Android ImageView NullPointerException

发布于 2024-10-06 17:32:58 字数 798 浏览 2 评论 0原文

我有两个图像，一个是红灯，一个是绿灯。我有一个自定义 ListView，我想在列表项处于非活动状态时显示红灯，在列表项处于活动状态时显示绿灯。按下时会激活列表项。

这是我的代码

row.xml

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

main.java

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

执行设置图像资源的行时出现 NullPointerException 。所以我做了一些测试，我删除了 XML 文件中设置 src 的行，并尝试在主类中设置它。仍然是NPE。所以我尝试不改变资源，而只是改变alpha。还是NPE。

我不确定我做错了什么。文件 light_off.png 和 light_on.png 都位于 res/drawable-ldpi 中，当我在 XML 中指定它们时，它们中的任何一个都可以工作。但是我尝试对主文件中的 iconLight 进行的任何更改都会导致此 NPE。有什么想法吗？

原文

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.

Here is my 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

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

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.

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?

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非要怀念 2024-10-13 17:32:58

在该行中获取 NPE 的唯一方法...

iconLight.setImageResource(R.drawable.light_on);

是将 iconLight 设置为 null。所以，你的 findViewById 失败了。您在调用 findViewById 之前设置了布局吗？您确定 R.id.iconLight 位于 Activity 的根布局中吗？

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

iconLight.setImageResource(R.drawable.light_on);

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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戏蝶舞 2024-10-13 17:32:58

我也有同样的问题。这是帮助我理解的代码。它适用于对话框窗口，但也可能对您有帮助。

    final Dialog dialog = new Dialog(context);
    dialog.setContentView(R.layout.custom);
    dialog.setTitle("Title...");
    TextView text = (TextView) dialog.findViewById(R.id.text);
    text.setText("Android");
    ImageView image = (ImageView) dialog.findViewById(R.id.image);
    image.setImageResource(R.drawable.ic_launcher);

观察最后一行之前的线路。注意它是如何实例化 ImageView 的。
无论如何，对图像的每次更改都是在 setContentView 之后进行的。

I had the same problem. Here is a code that helped me understand. It is for a Dialog window but might help you too.

    final Dialog dialog = new Dialog(context);
    dialog.setContentView(R.layout.custom);
    dialog.setTitle("Title...");
    TextView text = (TextView) dialog.findViewById(R.id.text);
    text.setText("Android");
    ImageView image = (ImageView) dialog.findViewById(R.id.image);
    image.setImageResource(R.drawable.ic_launcher);

Watch the line before the last one. Notice how it instantiates the ImageView.
Anyway every change to the image is made after the setContentView.

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