Mybatis接口Mapper内的方法为啥不能重载吗

public class User {
 private Integer id;
 private String name;
 private int age;

 public User(Integer id, String name, int age) {
  this.id = id;
  this.name = name;
  this.age = age;
 }
 // getter setter
}

public interface UserMapper {
 public User getUserById(Integer id); 
}

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class MapperProxy implements InvocationHandler {

 @SuppressWarnings("unchecked")
 public <T> T newInstance(Class<T> clz) {
  return (T) Proxy.newProxyInstance(clz.getClassLoader(), new Class[] { clz }, this);
 }

 @Override
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  if (Object.class.equals(method.getDeclaringClass())) {
   try {
    // 诸如hashCode()、toString()、equals()等方法，将target指向当前对象this
    return method.invoke(this, args);
   } catch (Throwable t) {
   }
  }
  // 投鞭断流
  return new User((Integer) args[0], "zhangsan", 18);
 }
}

public static void main(String[] args) {
 MapperProxy proxy = new MapperProxy();

 UserMapper mapper = proxy.newInstance(UserMapper.class);
 User user = mapper.getUserById(1001);

 System.out.println("ID:" + user.getId());
 System.out.println("Name:" + user.getName());
 System.out.println("Age:" + user.getAge());

 System.out.println(mapper.toString());
}

public static void main(String[] args) {
  SqlSession sqlSession = MybatisSqlSessionFactory.openSession();
  try {
   StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
   List<Student> students = studentMapper.findAllStudents();
   for (Student student : students) {
    System.out.println(student);
   }
  } finally {
   sqlSession.close();
  }
 }

public interface StudentMapper {
 List<Student> findAllStudents();
 Student findStudentById(Integer id);
 void insertStudent(Student student);
}

public class MapperProxy<T> implements InvocationHandler, Serializable {

 private static final long serialVersionUID = -6424540398559729838L;
 private final SqlSession sqlSession;
 private final Class<T> mapperInterface;
 private final Map<Method, MapperMethod> methodCache;

 public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
  this.sqlSession = sqlSession;
  this.mapperInterface = mapperInterface;
  this.methodCache = methodCache;
 }

 @Override
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  if (Object.class.equals(method.getDeclaringClass())) {
   try {
    return method.invoke(this, args);
   } catch (Throwable t) {
    throw ExceptionUtil.unwrapThrowable(t);
   }
  }
  // 投鞭断流
  final MapperMethod mapperMethod = cachedMapperMethod(method);
  return mapperMethod.execute(sqlSession, args);
 }
 // ...

public class MapperProxyFactory<T> {

 private final Class<T> mapperInterface;

 @SuppressWarnings("unchecked")
 protected T newInstance(MapperProxy<T> mapperProxy) {
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
 }

public User getUserById(Integer id);
public User getUserById(Integer id, String name);