在安卓应用中，如果相机预览未显示出来，可能是由于以下几个原因：

1. 没有给予相机权限：在AndroidManifest.xml文件中添加相机权限：

2. 没有正确初始化相机：在Activity或Fragment中进行相机初始化和预览：

private SurfaceView mPreview;
private Camera mCamera;

private void initCamera() {
    mCamera = Camera.open();
    mPreview = findViewById(R.id.preview_surface_view);

    try {
        mCamera.setPreviewDisplay(mPreview.getHolder());
        mCamera.startPreview();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

@Override
protected void onResume() {
    super.onResume();
    if (mCamera == null) {
        initCamera();
    }
}

@Override
protected void onPause() {
    super.onPause();
    releaseCamera();
}

private void releaseCamera() {
    if (mCamera != null) {
        mCamera.setPreviewCallback(null);
        mCamera.stopPreview();
        mCamera.release();
        mCamera = null;
    }
}

3. SurfaceView创建和设置错误：确保SurfaceView已经正确创建并设置：

4. 相机方向不正确：在Activity或Fragment中设置相机预览方向：

private void setCameraDisplayOrientation() {
    Camera.CameraInfo info = new Camera.CameraInfo();
    Camera.getCameraInfo(cameraId, info);

    int rotation = getWindowManager().getDefaultDisplay().getRotation();
    int degrees = 0;
    switch (rotation) {
        case Surface.ROTATION_0:
            degrees = 0;
            break;
        case Surface.ROTATION_90:
            degrees = 90;
            break;
        case Surface.ROTATION_180:
            degrees = 180;
            break;
        case Surface.ROTATION_270:
            degrees = 270;
            break;
    }

    int result;
    if (info.facing == Camera.CameraInfo.CAMERA_FACING_FRONT) {
        result = (info.orientation + degrees) % 360;
        result = (360 - result) % 360;  // compensate the mirror
    } else {  // back-facing
        result = (info.orientation - degrees + 360) % 360;
    }

    mCamera.setDisplayOrientation(result);
}

这些解决方法可以帮助您解决安卓应用中相机预览未显示出来的问题。