我正在制作Android应用，它应该要求算术方程并检查答案。应该有不同的难度级别。从AlertDialog中选择难度级别时，我的应用会崩溃。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);
}
有人可以帮助我理解我的应用崩溃的原因吗？
这是日志：
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onCreate类PlayGame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();
}
答案 0 :(得分：0)
引起：java.lang.NullPointerException：尝试在org.example.braintraining.PlayGame.onCreate的空对象引用上调用虚方法'int android.os.Bundle.getInt（java.lang.String）'（ PlayGame.java:10 4）
在两种情况下，代码可能都失败了：
- 您正在通过传递playIntent开始新的活动。请检查PlayGame.java中的代码，在onCreate（）中你应该从Intent而不是Bundle中获取字符串值（级别），比如
String chosenLevel;
Intent i = this.getIntent();
if(i != null){
chosenLevel = i.getStringExtra("level");
}
2.如果您正在旋转屏幕，一旦您处于游戏活动中，您在活动被销毁之前未保存所需的值，并且在重新创建活动后尝试将其检索回来。
解决方法是使用put方法在onSaveInstanceState（）中存储值：
protected void onSaveInstanceState(Bundle bundle) {
super.onSaveInstanceState(bundle);
bundle.putString("value", chosenLevel);
}
从onCreate（）中的Bundle恢复值，或者你可以使用在onStart（）之后调用的onRestoreInstanceState（），而在onStart（）之前调用onCreate（）。：
public void onCreate(Bundle bundle) {
if (bundle!= null){
chosenValue = bundle.getString("value");
}
}