Object o; Object[] params; String myString;
switch(myString){
case "ClassA": o = new ClassA(params);
case "ClassB": o = new ClassB(params);
//ETC
}

Object o; Object[] params; String myString;
HashMap<String, Class<?>> map;
o = new map.get(myString)(params); //Obviously doesnt work

Object o; Object[] params; String myString;
HashMap<String, Class<?>> map;
o = map.get(myString).getConstructors()[0].newInstance(params);