應用程序與媒體播放器崩潰

Question

我的測驗應用程序崩潰時，預計聽到正確/錯誤的答案的聲音。這曾經工作，現在它不，我不明白爲什麼。有人能幫助我嗎？的代碼：

import java.util.List; 
import android.media.MediaPlayer; 
import android.os.Bundle; 
import android.app.Activity; 
import android.content.Intent; 
import android.util.Log; 
import android.view.Menu; 
import android.view.View; 
import android.widget.Button; 
import android.widget.RadioButton; 
import android.widget.RadioGroup; 
import android.widget.TextView; 
import android.widget.Toast; 
import com.example.vtorferreira.R; 


public class QuizActivity extends Activity { 

List<Question> quesList; 
int score=0; 
int qid=0; 
Question currentQ; 
TextView txtQuestion; 
RadioButton rda, rdb, rdc; 
Button butNext; 

@Override 

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_quiz); 

    final MediaPlayer wrong_answer= MediaPlayer.create(this, R.raw.wrong_answer); 
    final MediaPlayer correct_answer= MediaPlayer.create(this, R.raw.correct_answer); 


    DbHelper db=new DbHelper(this); 

    quesList=db.getAllQuestions(); 
    currentQ=quesList.get(qid); 
    txtQuestion=(TextView)findViewById(R.id.textView1); 

    rda=(RadioButton)findViewById(R.id.radio0); 
    rdb=(RadioButton)findViewById(R.id.radio1); 
    rdc=(RadioButton)findViewById(R.id.radio2); 

    butNext=(Button)findViewById(R.id.button1); 
    setQuestionView(); 
    butNext.setOnClickListener(new View.OnClickListener() { 
     @Override 

     public void onClick(View v) { 
      RadioGroup grp=(RadioGroup)findViewById(R.id.radioGroup1); 
      RadioButton answer=(RadioButton)findViewById(grp.getCheckedRadioButtonId()); 

      if(currentQ.getANSWER().equals(answer.getText())) 
      { 
       correct_answer.start(); 
       score++; 
       Toast.makeText(getApplicationContext(),"Correct!", Toast.LENGTH_SHORT).show(); 

      }else{ 
       wrong_answer.start(); 
       Toast.makeText(getApplicationContext(),"Wrong...", Toast.LENGTH_SHORT).show(); 
      } 
      if(qid<9){ 
       currentQ=quesList.get(qid); 
       setQuestionView(); 
      }else{ 
       Intent intent = new Intent(QuizActivity.this, ResultActivity.class); 
       Bundle b = new Bundle(); 
       b.putInt("score", score); //Your score 
       intent.putExtras(b); //Put your score to your next Intent 
       startActivity(intent); 
       finish(); 
      } 
     } 
    }); 
} 
@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    // Inflate the menu; this adds items to the action bar if it is present. 
    getMenuInflater().inflate(R.menu.activity_quiz, menu); 
    return true; 
} 
private void setQuestionView() 
{ 
    txtQuestion.setText(currentQ.getQUESTION()); 
    rda.setText(currentQ.getOPTA()); 
    rdb.setText(currentQ.getOPTB()); 
    rdc.setText(currentQ.getOPTC()); 
    qid++; 
} 
public void onBackPressed(){ 

} 
} 

日誌： 顯示java.lang.NullPointerException：嘗試上的空對象引用調用虛擬方法 '無效android.media.MediaPlayer.start（）'

預先感謝

Answer 1

你應該寫correct_answer.prepare();correct_answer.start();前