解决问题：List集合add元素，添加多个对象出现重复的问题

首先看下面一段代码:

 ArrayList<Person> people = new ArrayList<>();
            Class.forName("com.mysql.jdbc.Driver");
            Connection connection = null;
            Statement statement1  = null;
            ResultSet resultSet = null;
            Person person = null;
            try {
                connection = DriverManager.getConnection("jdbc:mysql://116.196.124.159/shischcool", "root", "root");


                statement1 = connection.createStatement();

                String sql = "select * from student3 ";
                resultSet = statement1.executeQuery(sql);
                person = new Person();
                int anInt = 0;
                String str ;
                while(resultSet.next())
                {
                   
                    anInt = resultSet.getInt(1);
                    person.setId(anInt);
                    anInt = resultSet.getInt(3);
                    person.setAge(anInt);
                    anInt = resultSet.getInt(6);
                    person.setMath(anInt);
                    anInt = resultSet.getInt(7);
                    person.setEnglish(anInt);
                    str = resultSet.getString("name");
                    person.setName(str);
                    str = resultSet.getString("sex");
                    person.setSex(str);
                    str = resultSet.getString("address");
                    person.setAddress(str);

                    System.out.println(person);
                    people.add(person);
                }

                System.out.println(people);

            } catch (SQLException e) {
                e.printStackTrace();
            }finally {
                if(statement1 != null) {
                    try {
                        statement1.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if(connection != null) {
                    try {
                        connection.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
    }

打印的结果是:

[Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}, Person{Id=8, Name='德玛西亚', Age=18, Sex='男', Address='南京', Math=56, English=65}]
看到这个结果我就不乐意了，我明明是想存入8个不一样的Person ，并且我在list的add 之前修改了person的属性值，为什么呢?

原来，list 存储的是引用，

虽然你每次都修改了属性值，并且重新添加到了list里面，但是对于最后一次的操作才是最后的赋值操作，前面的操作都是作废的，因为list去遍历的时候是去找找个引用的地址里面的值，所以这样list里面的引用值是一样的，那值肯定都是最后一次赋值的值咯，

解决方法:

person 变量可以只需要定义一次，但是可以new多次，对应不用的引用地址，这样就不会出现每次的值都是一样的

 ArrayList<Person> people = new ArrayList<>();
            Class.forName("com.mysql.jdbc.Driver");
            Connection connection = null;
            Statement statement1  = null;
            ResultSet resultSet = null;
            Person person = null;
            try {
                connection = DriverManager.getConnection("jdbc:mysql://116.196.124.159/shischcool", "root", "root");


                statement1 = connection.createStatement();

                String sql = "select * from student3 ";
                resultSet = statement1.executeQuery(sql);
                
                int anInt = 0;
                String str ;
                while(resultSet.next())
                {
                   person = new Person();
                    anInt = resultSet.getInt(1);
                    person.setId(anInt);
                    anInt = resultSet.getInt(3);
                    person.setAge(anInt);
                    anInt = resultSet.getInt(6);
                    person.setMath(anInt);
                    anInt = resultSet.getInt(7);
                    person.setEnglish(anInt);
                    str = resultSet.getString("name");
                    person.setName(str);
                    str = resultSet.getString("sex");
                    person.setSex(str);
                    str = resultSet.getString("address");
                    person.setAddress(str);

                    System.out.println(person);
                    people.add(person);
                }

                System.out.println(people);

            } catch (SQLException e) {
                e.printStackTrace();
            }finally {
                if(statement1 != null) {
                    try {
                        statement1.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
                if(connection != null) {
                    try {
                        connection.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
    }