Android：资产中带有位图的ArrayList

Question

I've been trying to do a list out of an images from the assets directory, and this is my code

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();
    }

But when ever this code is executed, my app crashes. What would be the problem?

logcat of the app :

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

I would say you get a NullPointerException here:

list.add(i, bitmap);

you dont create the ArrayList :

ArrayList<Bitmap> list;

try it with:

ArrayList<Bitmap> list = new ArrayList<>;

Answer 2

您需要先初始化阵列列表

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