EditText saved the value after device rotation automatically

Because the EditText is a focused view, so in PhoneWindow, it's state will be saved automatically in saveHierarchyState() method. You can see the code:

@Override
public Bundle saveHierarchyState() {
    Bundle outState = new Bundle();
    if (mContentParent == null) {
        return outState;
    }
    SparseArray<Parcelable> states = new SparseArray<Parcelable>();
    mContentParent.saveHierarchyState(states);
    outState.putSparseParcelableArray(VIEWS_TAG, states);
    // save the focused view id
    View focusedView = mContentParent.findFocus();
    if (focusedView != null) {
        if (focusedView.getId() != View.NO_ID) {
            outState.putInt(FOCUSED_ID_TAG, focusedView.getId());
        } else {
            if (false) {
                Log.d(TAG, "couldn't save which view has focus because the focused view "
                        + focusedView + " has no id.");
            }
        }
    }
    // save the panels
    SparseArray<Parcelable> panelStates = new SparseArray<Parcelable>();
    savePanelState(panelStates);
    if (panelStates.size() > 0) {
        outState.putSparseParcelableArray(PANELS_TAG, panelStates);
    }
    if (mActionBar != null) {
        outState.putBoolean(ACTION_BAR_TAG, mActionBar.isOverflowMenuShowing());
    }
    return outState;
}

and the code in TextView:

@Override
public Parcelable onSaveInstanceState() {
    Parcelable superState = super.onSaveInstanceState();
    // Save state if we are forced to
    final boolean freezesText = getFreezesText();
    boolean hasSelection = false;
    int start = -1;
    int end = -1;
    if (mText != null) {
        start = getSelectionStart();
        end = getSelectionEnd();
        if (start >= 0 || end >= 0) {
            // Or save state if there is a selection
            hasSelection = true;
        }
    }
    if (freezesText || hasSelection) {
        SavedState ss = new SavedState(superState);
        if (freezesText) {
            if (mText instanceof Spanned) {
                final Spannable sp = new SpannableStringBuilder(mText);
                if (mEditor != null) {
                    removeMisspelledSpans(sp);
                    sp.removeSpan(mEditor.mSuggestionRangeSpan);
                }
                ss.text = sp;
            } else {
                ss.text = mText.toString();
            }
        }
        if (hasSelection) {
            // XXX Should also save the current scroll position!
            ss.selStart = start;
            ss.selEnd = end;
        }
        if (isFocused() && start >= 0 && end >= 0) {
            ss.frozenWithFocus = true;
        }
        ss.error = getError();
        if (mEditor != null) {
            ss.editorState = mEditor.saveInstanceState();
        }
        return ss;
    }
    return superState;
}

So, if you remove the id of the EditTextView in your xml file:

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.android.a2_screen_orientation_change.MainActivity">

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>

You will see what you want! (Tanks for supplement from @Mike M.)