Support Map<String, String> & Map<String, MyString>

The following programs work but I like to combine MyComboBox &
MyComboBox2 into one class so that both Map<String, String> &
Map<String, MyString> data types are supported in the single combined
class.
How to do that?

public class MyComboBox extends LangComboBox implements
PropertyChangeListener {
  protected EventListenerList listenerList = new EventListenerList();
  private Set keySet = Collections.EMPTY_SET;

  public MyComboBox(Map items) {
        super(items.values().toArray());
        keySet = items.keySet();
  }
  //...
}

public class MyComboBox2 extends LangComboBox implements
PropertyChangeListener {
  protected EventListenerList listenerList = new EventListenerList();
  private Set keySet = Collections.EMPTY_SET;

  public MyComboBox2(Map<String, MyString> items) {
        super(MyStringUtil.sort(items).values().toArray());
        Map<String, MyString> itemsSorted = MyStringUtil.sort(items);
        keySet = itemsSorted.keySet();
  }
  //...
}

public class MyStringUtil
       public static Map<String, MyString> sort(
             Map<String, MyString> map) {
             //...
       }
}

// Calling program
Map<String, String> map1 = getdata("data1");
MyComboBox box1 = new MyComboBox(map1); // this is ok
Map<String, MyString> map2 = getdata2("data2");
MyComboBox2 box2 = new MyComboBox2(map2); // this is ok

MyString is a third party class which I don't have the source code. It
has the toString() method.
It does not implement CharSequence.
MyComboBox cannot have two constructors because of type erasure of the
generic type java.util.Map, the two methods are considered the same:
public MyComboBox(Map<String, String> items) {
   //...
   }
public MyComboBox(Map<String, MyString> items) {
      //...
}
// These are the compile errors:
Duplicate method MyComboBox(Map<String,String>) in type MyComboBox
client/swing/components MyComboBox.java
Duplicate method MyComboBox(Map<String,MyString>) in type MyComboBox
client/swing/components MyComboBox.java

The following code gets ClassCastException.
Map<String, String> map1 = getdata("data1");
// this will cause casting exception - Exception in thread "AWT-
EventQueue-0"
// java.lang.ClassCastException: java.lang.String cannot be cast to
mycom.lang.MyString
// at myproj.client.swing.util.MyStringUtil
$MapComparable.compare(MyStringUtil.java:25)
MyComboBox2 box1 = new MyComboBox2(map1);