#include <iostream>

using namespace std;

class User
{
 private:
  int m_id;
 public:
  User(int id=0){m_id = id;}
  void setId(int id){m_id = id;}
  int getId(){return m_id;}
};

void test0(User t){//按值傳參
 User s;
 t = s;
 t.setId(1002);
 cout << "test1:" << t.getId() << endl;
}

void test1(User *t){//按值傳參
 t = new User();//指針指向了一個(gè)新對(duì)象，外面實(shí)參沒變
 t->setId(1002);
 cout << "test1:" << t->getId() << endl;
}

void test2(User* & t){//按引用傳參
 t = new User();//指針指向了一個(gè)新對(duì)象，外面實(shí)參也跟著變了
 t->setId(1002);
 cout << "test2:" << t->getId() << endl;
}

int main(int argc, char const *argv[]) {
 cout<< "\npass by ref:"<<endl;
 User* t = new User();
 t->setId(1001);
 cout << t->getId() << endl;
 test2(t);
 cout << t->getId() << endl;

 cout<< "\npass by value:"<<endl;
 t = new User();
 t->setId(1001);
 cout << t->getId() << endl;
 test1(t);
 cout << t->getId() << endl;
 return 0;
}

pass by ref:
1001
test2:1002
1002

pass by value:
1001
test1:1002
1001

package com.pollyduan.bean;

@Data
public class User {
 private Integer id;

 public static void testObject(User t){
  t=new User();//指向了一個(gè)新對(duì)象，外面實(shí)參沒變
  t.setId(1002);
  System.out.println("testObject="+t);
 }

 @Test
 public void testObject(){
  User user=new User();
  user.setId(1001);
  System.out.println("user="+user);
  testObject(user);
  System.out.println("user="+user);
 }
}

user=User(id=1001)
testObject=User(id=1002)
user=User(id=1001)