java - 为什么 .setImageBitmap() 不将图像分配给 ImageView 对象？

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。

我有一种方法可以读取外部存储空间中的图像。我想将这些图像存储在 ImageView ArrayList 中。为此，我定义了要搜索的文件夹，并创建了一个位图变量，该变量分配了我想要转换为 ImageView 的图像，但是 .setImageBitmap(saveImage)没有分配给我的 Imageview 对象。

代码如下:

 private ArrayList<ImageView> getData() {

        ImageView singleImage = null;
        final ArrayList<ImageView> addImage = new ArrayList<>();
        File retrieveImages = new File(Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_PICTURES), "/WordResolver");

        for(File countImages : retrieveImages.listFiles()) {

            if(countImages.isFile() && (countImages.getName().endsWith(".jpg"))) {

                Bitmap saveImage = BitmapFactory.decodeFile(countImages.getAbsolutePath());
                singleImage.setImageBitmap(saveImage); //Fails here
                addImage.add(singleImage);
            }

        }

        return addImage;
    }

这是图像正在被读取并存储在 BitMap 对象中的证据。

这是抛出 NPE 错误的堆栈跟踪，即使 ImageView 应该分配有图像。

05-20 03:13:38.220 8641-8641/com.example.aaron.wordresolver E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.aaron.wordresolver, PID: 8641 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.aaron.wordresolver/com.example.aaron.wordresolver.PictureGallery}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) at android.app.ActivityThread.-wrap11(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageBitmap(android.graphics.Bitmap)' on a null object reference at com.example.aaron.wordresolver.PictureGallery.getData(PictureGallery.java:78) at com.example.aaron.wordresolver.PictureGallery.onCreate(PictureGallery.java:45) at android.app.Activity.performCreate(Activity.java:6237) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)  at android.app.ActivityThread.-wrap11(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:148)  at android.app.ActivityThread.main(ActivityThread.java:5417)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Answer 1

singleImage 未正确初始化，将其指定为 null 是不够的。

在OnCreate()中初始化singleImage:

singleImage = (ImageView)findViewById(R.id.image);

删除:

ImageView singleImage = null;

并在初始化后运行getData()。