Question

尝试创建一个Android应用程序来录制视频和捕获图像。到目前为止我已经按照教程取得了一些成功，但是我必须对android sdk缺乏了解才能解决这个问题。毫无疑问，一旦有人想出来，我会感到愚蠢，但最后我会在这里学习。

这是MainActivity.java

package example.myapplication;

import android.content.Context;
import android.content.pm.PackageManager;
import android.hardware.Camera;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity {

public MainActivity(Context context) {
}

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

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

/** Check if this device has a camera */
public boolean checkCameraHardware(Context context) {
    if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA)){
        // this device has a camera
        return true;
    } else {
        // no camera on this device
        return false;
    }
}

/** A safe way to get an instance of the Camera object. */
public static Camera getCameraInstance(){
    Camera c = null;
    try {
        c = Camera.open(); // attempt to get a Camera instance
    }
    catch (Exception e){
        // Camera is not available (in use or does not exist)
    }
    return c; // returns null if camera is unavailable
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

这是CameraPreview.java

package example.myapplication;

import android.content.Context;
import android.hardware.Camera;
import android.net.Uri;
import android.os.Environment;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.View;
import android.widget.Button;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;

/** A basic Camera preview class */
public class CameraPreview extends MainActivity implements SurfaceHolder.Callback {

private android.hardware.Camera.PictureCallback mPicture;
private Camera mCamera;
private CameraPreview mPreview;
private SurfaceHolder mHolder;
private SurfaceHolder holder;

public void setContentView(int activity_camera) {}
public static final int MEDIA_TYPE_IMAGE = 1;
public static final int MEDIA_TYPE_VIDEO = 2;
private static final String TAG = "CameraPreview";

public CameraPreview(Context context, Camera camera) {
    super(context);
    mCamera = camera;

    // Install a SurfaceHolder.Callback so we get notified when the
    // underlying surface is created and destroyed.
    mHolder = getHolder();
    mHolder.addCallback(this);
    // deprecated setting, but required on Android versions prior to 3.0
    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}

public void surfaceCreated(SurfaceHolder holder) {
    // The Surface has been created, now tell the camera where to draw the preview.
    try {
        mCamera.setDisplayOrientation(90);
        mCamera.setPreviewDisplay(holder);
        mCamera.startPreview();
    } catch (IOException e) {
        Log.d(TAG, "Error setting camera preview: " + e.getMessage());
    }
}

public void surfaceDestroyed(SurfaceHolder holder) {
    // empty. Take care of releasing the Camera preview in your activity.
}

public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
    // If your preview can change or rotate, take care of those events here.
    // Make sure to stop the preview before resizing or reformatting it.

    if (mHolder.getSurface() == null){
        // preview surface does not exist
        return;
    }

    // stop preview before making changes
    try {
        mCamera.stopPreview();
    } catch (Exception e){
        // ignore: tried to stop a non-existent preview
    }

    // set preview size and make any resize, rotate or
    // reformatting changes here

    // start preview with new settings
    try {
        mCamera.setPreviewDisplay(mHolder);
        mCamera.startPreview();

    } catch (Exception e){
        Log.d(TAG, "Error starting camera preview: " + e.getMessage());
    }
}

public void onPictureTaken(byte[] data, Camera camera) {
    File pictureFile = getOutputMediaFile(MEDIA_TYPE_IMAGE);
    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 (FileNotFoundException e) {
        Log.d(TAG, "File not found: " + e.getMessage());

    } catch (IOException e) {
        Log.d(TAG, "Error accessing file: " + e.getMessage());
    }
}

/** Create a file Uri for saving an image or video */
private static Uri getOutputMediaFileUri(int type){
    return Uri.fromFile(getOutputMediaFile(type));
}

/** Create a File for saving an image or video */
private static File getOutputMediaFile(int type){
    // To be safe, you should check that the SDCard is mounted
    // using Environment.getExternalStorageState() before doing this.

    File mediaStorageDir = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_PICTURES), "MyCameraApp");
    // This location works best if you want the created images to be shared
    // between applications and persist after your app has been uninstalled.

    // Create the storage directory if it does not exist
    if (! mediaStorageDir.exists()){
        if (! mediaStorageDir.mkdirs()){
            Log.d("MyCameraApp", "failed to create directory");
            return null;
        }
    }

    // Create a media file name
    String timeStamp = new SimpleDateFormat("yyyyMMdd_HH:mm:ss").format(new Date());
    File mediaFile;
    if (type == MEDIA_TYPE_IMAGE){
        mediaFile = new File(mediaStorageDir.getPath() + File.separator +
                "IMG_"+ timeStamp + ".jpg");
    } else if(type == MEDIA_TYPE_VIDEO) {
        mediaFile = new File(mediaStorageDir.getPath() + File.separator +
                "VID_"+ timeStamp + ".mp4");
    } else {
        return null;
    }

    return mediaFile;
}

public void onClick() {
    // Add a listener to the Capture button
    Button captureButton = (Button) findViewById(R.id.button_capture);
    captureButton.setOnClickListener(new View.OnClickListener() {

        public void onClick(View arg0){
            // get an image from the camera
            mCamera.takePicture(null, null, mPicture);
        }
    });
}

public SurfaceHolder getHolder() {
    return holder;
}

public void setHolder(SurfaceHolder holder) {
    this.holder = holder;
}
}

这是AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="example.myapplication" >
<uses-permission android:name="android.permission.CAMERA" />
<uses-feature android:name="android.hardware.camera2" />
<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

和logcat

Process: example.myapplication, PID: 20671
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{example.myapplication/example.myapplication.MainActivity}: java.lang.InstantiationException: can't instantiate class example.myapplication.MainActivity; no empty constructor
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2514)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2653)
        at android.app.ActivityThread.access$800(ActivityThread.java:156)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1355)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:157)
        at android.app.ActivityThread.main(ActivityThread.java:5872)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674)
        at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.InstantiationException: can't instantiate class installation04.myapplication.MainActivity; no empty constructor
        at java.lang.Class.newInstanceImpl(Native Method)
        at java.lang.Class.newInstance(Class.java:1208)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1079)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2505)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2653)
        at android.app.ActivityThread.access$800(ActivityThread.java:156)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1355)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:157)
        at android.app.ActivityThread.main(ActivityThread.java:5872)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:674)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

需要排序的一些事情：

请从扩展Activity的类中删除所有构造函数。 - 活动不会这样。请查看Activity Life cycle的工作原理。
请将CameraPreview活动添加到您的清单中。

Answer 2

删除MainActivity构造函数

public MainActivity(Context context) {
}

请仔细阅读stacktrace：

java.lang.RuntimeException：无法实例化活动 ComponentInfo {example.myapplication / example.myapplication.MainActivity}： java.lang.InstantiationException：无法实例化类 example.myapplication.MainActivity;没有空构造函数

Answer 3

您已在Activity中创建了构造函数。从您的活动中删除构造函数并使用Lifecycle回调根据android指南，你不应该在Activity类中创建构造函数，因为android OS创建了Activity类的对象，并且它使用类的默认空构造函数来创建对象。

构建一个Android相机应用程序

3 个答案: