Java Android应用程序崩溃，代码中没有错误

我正在制作一个Android应用程序，它应该询问算术公式并检查答案。也应该有不同的难度。从

AlertDialog

选择难度级别时，我的应用程序崩溃。我在Android Studio中没有错误

以下是选择级别的代码：

public void onClick(View view) {
    if(view.getId()==R.id.play_btn){
        //play button
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setTitle("Choose a level")
                .setSingleChoiceItems(levelNames, 0, new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                        //start gameplay
                        startPlay(which);
                    }
                });
        AlertDialog ad = builder.create();
        ad.show();
    }

private void startPlay(int chosenLevel){
    //start gameplay
    Intent playIntent = new Intent(this, PlayGame.class);
    playIntent.putExtra("level", chosenLevel);
    this.startActivity(playIntent);
}

有人能帮我理解为什么我的应用程序会崩溃吗

以下是日志：

9758-9758/org.example.braintraining E/AndroidRuntime: FATAL EXCEPTION: main
                                                                     Process: org.example.braintraining, PID: 9758
                                                                     java.lang.RuntimeException: Unable to start activity ComponentInfo{org.example.braintraining/org.example.braintraining.PlayGame}: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.os.Bundle.getInt(java.lang.String)' on a null object reference
                                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2693)
                                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758)
                                                                         at android.app.ActivityThread.access$900(ActivityThread.java:177)
                                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448)
                                                                         at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                         at android.os.Looper.loop(Looper.java:145)
                                                                         at android.app.ActivityThread.main(ActivityThread.java:5942)
                                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                                         at java.lang.reflect.Method.invoke(Method.java:372)
                                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183)
                                                                      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.os.Bundle.getInt(java.lang.String)' on a null object reference
                                                                         at org.example.braintraining.PlayGame.onCreate(PlayGame.java:104)
                                                                         at android.app.Activity.performCreate(Activity.java:6289)
                                                                         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
                                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758) 
                                                                         at android.app.ActivityThread.access$900(ActivityThread.java:177) 
                                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448) 
                                                                         at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                         at android.os.Looper.loop(Looper.java:145) 
                                                                         at android.app.ActivityThread.main(ActivityThread.java:5942) 
                                                                         at java.lang.reflect.Method.invoke(Native Method) 
                                                                         at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388) 
                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183) 

以下是

PlayGame

类的

onCreate

方法的代码：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_playgame);
    gamePrefs = getSharedPreferences(GAME_PREFS, 0);

    //text and image views
    question = (TextView)findViewById(R.id.question);
    answerTxt = (TextView)findViewById(R.id.answer);
    response = (ImageView)findViewById(R.id.response);
    scoreTxt = (TextView)findViewById(R.id.score);

    //hide tick cross initially
    response.setVisibility(View.INVISIBLE);

    //number, enter and clear buttons
    btn1 = (Button)findViewById(R.id.btn1);
    btn2 = (Button)findViewById(R.id.btn2);
    btn3 = (Button)findViewById(R.id.btn3);
    btn4 = (Button)findViewById(R.id.btn4);
    btn5 = (Button)findViewById(R.id.btn5);
    btn6 = (Button)findViewById(R.id.btn6);
    btn7 = (Button)findViewById(R.id.btn7);
    btn8 = (Button)findViewById(R.id.btn8);
    btn9 = (Button)findViewById(R.id.btn9);
    btn0 = (Button)findViewById(R.id.btn0);
    enterBtn = (Button)findViewById(R.id.enter);
    clearBtn = (Button)findViewById(R.id.clear);

    //listen for clicks
    btn1.setOnClickListener(this);
    btn2.setOnClickListener(this);
    btn3.setOnClickListener(this);
    btn4.setOnClickListener(this);
    btn5.setOnClickListener(this);
    btn6.setOnClickListener(this);
    btn7.setOnClickListener(this);
    btn8.setOnClickListener(this);
    btn9.setOnClickListener(this);
    btn0.setOnClickListener(this);
    enterBtn.setOnClickListener(this);
    clearBtn.setOnClickListener(this);

    //get passed level number
    if(savedInstanceState!=null){
        //restore state
    }
    else{
        Bundle extras = getIntent().getExtras();
        if(extras !=null)
        {
            int passedLevel = extras.getInt("level", -1);
            if(passedLevel>=0) level = passedLevel;
            level=savedInstanceState.getInt("level");
            int exScore = savedInstanceState.getInt("score");
            scoreTxt.setText("Score: "+exScore);
        }
    }

    //initialize random
    random = new Random();
    //play
    chooseQuestion();
}

---

原因：java.lang.NullPointerException:试图在org.example.braintraining.PlayGame.onCreate（PlayGame.java:10）上的空对象引用上调用虚拟方法“int android.os.Bundle.getInt（java.lang.String）”‌​(四)

在两种情况下，代码可能会失败：

您正在通过传递playIntent开始新活动。请检查PlayGame.java中的代码，在onCreate（）中，您应该从Intent而不是Bundle中获取字符串值（级别），例如

2.如果您正在旋转屏幕，则在游戏活动中，您没有在活动被销毁之前保存所需的值，并在活动重新创建后尝试将其检索回来

解决方案是使用put方法在onSaveInstanceState（）中存储值：

并从onCreate（）中的Bundle还原值，或者可以使用onRestoreInstanceState（），它在onStart（）之后调用，而onCreate（）在onStart（）之前调用：

---

你能添加崩溃日志吗？我想你忘了在Androidmanifest.xmlYes中定义PlayGame活动了，我添加了日志。@savepopulation它在那里：

原因：java.lang.NullPointerException:尝试调用虚拟方法'int android.os.Bundle.getInt（java.lang.String）'在org.example.braintraining.PlayGame.onCreate（PlayGame.java:104）上的空对象引用上

请解释此错误消息让您感到困惑的地方，它告诉您错误是什么以及发生在哪里。

 String chosenLevel;
 Intent i = this.getIntent();
 if(i != null){
  chosenLevel = i.getStringExtra("level");
 }

protected void onSaveInstanceState(Bundle bundle) {
super.onSaveInstanceState(bundle);
bundle.putString("value", chosenLevel);
} 

public void onCreate(Bundle bundle) {
if (bundle!= null){
chosenValue = bundle.getString("value");
 } 
}