Android studioh建立SubMenu显示菜单栏

Android studioh建立SubMenu运行不显示菜单栏

img


上图是运行截图，不知道怎么修改才能显示子菜单栏 。

这是Java文件

public class submenu extends AppCompatActivity {
    /** Called when the activity is first created. */
    private Button button;
    private TextView textView;
    @Override
    public void onCreate( Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_activity);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.sub_menu, menu);
        return true;
    }
    public boolean onOptionsItemSelected(MenuItem item){
        TextView label=(TextView) findViewById(R.id.label);
            if(item.getItemId()==R.id.main_menu_0){
                label.setText("设置"+item.getItemId());
                return true;}
            if (item.getItemId()==R.id.main_menu_0_0){
                label.setText("打印"+item.getItemId());
                return true;}
            if (item.getItemId()==R.id.main_menu_1){
                label.setText("新建"+item.getItemId());
                return true;}
            if (item.getItemId()==R.id.main_menu_1_0){
                label.setText("邮件"+item.getItemId());
                return true;}
            if (item.getItemId()==R.id.main_menu_1_1){
                label.setText("订阅"+item.getItemId());
                return true; }
        return true;
    }

}

这是menu.xml文件

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/main_menu_0"

        android:title="@string/action_setting">
        <menu>
            <item
                android:id="@+id/main_menu_0_0"

                android:title="@string/action_print" />
        </menu>

    </item>
    <item
        android:id="@+id/main_menu_1"

        android:title="@string/action_new">
        <menu>
            <item
                 android:id="@+id/main_menu_1_0"

                 android:title="@string/action_email"/>
            <item
                 android:id="@+id/main_menu_1_1"

                 android:title="@string/action_look"/>
        </menu>
    </item>
</menu>

这是layout下的main_activity.xml文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical">
    <TextView
        android:id="@+id/label"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello"/>
</LinearLayout>

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