Java 如何将手机图库中的图像输入应用程序

我想从我的库中添加一个图像，然后保存它。 当我在AddImage活动中单击setOnClickListener时，我的应用程序崩溃。 我将这段代码添加到一个新项目中，然后它工作得很好，但我不知道为什么它在我的主项目中不工作

我正在使用MediaStore

public class AddImage extends Activity {
ImageView viewImage;
Button b;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b=findViewById(R.id.button);
    viewImage=findViewById(R.id.camera);
    b.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            selectImage();
        }
    });
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds options to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}
private void selectImage() {
    final CharSequence[] options = { "Take Photo", "Choose from Gallery","Cancel" };
    AlertDialog.Builder builder = new AlertDialog.Builder(AddImage.this);
    builder.setTitle("Add Photo!");
    builder.setItems(options, new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int item) {
            if (options[item].equals("Take Photo"))
            {
                Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
                File f = new File(android.os.Environment.getExternalStorageDirectory(), "temp.jpg");
                intent.putExtra(MediaStore.EXTRA_OUTPUT, Uri.fromFile(f));
                startActivityForResult(intent, 1);
            }
            else if (options[item].equals("Choose from Gallery"))
            {
                Intent intent = new   Intent(Intent.ACTION_PICK,android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
                startActivityForResult(intent, 2);
            }
            else if (options[item].equals("Cancel")) {
                dialog.dismiss();
            }
        }
    });
    builder.show();
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (resultCode == RESULT_OK) {
        if (requestCode == 1) {
            File f = new File(Environment.getExternalStorageDirectory().toString());
            for (File temp : f.listFiles()) {
                if (temp.getName().equals("temp.jpg")) {
                    f = temp;
                    break;
                }
            }
            try {
                Bitmap bitmap;
                BitmapFactory.Options bitmapOptions = new BitmapFactory.Options();
                bitmap = BitmapFactory.decodeFile(f.getAbsolutePath(),
                        bitmapOptions);
                viewImage.setImageBitmap(bitmap);
                String path = android.os.Environment
                        .getExternalStorageDirectory()
                        + File.separator
                        + "Phoenix" + File.separator + "default";
                f.delete();
                OutputStream outFile = null;
                File file = new File(path, String.valueOf(System.currentTimeMillis()) + ".jpg");
                try {
                    outFile = new FileOutputStream(file);
                    bitmap.compress(Bitmap.CompressFormat.JPEG, 85, outFile);
                    outFile.flush();
                    outFile.close();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        } else if (requestCode == 2) {
            Uri selectedImage = data.getData();
            String[] filePath = { MediaStore.Images.Media.DATA };
            Cursor c = getContentResolver().query(selectedImage,filePath, null, null, null);
            c.moveToFirst();
            int columnIndex = c.getColumnIndex(filePath[0]);
            String picturePath = c.getString(columnIndex);
            c.close();
            Bitmap thumbnail = (BitmapFactory.decodeFile(picturePath));
            viewImage.setImageBitmap(thumbnail);
        }
    }
}

}

日志：

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.completepractice.AddImage.onCreate(AddImage.java:34)
    at android.app.Activity.performCreate(Activity.java:7802)
    at android.app.Activity.performCreate(Activity.java:7791)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
    at android.os.Handler.dispatchMessage(Handler.java:107) 
    at android.os.Looper.loop(Looper.java:214) 
    at android.app.ActivityThread.main(ActivityThread.java:7356) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

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请帮助我，因为我是这个领域的新手

请检查

b=findViewById（R.id.button）属于activity_main.xml

当findViewById引用的id值不是setContentView（）中定义的XML的一部分时，会产生空对象引用。
使用带有Android Oreo的仿真器，看看它是否有效。如果是，则您可能必须深入研究存储权限。我建议您提出一个新问题，其中包含特定的logcat错误和相关代码。我应该编辑此问题还是创建一个新问题？