Question

我在数据库sqlite中获取字符串theme_name，然后使用主题名称在库中设置图像。但是我的nullpointer异常有一些错误。你能告诉我为什么会这样吗？我该怎么办？谢谢

这是我的代码

 public class MybookActivity_Gallery extends Activity{

private BooksDB db;
private Context context;
private HashMap< String, ArrayList<String>> hm;
private ImageView imgOldSelected;
private int resID;
private ImageView img_shelf;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);

    setContentView(R.layout.layout_mybookactivity_gallery);
    context = this;
    db = new BooksDB(context);
    ArrayList<String> list_bokID = new ArrayList<String>();
    ArrayList<String> list_title = new ArrayList<String>();
    ArrayList<String> list_theme = new ArrayList<String>();
    hm = new HashMap<String, ArrayList<String>>();

    //get Theme_name
    hm = db.getBookTheme();
    if(hm.isEmpty() == true){
        System.out.println("not have data");
    }else{
        //prepare theme, title, book_id 
        list_theme = hm.get("theme");
        list_title = hm.get("title");
        list_bokID = hm.get("bokID");
    }

    LinearLayout gallery = (LinearLayout)findViewById(R.id.images_gallery);
    //image fullsize 
    final ImageView imgFullSize = (ImageView)findViewById(R.id.full_size_image);

    for (String themeName : list_theme) {
        //prepare image in layout gallery 
        ImageView img_shelf = new ImageView(this);
        img_shelf.setLayoutParams(new LinearLayout.LayoutParams(150, 150));
        img_shelf.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
        img_shelf.setPadding(8, 8, 8, 8);
        //set image resource
        resID = getResources().getIdentifier(themeName, "drawable", getPackageName());
        img_shelf.setImageResource(resID);


    }

    // img_shelf set it in on click
                img_shelf.setOnClickListener(new View.OnClickListener() {

                    @Override
                    public void onClick(View v) {

                        ImageView selectedImage = (ImageView) v;
                        Bitmap bitmap = ((BitmapDrawable) selectedImage.getDrawable()).getBitmap();

                        if(imgOldSelected != null){
                            imgOldSelected.setBackgroundColor(Color.TRANSPARENT);
                        }
                        selectedImage.setBackgroundColor(Color.LTGRAY);

                    }
                });

                gallery.addView(img_shelf);
}

}

这是我的logcat错误

 02-21 16:12:02.421: E/AndroidRuntime(29116): FATAL EXCEPTION: main
 02-21 16:12:02.421: E/AndroidRuntime(29116): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.splash.bookguk_project/category_viewbook_sub2_1.MybookActivity_Gallery}: java.lang.NullPointerException
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread.access$700(ActivityThread.java:140)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.os.Handler.dispatchMessage(Handler.java:99)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.os.Looper.loop(Looper.java:137)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread.main(ActivityThread.java:4921)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at java.lang.reflect.Method.invokeNative(Native Method)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at java.lang.reflect.Method.invoke(Method.java:511)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1038)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:805)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at dalvik.system.NativeStart.main(Native Method)
 02-21 16:12:02.421: E/AndroidRuntime(29116): Caused by: java.lang.NullPointerException
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at category_viewbook_sub2_1.MybookActivity_Gallery.onCreate(MybookActivity_Gallery.java:73)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.Activity.performCreate(Activity.java:5206)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)
 02-21 16:12:02.421: E/AndroidRuntime(29116):   ... 11 more

谢谢

Answer 1

你有这个ImageView delcared一个初始化的内部循环

   for (String themeName : list_theme) {
    //prepare image in layout gallery 
    ImageView img_shelf = new ImageView(this);
    //this iamgeview is not added to gridview or to the layout
   }

所以你有一个NullPointerException @

   img_shelf.setOnClickListener(new View.OnClickListener() { // outside for loop

coz

   private ImageView img_shelf;  // is declared not initialized
   @Override
   protected void onCreate(Bundle savedInstanceState) {

在for循环中声明和初始化的ImageView也不会添加到gridview或activity中。

Answer 2

您在类上定义了img_shelf变量，然后在循环中再次重新定义它（因此它仅在循环范围内实例化）：

for (String themeName : list_theme) {
    //prepare image in layout gallery 
    ImageView img_shelf = new ImageView(this);
    ...
}

这不会按预期工作。

我不确定您是如何使用它的，因为您似乎创建了多个ImageView，但随后将OnClickListener仅附加到其中一个OnClickListener。您需要将ImageView的代码移动到循环中，或者在类上创建一个{{1}}的变量列表。

带图像库的Nullpointer异常

2 个答案: