在Mainactivity中，使用Camera API或者CameraX进行相机预览和拍照操作，并将拍照后的图片保存到公共文件夹中，具体代码

使用Camera API进行相机预览和拍照操作的代码如下：

public class MainActivity extends AppCompatActivity implements SurfaceHolder.Callback, Camera.PictureCallback {

    private Camera mCamera;
    private SurfaceView mSurfaceView;
    private SurfaceHolder mHolder;
    private Button mCaptureButton;

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

        mSurfaceView = findViewById(R.id.surfaceView);
        mHolder = mSurfaceView.getHolder();
        mHolder.addCallback(this);

        mCaptureButton = findViewById(R.id.captureButton);
        mCaptureButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mCamera.takePicture(null, null, MainActivity.this);
            }
        });
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        try {
            mCamera = Camera.open();
            mCamera.setPreviewDisplay(holder);
            mCamera.startPreview();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
        // empty
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        mCamera.stopPreview();
        mCamera.release();
        mCamera = null;
    }

    @Override
    public void onPictureTaken(byte[] data, Camera camera) {
        File pictureFile = getOutputMediaFile();
        if (pictureFile == null) {
            Log.d(TAG, "Error creating media file, check storage permissions");
            return;
        }

        try {
            FileOutputStream fos = new FileOutputStream(pictureFile);
            fos.write(data);
            fos.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

        mCamera.startPreview();
    }

    private static File getOutputMediaFile(){
        File mediaStorageDir = new File(Environment.getExternalStoragePublicDirectory(
                Environment.DIRECTORY_PICTURES), "CameraDemo");

        if (!mediaStorageDir.exists()){
            if (!mediaStorageDir.mkdirs()){
                return null;
            }
        }

        String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());
        File mediaFile = new File(mediaStorageDir.getPath() + File.separator +
                "IMG_"+ timeStamp + ".jpg");

        return mediaFile;
    }
}

使用CameraX进行相机预览和拍照操作的代码如下：

public class MainActivity extends AppCompatActivity {

    private PreviewView mPreviewView;
    private Button mCaptureButton;

    private ImageCapture mImageCapture;
    private Executor mExecutor = Executors.newSingleThreadExecutor();

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

        mPreviewView = findViewById(R.id.previewView);
        mCaptureButton = findViewById(R.id.captureButton);

        mCaptureButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                captureImage();
            }
        });

        startCamera();
    }

    private void startCamera() {
        ListenableFuture<ProcessCameraProvider> cameraProviderFuture = ProcessCameraProvider.getInstance(this);
        cameraProviderFuture.addListener(new Runnable() {
            @Override
            public void run() {
                try {
                    ProcessCameraProvider cameraProvider = cameraProviderFuture.get();
                    bindPreview(cameraProvider);
                } catch (ExecutionException | InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, ContextCompat.getMainExecutor(this));
    }

    private void bindPreview(ProcessCameraProvider cameraProvider) {
        Preview preview = new Preview.Builder().build();
        CameraSelector cameraSelector = new CameraSelector.Builder()
                .requireLensFacing(CameraSelector.LENS_FACING_BACK)
                .build();

        preview.setSurfaceProvider(mPreviewView.createSurfaceProvider());

        mImageCapture = new ImageCapture.Builder()
                .setCaptureMode(ImageCapture.CAPTURE_MODE_MINIMIZE_LATENCY)
                .build();

        cameraProvider.unbindAll();

        cameraProvider.bindToLifecycle(this, cameraSelector, preview, mImageCapture);
    }

    private void captureImage() {
        File outputDirectory = getOutputDirectory();
        if (!outputDirectory.exists() && !outputDirectory.mkdirs()) {
            Log.e(TAG, "Failed to create output directory");
            return;
        }

        ImageCapture.OutputFileOptions outputFileOptions = new ImageCapture.OutputFileOptions.Builder(
                new File(outputDirectory, "IMG_" + System.currentTimeMillis() + ".jpg"))
                .build();

        mImageCapture.takePicture(outputFileOptions, mExecutor, new ImageCapture.OnImageSavedCallback() {
            @Override
            public void onImageSaved(@NonNull ImageCapture.OutputFileResults outputFileResults) {
                Toast.makeText(MainActivity.this, "Image saved successfully", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onError(@NonNull ImageCaptureException exception) {
                Log.e(TAG, "Error capturing image: " + exception.getMessage());
            }
        });
    }

    private File getOutputDirectory() {
        return new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES), "CameraXDemo");
    }
}

注意：使用CameraX需要在AndroidManifest.xml中添加以下权限：

<uses-permission android:name="android.permission.CAMERA" />
<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

同时需要在build.gradle文件中添加以下依赖：

implementation "androidx.camera:camera-camera2:1.0.0-rc01"
implementation "androidx.camera:camera-lifecycle:1.0.0-rc01"
implementation "androidx.camera:camera-view:1.0.0-alpha23"