Android自定義相機，獲取空指針異常？

Question

我是新來的Android編碼和我在Android開發人員網站，在那裏他們解釋如何創建自定義相機。我擁有與網站上的所有內容完全相同的內容，但我始終在mCamera.setPreviewDisplay（持有者）處獲取空指針; - 第31行：CameraPreview.java

http://developer.android.com/guide/topics/media/camera.html

這裏是activity_main.xml中：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    tools:context=".MainActivity" > 

    <FrameLayout 
     android:id="@+id/camera_preview" 
     android:layout_width="fill_parent" 
     android:layout_height="fill_parent" 
     android:layout_weight="1"/> 
</LinearLayout> 

現在MainActivity.java：

package com.example.customcamera; 

import android.hardware.Camera; 
import android.os.Bundle; 
import android.app.Activity; 
import android.view.Menu; 
import android.widget.FrameLayout; 

public class MainActivity extends Activity { 

    private Camera mCamera; 
    private CameraPreview mPreview; 

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

     // Create an instance of Camera 
     mCamera = getCameraInstance(); 

     // Create our Preview view and set it as the content of our activity. 
     mPreview = new CameraPreview(this, mCamera); 
     FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview); 
     preview.addView(mPreview); 
    } 

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

    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 
    } 
} 

CameraPreview.java：

package com.example.customcamera; 

import java.io.IOException; 

import android.content.Context; 
import android.hardware.Camera; 
import android.util.Log; 
import android.view.SurfaceHolder; 
import android.view.SurfaceView; 

/** A basic Camera preview class */ 
public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback { 
    private SurfaceHolder mHolder; 
    private Camera mCamera; 
    private 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.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()); 
     } 
    } 
} 

Answer 1

個getCameraInstance狀態return c; // returns null if camera is unavailable。它是否返回null？你檢查過了嗎？