Android：带有来自资产的位图的 ArrayList

Question

我一直在尝试从资产目录中的图像中列出一个列表， 这是我的代码

ArrayList<Bitmap> list = new ArrayList<>();

    try{
        String[] PathFiles = assetManager.list("img");
        for (int i = 0; i < PathFiles.length; i++) {
            InputStream is = assetManager.open("img/" + PathFiles[i]);
            Bitmap bitmap = BitmapFactory.decodeStream(is);
            list.add(i, bitmap);
        }
    }
    catch(IOException e)
    {
        e.printStackTrace();
    }

但是，一旦执行此代码，我的应用就会崩溃。 会有什么问题？

应用程序的logcat：

10-29 18:19:06.712  18272-18272/com.example.mohammed.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.mohammed.myapplication, PID: 18272
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mohammed.myapplication/com.example.mohammed.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String[] android.content.res.AssetManager.list(java.lang.String)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2790)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2855)
            at android.app.ActivityThread.access$900(ActivityThread.java:181)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1474)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:6117)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String[] android.content.res.AssetManager.list(java.lang.String)' on a null object reference
            at com.example.mohammed.myapplication.MainActivity.onCreate(MainActivity.java:49)
            at android.app.Activity.performCreate(Activity.java:6374)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2743)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2855)
            at android.app.ActivityThread.access$900(ActivityThread.java:181)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1474)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:6117)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

Answer 1

我会说你在这里得到一个NullPointerException：

list.add(i, bitmap);

你没有创建ArrayList:

ArrayList<Bitmap> list;

尝试一下：

ArrayList<Bitmap> list = new ArrayList<>;

Answer 2

你需要先初始化你的数组列表

ArrayList<Bitmap> list=new ArrayList<>();