源，但只是改变了阿尔法。尽管如此NPE。

I get a NullPointerException executing the line that sets the image resource. So I did a little testing, I deleted the line setting the src in the XML file and just tried to set it in the main class. Still a NPE. So I tried not changing the resource, but just changing the alpha. Still NPE.

我不知道我在做什么错。该文件 light_off.png 和 light_on.png 都在 RES /绘，LDPI 和其中任何一个，当我在XML中指定他们的工作。但任何改变我试图做出 iconLight 在主文件导致此NPE。任何想法？

I'm not sure what I'm doing wrong. The files light_off.png and light_on.png are both in res/drawable-ldpi and either of them work when I specify them in the XML. But any change I attempt to make to iconLight in the main file causes this NPE. Any ideas?

推荐答案

要得到就行了NPE的唯一途径...

The only way to get a NPE in the line...

iconLight.setImageResource(R.drawable.light_on);

是 iconLight 的为空。所以，你的findViewById失败。你已经确立了自己的布局，你打电话之前findViewById？你的确定的R.id.iconLight是在活动的根布局？

Is for iconLight to be null. So, your findViewById is failing. Have you set your layout before you call findViewById? Are you sure R.id.iconLight is in the Activity's root layout?

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