java 由身份证号生成出生日期

package com.hrms.util;
public class HrmsMath {
     public static String getBirthday(String cardID) {
    String birth = null;
    StringBuffer tempStr=null;
        if(cardID!=null&&cardID.trim().length()>0){
                if(cardID.trim().length()==15){ //2000年以前出生的
                        tempStr=new StringBuffer(cardID.substring(6, 12));
                        tempStr.insert(4, '-');
                        tempStr.insert(2, '-');
                        tempStr.insert(0, "19");
                }else if(cardID.trim().length()==18){
                        tempStr=new StringBuffer(cardID.substring(6, 14));
                        tempStr.insert(6, '-');
                        tempStr.insert(4, '-');
                }                       
        }
        if(tempStr!=null&&tempStr.toString().trim().length()>0){
        birth = tempStr.toString();
        }
    return birth ;
    }
}