android - 无法解决方法获取系统服务问题

我遇到了有关getSystemService方法的问题，因为Android Studio IDE一直告诉我它无法解决此方法。据我所知，它可能来自这样一个事实，即我的类不包含活动。我正在阅读有关getSystemService问题的相关主题，但我根本找不到任何匹配的解决方案。

我把我的代码放在上面

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.graphics.Rect;
import android.hardware.Camera;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

private Camera mCamera;
private CameraView mCameraView;

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


@Override
protected void onResume(){
    super.onResume();

    try{
        mCamera = Camera.open(1);
        mCameraView = new CameraView(this, mCamera);
        setContentView(mCameraView);
    } catch (Exception e){
        finish();
    }
}

@Override
protected void onPause(){
    if(mCamera != null){
        mCamera.release();
        mCamera = null;
    }
    super.onPause();


   }

}

上面类中大胆的一个是我试图弄清楚的那个

import android.content.Context;
import android.hardware.Camera;
import android.hardware.Camera.Parameters;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.view.Surface;
import android.view.Display;
import android.view.WindowManager;
import java.io.IOException;
import android.app.Activity;
import java.util.List;


public class CameraView extends SurfaceView implements SurfaceHolder.Callback{


private Camera mCamera;
private View mView;
private WindowManager display;


public CameraView(Context context, Camera mCamera) {
    super(context);

    this.mCamera = mCamera;
    mCamera.setDisplayOrientation(90);

    SurfaceHolder holder = getHolder();
    holder.addCallback(this);
    holder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);

}


@Override
public void surfaceCreated(SurfaceHolder holder) {
    try{
        mCamera.setPreviewDisplay(holder);
        mCamera.startPreview();
    } catch (IOException e) {
        Log.e("cameraPreview", "The failure of the camera settings");
    }
}


@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    Camera.Parameters params = mCamera.getParameters();
    List<Camera.Size> sizes = params.getSupportedPreviewSizes();
    Camera.Size optionalSize = getOptimalPreviewSize(sizes, width, height);
    params.setPreviewSize(optionalSize.width, optionalSize.height);
    mCamera.setParameters(params);

    boolean isPreviewRunning = true;

    if (isPreviewRunning)
    {
        mCamera.stopPreview();
    }

    Parameters parameters = mCamera.getParameters();
    Display display = ((WindowManager) getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();  

    if(display.getRotation() == Surface.ROTATION_0)
    {
        parameters.setPreviewSize(height, width);
        mCamera.setDisplayOrientation(90);
    }

    if(display.getRotation() == Surface.ROTATION_90)
    {
        parameters.setPreviewSize(width, height);
    }

    if(display.getRotation() == Surface.ROTATION_180)
    {
        parameters.setPreviewSize(height, width);
    }

    if(display.getRotation() == Surface.ROTATION_270)
    {
        parameters.setPreviewSize(width, height);
        mCamera.setDisplayOrientation(180);
    }

    mCamera.setParameters(parameters);
    previewCamera();
}

public void previewCamera()
{
    try
    {
        mCamera.setPreviewDisplay();
        mCamera.startPreview();
        boolean isPreviewRunning = true;
    }
    catch(Exception e)
    {
        Log.d(APP_CLASS, "Cannot start preview", e);
    }
}

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

private Camera.Size getOptimalPreviewSize(List<Camera.Size> sizes, int w, int h) {

    final double ASPECT_TOLERANCE = 0.1;
    double targetRatio=(double)h / w;

    if (sizes == null) return null;

    Camera.Size optimalSize = null;
    double minDiff = Double.MAX_VALUE;

    int targetHeight = h;

    for (Camera.Size size : sizes) {
        double ratio = (double) size.width / size.height;
        if (Math.abs(ratio - targetRatio) > ASPECT_TOLERANCE) continue;
        if (Math.abs(size.height - targetHeight) < minDiff) {
            optimalSize = size;
            minDiff = Math.abs(size.height - targetHeight);
        }
    }

    if (optimalSize == null) {
        minDiff = Double.MAX_VALUE;
        for (Camera.Size size : sizes) {
            if (Math.abs(size.height - targetHeight) < minDiff) {
                optimalSize = size;
                minDiff = Math.abs(size.height - targetHeight);
            }
        }
    }
    return optimalSize;


  }



}

你能告诉我应该如何避免这件事吗？

谢谢！