Ralph's answer is fabulous. I would rather use StringBuilder class to build/decorate the String because the usage of it is more look like Builder pattern.
public String decorateTheString(String orgStr){
StringBuilder builder = new StringBuilder();
builder.append(orgStr);
builder.deleteCharAt(orgStr.length()-1);
builder.insert(0,builder.hashCode());
return builder.toString();
}
It can be use as a helper/builder to build the String, not the String itself.