这个功能的实现使用的技术比较落后，但是对于我这样的初学者来说还是足够的

 直接上代码

整个代码都在MainAvtivity中，测试使用的activity十分简单，最重要的是有两个id即可。

package com.example.takephotos;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.PixelFormat;
import android.hardware.Camera;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.provider.MediaStore;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.view.WindowManager;
import android.widget.Button;
import android.widget.Toast;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {
    private Camera camera;//摄像头对象
    private boolean isPreview=false;//是否为预览状态
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);//设置全屏显示
        if(!android.os.Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED
        )){
            Toast.makeText(this,"请安装sd卡",Toast.LENGTH_SHORT).show();
        }
        //打开摄像头并预览
        SurfaceView surfaceView=findViewById(R.id.surfaceView);//用于显示摄像头预览
        SurfaceHolder surfaceHolder = surfaceView.getHolder();
        surfaceHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
        Button preview=findViewById(R.id.preview);
        preview.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(!isPreview){
                    camera=Camera.open();//打开摄像头
                    isPreview=true;
                }
                try {
                    camera.setPreviewDisplay(surfaceHolder);
                    Camera.Parameters parameters=camera.getParameters();
                    parameters.setPictureFormat(PixelFormat.JPEG);
                    parameters.set("jpeg-quality",80);
                    camera.setParameters(parameters);
                    camera.startPreview();
                    camera.autoFocus(null);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        });
        //实现拍照功能
        Button takephoto=findViewById(R.id.takephoto);
        takephoto.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (camera!=null){
                    camera.takePicture(null,null,jpeg);
                }
            }
        });

    }
    Camera.PictureCallback jpeg = new Camera.PictureCallback() {
        @Override
        public void onPictureTaken(byte[] bytes, Camera camera) {
            Bitmap bitmap= BitmapFactory.decodeByteArray(bytes,0,bytes.length);
            camera.stopPreview();//停止预览
            isPreview=false;
            File appDir=new File(Environment.getExternalStorageDirectory(),"/DCIM/Camera");
            String fileName=System.currentTimeMillis()+".jpg";
            File file=new File(appDir,fileName);
            try {
                FileOutputStream fos=new FileOutputStream(file);
                bitmap.compress(Bitmap.CompressFormat.JPEG,100,fos);
                fos.flush();
                fos.close();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            //将相片插入系统图库
            try {
                MediaStore.Images.Media.insertImage(MainActivity.this.getContentResolver(),
                        file.getAbsolutePath(),fileName,null);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
            //图库更新
            MainActivity.this.sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE,
                    Uri.parse("file://"+"")));
            Toast.makeText(MainActivity.this,"保存至"+file,Toast.LENGTH_LONG).show();
            resetCamera();//重新预览
        }
    };

    private void resetCamera() {
        if(!isPreview){
            camera.startPreview();//预览
            isPreview=true;
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        if(camera!=null){
            camera.stopPreview();
            camera.release();//释放资源
        }
    }
}

注：这里面使用到了sd卡，并不一定使用真机，我使用的是逍遥模拟器（这个无论打游戏还是学习都挺好用的）

实现效果：电脑摄像头是前置的