Custom menu icon not displaying

I am trying the implement the custom menu shown here instead i just get a list of items alone without any icon. All the icons required to are placed in the correct folder and yet it doesn't work. Please can some tell me wat i'm doing wrong. My code is shown below and thanks in advance.

MeunTest.java

public class MenuTest extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_menu_test);
}

// Initiating Menu XML file (menu.xml)
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
    MenuInflater menuInflater = getMenuInflater();
    menuInflater.inflate(R.menu.main, menu);
    return true;
}

/**
 * Event Handling for Individual menu item selected
 * Identify single menu item by it's id
 * */
@Override
public boolean onOptionsItemSelected(MenuItem item)
{

    switch (item.getItemId())
    {
        case R.id.menu_bookmark:
            // Single menu item is selected do something
            // Ex: launching new activity/screen or show alert message
            Toast.makeText(MenuTest.this, "Bookmark is Selected", Toast.LENGTH_SHORT).show();
            return true;

        case R.id.menu_save:
            Toast.makeText(MenuTest.this, "Save is Selected", Toast.LENGTH_SHORT).show();
            return true;

        case R.id.menu_search:
            Toast.makeText(MenuTest.this, "Search is Selected", Toast.LENGTH_SHORT).show();
            return true;

        case R.id.menu_share:
            Toast.makeText(MenuTest.this, "Share is Selected", Toast.LENGTH_SHORT).show();
            return true;

        case R.id.menu_delete:
            Toast.makeText(MenuTest.this, "Delete is Selected", Toast.LENGTH_SHORT).show();
            return true;

        case R.id.menu_preferences:
            Toast.makeText(MenuTest.this, "Preferences is Selected", Toast.LENGTH_SHORT).show();
            return true;

        default:
            return super.onOptionsItemSelected(item);
    }
}
 }

menu.xml

<?xml version="1.0" encoding="utf-8"?>
  <menu xmlns:android="http://ift.tt/nIICcg">
<!-- Single menu item
     Set id, icon and Title for each menu item
-->
<item android:id="@+id/menu_bookmark"
    android:icon="@drawable/icon_bookmark"
    android:title="Bookmark" />
<item android:id="@+id/menu_save"
    android:icon="@drawable/icon_save"
    android:title="Save" />
<item android:id="@+id/menu_search"
    android:icon="@drawable/icon_search"
    android:title="Search" />
<item android:id="@+id/menu_share"
    android:icon="@drawable/icon_share"
    android:title="Share" />
<item android:id="@+id/menu_delete"
    android:icon="@drawable/icon_delete"
    android:title="Delete" />
<item android:id="@+id/menu_preferences"
    android:icon="@drawable/icon_preferences"
    android:title="Preferences" />
 </menu>