java 笔记（四）

书接上文

41.关于形参

public class Test {static void ParTest1(int a,char b,int[] c,char[] d,String e,String [] f) {a = 2;b = 'a';c[1] = 4;d[1] = 's';e = "ZXcZXcZX";f[1] = "adsfasd";} public void ParTest2(int a,char b,int[] c,char[] d,String e,String [] f) {a = 2;b = 'a';c[1] = 4;d[1] = 's';e = "ZXcZXcZX";f[1] = "adsfasd";} public static void main(String[] args) {// TODO Auto-generated method stubint a = 1;char b = 'c';int [] c = {1,2,3};char [] d = {'s','h','l'};String str = "a string";String [] strarr = {"first","second","third"};ParTest1(a, b, c, d, str, strarr);System.out.println(a+" "+b+" "+c[1]+" "+d[1]+" "+str+" "+strarr[1]);//Test t = new Test();//t.ParTest2(a, b, c, d, str, strarr);//System.out.println(a+" "+b+" "+c[1]+" "+d[1]+" "+str+" "+strarr[1]);}}

1 c 4 s a string adsfasd

42.在做项目时，经常通过继承来简化和规范编程，因为经常要从数据库中提取数据，所以经常把数据库中的一个个表在项目中定义成实体类，其中有一个BaseEntity，被其他实体继承，其中包括每个表中共有的字段，比如修改时间，修改人等

package com.Entity;import java.util.Date;public abstract class BaseEntity {private String uId;//主键，用户IDprivate Date creationDate;//信息录入时间private Date modifiedDate;//最后一次修改的时间private String modifiedUserId;//最后一次修改的人的IDpublic String getuId() {return uId;}public void setuId(String uId) {this.uId = uId;}public Date getCreationDate() {return creationDate;}public void setCreationDate(Date creationDate) {this.creationDate = creationDate;}public Date getModifiedDate() {return modifiedDate;}public void setModifiedDate(Date modifiedDate) {this.modifiedDate = modifiedDate;}public String getModifiedUserId() {return modifiedUserId;}public void setModifiedUserId(String modifiedUserId) {this.modifiedUserId = modifiedUserId;}}

package com.Entity;public class T_Tel extends BaseEntity {private String telId;private String telNumber;public String getTelId() {return telId;}public void setTelId(String telId) {this.telId = telId;}public String getTelNumber() {return telNumber;}public void setTelNumber(String telNumber) {this.telNumber = telNumber;}}

package com.Entity;public class T_User extends BaseEntity {private String firstName;private String lastName;public String getFirstName() {return firstName;}public void setFirstName(String firstName) {this.firstName = firstName;}public String getLastName() {return lastName;}public void setLastName(String lastName) {this.lastName = lastName;}}

package com.Dao;import com.Entity.BaseEntity;public abstract class BaseDao <T extends BaseEntity>{public abstract void insert(T t);public abstract void update(T t);public abstract T selectByPK(long PK);public abstract void delete(T t);public abstract void deleteByPK(long PK);}

package com.Dao;import com.Entity.T_Tel;public class TelDao extends BaseDao<T_Tel> {@Overridepublic void insert(T_Tel t) {// TODO Auto-generated method stub}@Overridepublic void update(T_Tel t) {// TODO Auto-generated method stub}@Overridepublic T_Tel selectByPK(long PK) {// TODO Auto-generated method stubreturn null;}@Overridepublic void delete(T_Tel t) {// TODO Auto-generated method stub}@Overridepublic void deleteByPK(long PK) {// TODO Auto-generated method stub}}

package com.Dao;import com.Entity.T_User;public class UserDao extends BaseDao<T_User> {@Overridepublic void insert(T_User t) {// TODO Auto-generated method stub}@Overridepublic void update(T_User t) {// TODO Auto-generated method stub}@Overridepublic T_User selectByPK(long PK) {// TODO Auto-generated method stubreturn null;}@Overridepublic void delete(T_User t) {// TODO Auto-generated method stub}@Overridepublic void deleteByPK(long PK) {// TODO Auto-generated method stub}}

43.之前总是在想，学会了sql,我要怎么做到通过java代码从数据库提取出代码并显示在控制台，因为做网页或者很多项目经常需要把数据进行增删改查，并显示在网页中，别忘了需要导入connection的jar包

employee表是这个样子的

import java.sql.Connection;import java.sql.DriverManager;import java.sql.PreparedStatement;import java.sql.ResultSet;import java.sql.SQLException;public class GetInfoFroDB {public static void main(String[] args) {// TODO Auto-generated method stubString url = "jdbc:mysql://localhost/activity";String user = "root";String password = "root";try {Class.forName("com.mysql.jdbc.Driver");Connection conn = DriverManager.getConnection(url, user, password);PreparedStatement pstmt = conn.prepareStatement("select firstname,lastname from employee where id > ?");pstmt.setString(1, "3");ResultSet rs = pstmt.executeQuery();while (rs.next()) {System.out.println(rs.getString(1) + " " + rs.getString(2));}pstmt.close();conn.close();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (SQLException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}