java 程序 与数据库连接问题

Question

public class Test11 {
public static final String DRIVER = "com.mysql.jdbc.Driver";
public static final String URL = "jdbc:mysql://localhost:3306/MySQL";
public static final String USERNAME = "root";
public static final String PASSWORD = "saber";

Connection connection = null;
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;

public Connection getConnection() throws Exception {
    Class.forName(DRIVER);
    connection = DriverManager.getConnection(URL,USERNAME,PASSWORD);
    System.out.println("连接成功");
    return connection;
}

public ResultSet executeQuery(String sql) throws Exception {
    connection = this.getConnection();
    preparedStatement = connection.prepareStatement(sql);
    resultSet = preparedStatement.executeQuery();
    return resultSet;

}

public int executeUpdate(String sql,Object[] obj) throws Exception {
    connection = this.getConnection();
    preparedStatement = connection.prepareStatement(sql);
    for(int i =0;i<obj.length;i++){
        preparedStatement.setObject(i+1, obj[i]);
    }
    return preparedStatement.executeUpdate();
}  //这一段代码能不能帮忙解释一下，这样写的作用。不是很理解~

public void closeAll() throws Exception {
    if(null != resultSet){
        resultSet.close();
    }
    if(null != preparedStatement){
        preparedStatement.close();
    }
    if(null != connection){
        connection.close();
    }
}

新手，有点不懂这段代码我标的部分的意义，请逐句解释下，求指导。