如何将项目添加到顶部ActionBar？

<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/action_example"
        android:icon="@drawable/ic_example"
        android:title="@string/example_title"
        app:showAsAction="ifRoom"/>
</menu>

import android.view.Menu;
import android.view.MenuInflater;

// ...

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.menu_main, menu);
    return true;
}

import android.view.MenuItem;

// ...

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_example) {
        // Do something when the user clicks on the example item
        return true;
    }

    return super.onOptionsItemSelected(item);
}