尝试在Android中旋转时保存和还原实例状态时发生NullPointerException

Question

我正在做一个练习，其中每当调用生命周期函数（onStart，onResume等）时，应用程序都会进行记录。 这一切都有效，直到我尝试实现onSaveInstanceState为止，在其中它告诉我不能对空对象引用应用.getText（）

这是我的代码

private TextView creatCount;
private TextView createTime;
private TextView startCount;
private TextView startTime;
private TextView resumeCount;
private TextView resumeTime;
private TextView pauseCount;
private TextView pauseTime;
private TextView stopCOunt;
private TextView stopTime;
private TextView destroyCount;
private TextView destroyTime;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_lifecycle);
    creatCount = (TextView)findViewById(R.id.create_count);
    createTime = (TextView)findViewById(R.id.create_time);
    increment(creatCount);
    timeStamp(createTime);

    if (savedInstanceState != null){
        creatCount.setText(savedInstanceState.getString("create_count"));
        createTime.setText(savedInstanceState.getString("create_time"));
        startCount.setText(savedInstanceState.getString("start_count"));
        startTime.setText(savedInstanceState.getString("start_time"));
        resumeCount.setText(savedInstanceState.getString("resume_count"));
        resumeTime.setText(savedInstanceState.getString("resume_time"));
        pauseCount.setText(savedInstanceState.getString("pause_count"));
        pauseTime.setText(savedInstanceState.getString("pause_time"));
        stopCOunt.setText(savedInstanceState.getString("stop_count"));
        stopTime.setText(savedInstanceState.getString("stop_time"));
        destroyCount.setText(savedInstanceState.getString("destroy_count"));
        destroyTime.setText(savedInstanceState.getString("destroy_time"));
    }
}

public void onStart(){
    super.onStart();
    startCount = (TextView)findViewById(R.id.start_count);
    startTime = (TextView)findViewById(R.id.start_time);
    increment(startCount);
    timeStamp(startTime);
}

public void onResume(){
    super.onResume();
    resumeCount = (TextView)findViewById(R.id.resume_count);
    resumeTime = (TextView)findViewById(R.id.resume_time);
    increment(resumeCount);
    timeStamp(resumeTime);
}

public void onPause(){
    super.onPause();
    pauseCount = (TextView)findViewById(R.id.pause_count);
    pauseTime = (TextView)findViewById(R.id.pause_time);
    increment(pauseCount);
    timeStamp(pauseTime);
}

public void onStop(){
    super.onStop();
    stopCOunt = (TextView)findViewById(R.id.stop_count);
    stopTime = (TextView)findViewById(R.id.stop_time);
    increment(stopCOunt);
    timeStamp(stopTime);
}

public void onDestroy(){
    super.onDestroy();
    destroyCount = (TextView)findViewById(R.id.destroy_count);
    destroyTime = (TextView)findViewById(R.id.destroy_time);
    increment(destroyCount);
    timeStamp(destroyTime);
}

public void increment(TextView view){
    String number = view.getText().toString();
    int count = Integer.parseInt(number);
    count++;
    view.setText(String.valueOf(count));
}

public void timeStamp(TextView view){
    String currentTime = DateFormat.getTimeInstance().format(new Date());
    view.setText(currentTime);
}

public void onSaveInstanceState(Bundle savedInstanceState){
    savedInstanceState.putString("create_count", creatCount.getText().toString());
    savedInstanceState.putString("create_time", createTime.getText().toString());
    savedInstanceState.putString("start_count", startCount.getText().toString());
    savedInstanceState.putString("start_time", startTime.getText().toString());
    savedInstanceState.putString("resume_count", resumeCount.getText().toString());
    savedInstanceState.putString("resume_time", resumeTime.getText().toString());
    savedInstanceState.putString("pause_count", pauseCount.getText().toString());
    savedInstanceState.putString("pause_time", pauseTime.getText().toString());
    savedInstanceState.putString("stop_count", stopCOunt.getText().toString());
    savedInstanceState.putString("stop_time", stopTime.getText().toString());
    savedInstanceState.putString("destroy_count", destroyCount.getText().toString());
    savedInstanceState.putString("destroy_time", destroyTime.getText().toString());

    super.onSaveInstanceState(savedInstanceState);
    }
}

所有默认文本值都在xml中，并设置为“ 0”

这是logcat

02-02 08:29:26.971 24397-24397/com.example.jmcervantes02.jan_24 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.example.jmcervantes02.jan_24, PID: 24397
                                                                              java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.CharSequence android.widget.TextView.getText()' on a null object reference
                                                                                  at com.example.jmcervantes02.jan_24.LifecycleActivity.onSaveInstanceState(LifecycleActivity.java:115)
                                                                                  at android.app.Activity.performSaveInstanceState(Activity.java:1482)
                                                                                  at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1301)
                                                                                  at android.app.ActivityThread.callCallActivityOnSaveInstanceState(ActivityThread.java:4625)
                                                                                  at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3935)
                                                                                  at android.app.ActivityThread.handleStopActivity(ActivityThread.java:4000)
                                                                                  at android.app.ActivityThread.-wrap25(ActivityThread.java)
                                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1568)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                  at android.os.Looper.loop(Looper.java:163)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:6342)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:880)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:770)

Answer 1

好的，首先让我们了解活动的生命周期。 在此链接上，您可以查看更多详细信息。 但现在：

法案。 启动-> onCreate-> onStart-> onResume->运行中...

提示：始终在活动的onCreate方法中设置View的元素（或在使用片段的情况下，在onCreateView方法中设置View的元素）。

请牢记上面的生命周期，并查看您的“ onCreate”方法。 您是否意识到您正在从TextView调用.setText方法，甚至没有获得它们的引用？

    startCount.setText(savedInstanceState.getString("start_count"));
    startTime.setText(savedInstanceState.getString("start_time"));
    resumeCount.setText(savedInstanceState.getString("resume_count"));
    resumeTime.setText(savedInstanceState.getString("resume_time"));
    pauseCount.setText(savedInstanceState.getString("pause_count"));
    pauseTime.setText(savedInstanceState.getString("pause_time"));
    stopCOunt.setText(savedInstanceState.getString("stop_count"));
    stopTime.setText(savedInstanceState.getString("stop_time"));
    destroyCount.setText(savedInstanceState.getString("destroy_count"));
    destroyTime.setText(savedInstanceState.getString("destroy_time"));

在这里，您要在无参考元素中设置文本。