java 操作实例

1、无重复字符的最长子串

输入: "abcabcbb"
输出: 3
解释: 因为无重复字符的最长子串是 "abc"，所以其长度为 3。

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int max = 0;
        int curNoStrIndexStart = 0;
        Map<String,Integer> map = new HashMap<>();
        for (int i=0;i < s.length();i++) {
            String index = String.valueOf(s.charAt(i));
            if (map.containsKey(index)) {
                curNoStrIndexStart = Math.max(map.get(index),curNoStrIndexStart);
            }
            max = Math.max(max,i - curNoStrIndexStart +1);
            map.put(index,i + 1);
        }
        return max;
    }
}

2、mysql获取表的元数据信息

import java.io.*;
import java.sql.*;
import java.util.*;
 
public class DBEntityInit {
 
    private static String propName = "db.properties";
 
    private static Connection connection = null;
 
    private static Map<String, List<Column>> tableData = new HashMap<>();
 
    static {
       connection = getConnection(propName);
    }
 
    public static Connection getConnection(String propName){
        Connection conn = null;
        try {
            InputStream in =  new FileInputStream(new File(propName));
            Properties properties = new Properties();
            properties.load(in);
            String url = properties.getProperty("jdbc.url");
            String username = properties.getProperty("jdbc.username");
            String driver = properties.getProperty("jdbc.driver");
            String password = properties.getProperty("jdbc.password");
            Class.forName(driver);
            conn = DriverManager.getConnection(url,username,password);
        } catch (SQLException | ClassNotFoundException | IOException e) {
            e.printStackTrace();
        }
        return conn;
    }
 
    public static void initTableData() {
        ResultSet tables = null;
        try {
            DatabaseMetaData metaData = connection.getMetaData();
            tables = metaData.getTables(null, null, null, new String[]{"TABLE"});
            while (tables.next()) {
                tableData.put(tables.getString(3),new ArrayList<>());
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (tables != null) {
                    tables.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static void initTableColumn(String tableName) {
        List<Column> columns = tableData.get(tableName);
        PreparedStatement pst = null;
        try {
            String sql = "select * from " + tableName;
            pst = connection.prepareStatement(sql);
            ResultSetMetaData metaData = pst.getMetaData();
            int columnCount = metaData.getColumnCount();
            for (int i=0 ; i < columnCount; i++) {
                Column column = new Column();
                column.setName(metaData.getColumnName(i+1));
                column.setType(metaData.getColumnTypeName(i+1));
                columns.add(column);
            }
            //tableData.put(tableName,columns);
 
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (pst != null) {
                    pst.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static void initComment(String tableName) {
        //与数据库的连接
        PreparedStatement pst = null;
        String tableSql = "select * from" + tableName;
        List<String> comments = new ArrayList<>();//列名注释集合
        ResultSet rs = null;
        List<Column> columns = tableData.get(tableName);
        try {
            pst = connection.prepareStatement(tableSql);
            rs = pst.executeQuery("show full columns from " + tableName);
            while (rs.next()) {
                String comment = rs.getString("Comment");
                comments.add(comment);
            }
 
            for (int i=0 ;i < columns.size() ;i++) {
                Column column = columns.get(i);
                column.setComment(comments.get(i));
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
 
        }
    }
 
    public static void closeConnection(Connection conn) {
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
 
    public static Map<String, List<Column>> init() {
        initTableData();
        for (String key : tableData.keySet()) {
            initTableColumn(key);
            initComment(key);
        }
        closeConnection(connection);
        return tableData;
    }
 
    public static String getPropName() {
        return propName;
    }
 
    public static void setPropName(String propName) {
        DBEntityInit.propName = propName;
    }
 
    public static void main(String[] args) {
 
        init();
        for (Map.Entry<String,List<Column>> entry : tableData.entrySet()) {
            List<Column> value = entry.getValue();
            System.out.println("--------------表名" + entry.getKey());
            for (Column column : value) {
                System.out.print(column.getName() + "---");
                System.out.print(column.getType() + "---");
                System.out.print(column.getComment());
                System.out.println();
            }
        }
 
    }
 
//    public static void initColumnComment(String tableName) {
//
//        try {
//            List<Column> cList = tableData.get(tableName);
//            DatabaseMetaData metaData = connection.getMetaData();
//            ResultSet columns = metaData.getColumns(null, null, tableName, "%");
//
//            while (columns.next()) {
//                Column column = new Column();
//                String  name = columns.getString("COLUMN_NAME");
//                String  type = columns.getString("TYPE_NAME");
//                String  comment = columns.getString("REMARKS");
//                column.setName(name);
//                column.setType(type);
//                column.setComment(comment);
//                cList.add(column);
//            }
//        } catch (SQLException e) {
//            e.printStackTrace();
//        }
//
//    }
 
}

相关实体

public class Column {
 
    private String name;
    private String type;
    private String comment;
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        name = name;
    }
 
    public String getType() {
        return type;
    }
 
    public void setType(String type) {
        this.type = type;
    }
 
    public String getComment() {
        return comment;
    }
 
    public void setComment(String comment) {
        this.comment = comment;
    }
}

3、日期的加减操作

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
 
public class DateCalculate {
 
    private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
 
    /**
     * 获取形如yyyy-MM-dd HH:mm:ss
     * @param date
     * @return
     */
    public static String datetimeToString(Date date) {
        return sdf.format(date);
    }
 
    /**
     * 根据时间字符串获取日期
     * @param dateString
     * @return
     * @throws ParseException
     */
    public static Date stringToDatetime(String dateString) throws ParseException {
        return sdf.parse(dateString);
    }
 
    /**
     * 获取本月最后一天
     * @return
     */
    public static Date getMonthStartDate(Date date) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.set(Calendar.DAY_OF_MONTH,1);
        return calendar.getTime();
    }
 
    /**
     * 获取本月最后一天
     * @return
     */
    public static Date getMonthEndDate(Date date) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.set(Calendar.DAY_OF_MONTH,calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
        return calendar.getTime();
    }
 
    /**
     * 获取指定日期所属周的开始时间
     * @param date
     * @return
     */
    public static Date getBeginWeekDate(Date date) {
        Calendar cal = Calendar.getInstance();
        cal.setTime(date);
        int dayOfWeek = cal.get(Calendar.DAY_OF_WEEK);
        if (dayOfWeek == 1) {
            dayOfWeek += 7;
        }
        cal.add(Calendar.DATE,2 - dayOfWeek);
        return cal.getTime();
    }
 
    /**
     * 距离指定日期所属周结束时间
     * @return
     */
    public static Date getEndWeekDate(Date date) {
        Calendar cal = Calendar.getInstance();
        cal.setTime(date);
        int dayOfWeek = cal.get(Calendar.DAY_OF_WEEK);
        if (dayOfWeek == 1) {
            dayOfWeek += 7;
        }
        cal.add(Calendar.DATE,8 - dayOfWeek);
        return cal.getTime();
    }
 
    /**
     * 对指定日期进行年份加减操作
     * @param date
     * @param num
     * @return
     */
    public static Date calculateDateOfYear(Date date,Integer num) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.YEAR,num);
        return calendar.getTime();
    }
 
    /**
     * 对指定日期月份进行加减操作
     * @param date
     * @param num
     * @return
     */
    public static Date calculateDateOfMonth(Date date,Integer num) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.MONTH,num);
        return calendar.getTime();
    }
 
    /**
     * 对指定日期天数进行加减操作
     * @param date
     * @param num 负整数 正整数
     * @return
     */
    public static Date calculateDateOfDay(Date date,Integer num) {
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(date);
        calendar.add(Calendar.DAY_OF_MONTH,num);
        return calendar.getTime();
    }
 
    public static void main(String[] args) throws ParseException {
 
        System.out.println(datetimeToString(getMonthStartDate(sdf.parse("2019-12-04 12:09:52"))));
        System.out.println(datetimeToString(getEndWeekDate(sdf.parse("2019-12-04 12:09:52"))));
        System.out.println(datetimeToString(calculateDateOfYear(stringToDatetime("2019-12-04 12:09:52"),-2)));
    }
}