Java - Warning de un vector

este es mi codigo:
public Integer minClave(Integer serie) {
Hashtable t;

if (tablasSeries.containsKey(serie))
t=(Hashtable) tablasSeries.get(serie);
else{
return -9999999;
}


Vector v = new Vector(t.keySet()); <- Warning
Collections.sort(v); <- Warning
if (v.size() == 0)
return 9999;
else
return (Integer)v.get(0);
}

he intentado hacer lo que me dices pero cuando lo hago en la linea de return me da error puesto que los tipos son incompatibles.

la variable tablasSeries es:

Hashtable <Integer, Hashtable<Object, Object> > tablasSeries;

¿Qué warnings te da?

aqui tienes los warning

316: warning: [unchecked] unchecked call to Vector(java.util.Collection<? extends E>) as a member of the raw type java.util.Vector
Vector v = new Vector(t.keySet());

317: warning: [unchecked] unchecked conversion found : java.util.Vector required: java.util.List<T>
Collections.sort(v);

317: warning: [unchecked] unchecked method invocation: <T>sort(java.util.List<T>) in java.util.Collections is applied to (java.util.Vector)
Collections.sort(v);

350: warning: [unchecked] unchecked call to Vector(java.util.Collection<? extends E>) as a member of the raw type java.util.Vector
Vector v = new Vector(t.keySet());

:351: warning: [unchecked] unchecked conversion found : java.util.Vector
required: java.util.List<T>
Collections.sort(v);

351: warning: [unchecked] unchecked method invocation: <T>sort(java.util.List<T>) in java.util.Collections is applied to (java.util.Vector)
Collections.sort(v);
6 warnings