录制音频播放器android时出现NullPointerException

Question

package com.example.projectlayout;

import java.io.File;

import android.media.MediaRecorder;

import android.os.Bundle;

import android.os.Environment;

import android.os.SystemClock;

import android.app.Activity;

import android.content.Intent;

import android.view.Menu;

import android.view.View;

import android.widget.Chronometer;

import android.widget.Toast;


public class MainActivity extends Activity {

Chronometer time;
 int i=0;
MediaRecorder recorder;
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    time=(Chronometer)findViewById(R.id.chronometer1);
    String name="myrecord"+i++;
    File sdcard=Environment.getExternalStorageDirectory();
    System.out.println(sdcard.toString());
    File recording=new File(sdcard,"videoRecordingFileZ");
    if(!recording.exists())
    {
    System.out.println("inside if");
    recording.mkdir();
    }
String rec=recording.getAbsolutePath()+"/Record"+".3GP";
    try{                
    recorder.setAudioSource(MediaRecorder.AudioSource.MIC);
    recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
  recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
    recorder.setOutputFile(rec);                                    
    recorder.prepare();
    }
    catch(Exception e){
    System.out.println(e);
    recorder.start();
    }}
   public void startrecording(View v)
    {
        Toast.makeText(MainActivity.this, "START RECORDING", Toast.LENGTH_LONG).show();
        time.setBase(SystemClock.elapsedRealtime());
        time.start();
    }
public void stoprecording(View v)
    {
        Toast.makeText(MainActivity.this, "STOP RECORDING", Toast.LENGTH_LONG).show();
        time.stop();
    }
    public void showrecording(View v)
    {
        Toast.makeText(MainActivity.this, "Show RECORDING", Toast.LENGTH_LONG).show();
        Intent i=new Intent(MainActivity.this,Showlist.class);
        startActivity(i);
    }
@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;
}}

logcat的：

03-27 00:49:22.224: E/AndroidRuntime(470): FATAL EXCEPTION: main

03-27 00:49:22.224: E/AndroidRuntime(470): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.projectlayout/com.example.projectlayout.MainActivity}: java.lang.NullPointerException

03-27 00:49:22.224: E/AndroidRuntime(470): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.projectlayout/com.example.projectlayout.MainActivity}: java.lang.NullPointerException

Answer 1

如果您想获得异常帮助，您确实需要提供完整的错误消息和调用堆栈。

话虽如此，我可以在您的代码中看到问题（假设您提供了所有相关部分）。

'recorder'成员变量永远不会赋值，但在onCreate（）的try AND catch子句中被取消引用。可能是catch子句中的dereference引起了NullPointerException。

Answer 2

您没有初始化MediaRecorder：

 MediaRecorder recorder;

您应该在onCreate方法中执行此操作：

 recorder = new MediaRecorder();

在使用recorder之前或者你得到NullPointerException。

BTW：改进代码缩进（在Eclipse Ctrl + I keybord快捷方式）以提高代码的可读性

---

关于下一个问题：IllegalStateException：

从android文档中检查MediaRecorder state diagram：

和提供的示例代码：

MediaRecorder recorder = new MediaRecorder();
 recorder.setAudioSource(MediaRecorder.AudioSource.MIC);
 recorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
 recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
 recorder.setOutputFile(PATH_NAME);
 recorder.prepare();
 recorder.start();   // Recording is now started
 ...
 recorder.stop();
 recorder.reset();   // You can reuse the object by going back to setAudioSource() step
 recorder.release(); // Now the object cannot be reused