Question

我有我的MainActivity，我扩展了FragmentActivity。我还有其他3个片段活动，我使用此代码而不是onCreate片段的原因（我将我的应用程序从Activity更改为Fragment）。

    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

以下是我如何扩展我的活动：

public class MainActivity extends FragmentActivity {

FragmentAdapter mAdapter;
ViewPager mPager;
PageIndicator mIndicator;
int Number = 0;
private TextView mCalculatorDisplay;
private Calculation mCalculation;
DecimalFormat df = new DecimalFormat("@##,###,###,###");

/** SharedPreferences */
public static final String MyPREFERENCES = "MyPrefs";
SharedPreferences sharedpreferences;
int currentOrientation;

这是我在MainActivity中的onCreate：

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
            WindowManager.LayoutParams.FLAG_FULLSCREEN);

    setContentView(R.layout.activity_main);

    mAdapter = new FragmentAdapter(getSupportFragmentManager());

    mPager = (ViewPager) findViewById(R.id.pager);
    mPager.setAdapter(mAdapter);

    mIndicator = (TitlePageIndicator) findViewById(R.id.indicator);
    mIndicator.setViewPager(mPager);


}

这里是onSaveInstanceState：

    @Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putDouble("OPERAND", mCalculation.getResult());
    outState.putDouble("MEMORY", mCalculation.getMemory());
    outState.putInt("currentOrientation", Integer.valueOf(currentOrientation));

}

这里是onRestoreInstanceState：

    @Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    mCalculation.setOperand(savedInstanceState.getDouble("OPERAND"));
    mCalculation.setMemory(savedInstanceState.getDouble("MEMORY"));
    mCalculatorDisplay.setText(df.format(mCalculation.getResult()));

    currentOrientation = savedInstanceState.getInt("currentOrientation");

}

最后这是我使用旋转按钮的菜单：

    @Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {

    case R.id.menuAbout:
        startActivity(new Intent("com.this.calculation.ABOUT"));
        return true;

        /** Rotation */
    case R.id.menuRotate:
        SharedPreferences preferences = PreferenceManager
                .getDefaultSharedPreferences(this);
        Editor editor = preferences.edit();
        if (getResources().getConfiguration().orientation == Configuration.ORIENTATION_PORTRAIT) {
            setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
            editor.putInt("orientation",
                    ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE);
        } else {
            setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
            editor.putInt("orientation",
                    ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
        }
        editor.commit();

        break;

    }
    return false;
}

除非我尝试使用菜单按钮旋转我的应用程序，否则一切正常。

它给我发了错误：

E / AndroidRuntime（467）：at com.this.calculation.MainActivity.onSaveInstanceState（MainActivity.java:60）

更新日志：

03-08 13:35:50.998: W/dalvikvm(623): threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
03-08 13:35:51.068: E/AndroidRuntime(623): FATAL EXCEPTION: main
03-08 13:35:51.068: E/AndroidRuntime(623): java.lang.NullPointerException
03-08 13:35:51.068: E/AndroidRuntime(623):  at mini.worlld.ecalculation.MainActivity.onSaveInstanceState(MainActivity.java:59)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.Activity.performSaveInstanceState(Activity.java:1113)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.Instrumentation.callActivityOnSaveInstanceState(Instrumentation.java:1185)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3325)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.ActivityThread.access$700(ActivityThread.java:123)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1151)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.os.Looper.loop(Looper.java:137)
03-08 13:35:51.068: E/AndroidRuntime(623):  at android.app.ActivityThread.main(ActivityThread.java:4424)
03-08 13:35:51.068: E/AndroidRuntime(623):  at java.lang.reflect.Method.invokeNative(Native Method)
03-08 13:35:51.068: E/AndroidRuntime(623):  at java.lang.reflect.Method.invoke(Method.java:511)
03-08 13:35:51.068: E/AndroidRuntime(623):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
03-08 13:35:51.068: E/AndroidRuntime(623):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
03-08 13:35:51.068: E/AndroidRuntime(623):  at dalvik.system.NativeStart.main(Native Method)

Answer 1

NPE是因为 mCalculation 变量为null。

在上面的代码中，您已声明它但未初始化它，因此在访问mCalculation.getResult（）时会出现错误。

像这样初始化它，

mCalculation = new Calculation(); //if you dont have any other constructors created

片段中的方向崩溃

1 个答案: