Android 当用户从库中选择背景时出现NullPointerException

你好，我想允许用户在我的应用程序中更改背景。试着去做，但要面对这个问题：

12-09 06:22:15.874 20413-20413/com.vadimsleonovs.horoscope E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.vadimsleonovs.horoscope, PID: 20413
java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=1, result=-1, data=Intent { dat=content://media/external/images/media/20 flg=0x1 }} to activity {com.vadimsleonovs.horoscope/com.vadimsleonovs.horoscope.activities.SettingsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.setBackground(android.graphics.drawable.Drawable)' on a null object reference
at android.app.ActivityThread.deliverResults(ActivityThread.java:3539)
at android.app.ActivityThread.handleSendResult(ActivityThread.java:3582)
at android.app.ActivityThread.access$1300(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1327)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.setBackground(android.graphics.drawable.Drawable)' on a null object reference
at com.vadimsleonovs.horoscope.activities.SettingsActivity.onActivityResult(SettingsActivity.java:76)
at android.app.Activity.dispatchActivityResult(Activity.java:6135)
at android.app.ActivityThread.deliverResults(ActivityThread.java:3535)
at android.app.ActivityThread.handleSendResult(ActivityThread.java:3582) 
at android.app.ActivityThread.access$1300(ActivityThread.java:144) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1327) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:135) 
at android.app.ActivityThread.main(ActivityThread.java:5221) 
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:899) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

守则是：

public class SettingsActivity extends AppCompatActivity {

    Button mBcgBtn;
    private static int RESULT_LOAD_IMAGE = 1;
    String picturePath;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_settings);

        mBcgBtn = (Button) findViewById(R.id.bcg_clr_btn);


        mBcgBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(
                        Intent.ACTION_PICK,
                        android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);

                startActivityForResult(i, RESULT_LOAD_IMAGE);
            }
        });

    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == RESULT_LOAD_IMAGE && resultCode == RESULT_OK && null != data) {
            Uri selectedImage = data.getData();
            String[] filePathColumn = { MediaStore.Images.Media.DATA };
           try {
               Cursor cursor = getContentResolver().query(selectedImage,
                       filePathColumn, null, null, null);
               cursor.moveToFirst();
               int columnIndex = cursor.getColumnIndex(filePathColumn[0]);
               picturePath  = cursor.getString(columnIndex);
               cursor.close();
           }catch(NullPointerException e){
               e.printStackTrace();
            }


            LinearLayout mHomeLayout = (LinearLayout) findViewById(R.id.activity_home);
            LinearLayout mDetailsPageLayout = (LinearLayout) findViewById(R.id.activity_details_page);


            Drawable d = new BitmapDrawable(getResources(), BitmapFactory.decodeFile(picturePath));
            mHomeLayout.setBackground(d);
        }


    }

}

---

此外，我还添加了“写入外部存储”权限，该代码在“空白项目”中非常有效，但在我的项目中根本不起作用。

它说mHomeLayout为空。也就是说，在活动设置布局中没有id为活动主页的线性布局

因此，您需要做的是将选定的可绘制路径保存在“设置”活动的“存储首选项”中。并在打开另一个活动时加载它，该活动具有id为“活动\主页”的线性布局。然后将其设置为背景

我希望这是你的问题