发表于： 2017-07-29 17:26:29
2 860
今天完成：
定名常量：named constant 表示从不改变的永久数据
当需要频繁使用一个常量时，可以用named constant
final datatype CONSTANTNAME = VALUE;
public class Area_1 {
public static void main(String[] args) {
final double PI = 3.14159 ;
double radius = 20 ;
double area = radius*radius*PI;
System.out.println("The area for the circle of radius "+radius+" is "+area);
}
}
查找数组中元素
import java.util.Random;
import java.util.Scanner;
public class findData_1 {
static int N = 20;
public static void main(String[] args) {
int[] arr = new int[N];
int x;
int f = -1;
Random r = new Random();
for (int i = 0; i < N; i++) {
arr[i] = r.nextInt(100);
}
System.out.println("随机生成的数据序列：");
for (int i = 0; i < N; i++) {
System.out.print(arr[i] + " ");
}
System.out.print("\n\n");
for(int j = 0 ; j < 3 ; j++) {
System.out.print("输入要查找的整数：");
@SuppressWarnings("resource")
Scanner input = new Scanner(System.in);
x = input.nextInt(); // 输入要查找的数
for (int i = 0; i < N; i++) {
f = -1;
if (x == arr[i]) {
f = i;
System.out.println("数据" + x + "位于数组的第" + (f + 1) + "个元素处。");
}
}
if (f < 0) {
System.out.println("没找到数据:" + x);
}
}
}
}
在if循环外，for循环内加上一句：
f = -1;
程序正常运行
明天计划：继续学习Spring+JDBC 调试程序 改BUG
遇到的问题：
Spring+jdbc:
项目结构：
部分代码：
package com.jinhege.customer.dao.impl;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.sql.DataSource;
import com.jinhege.customer.D.CustomerDAO;
import com.jinhege.customer.model.Customer;
public class JdbcCustomerDao implements CustomerDAO
{
private DataSource dataSource;
public void setDataSource(DataSource dataSource) {
this.dataSource = dataSource;
}
public void insert(Customer customer) {
String sql = "INSERT INTO CUSTOMER" + "(CUST_ID,NAME,AGE)VALUES(?,?,?)";
Connection conn = null;
try {
conn = dataSource.getConnection();
PreparedStatement ps = conn.prepareStatement(sql);
ps.setInt(1, customer.getCustId());
ps.setString(2, customer.getName());
ps.setInt(3, customer.getAge());
ps.executeUpdate();
//ps.close();
} catch (SQLException e) {
throw new RuntimeException(e);
} finally {
if (conn != null) {
try {
conn.close();
} catch (SQLException e) {
}
}
}
}
public Customer findByCustomerId(int custId) {
String sql = "SELECT *FROM CUSTOMER WHERE CUST_ID = ?";
Connection conn = null;
try {
conn = dataSource.getConnection();
PreparedStatement ps = conn.prepareStatement(sql);
ps.setInt(1, custId);
Customer customer = null;
ResultSet rs = ps.executeQuery();
if (rs.next()) customer = new Customer(
rs.getInt("CUST_ID"),
rs.getString("NAME"),
rs.getInt("Age")
);
rs.close();
ps.close();
return customer;
} catch (SQLException e) {
throw new RuntimeException(e);
} finally {
if (conn != null) {
try {
conn.close();
} catch (SQLException e) {
}
}
}
}
}
Console:
改了一下午没有改出来，网上找了很多资料，对比感觉应该没问题才对。明天再继续研究看看吧
七月 29, 2017 5:17:48 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@2e0fa5d3: startup date [Sat Jul 29 17:17:48 CST 2017]; root of context hierarchy
七月 29, 2017 5:17:48 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [application.xml]
七月 29, 2017 5:17:50 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [database/Spring-Datasource.xml]
七月 29, 2017 5:17:50 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [customer/Spring-Customer.xml]
七月 29, 2017 5:17:51 下午 org.springframework.jdbc.datasource.DriverManagerDataSource setDriverClassName
信息: Loaded JDBC driver: com.mysql.jdbc.Driver
Exception in thread "main" java.lang.RuntimeException: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'name' cannot be null
at com.jinhege.customer.dao.impl.JdbcCustomerDao.insert(JdbcCustomerDao.java:31)
at com.jinhege.common.App.main(App.java:16)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'name' cannot be null
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425)
at com.mysql.jdbc.Util.getInstance(Util.java:408)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:935)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3973)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3909)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2527)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2680)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2490)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1858)
at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(PreparedStatement.java:2079)
at com.mysql.jdbc.PreparedStatement.executeUpdateInternal(PreparedStatement.java:2013)
at com.mysql.jdbc.PreparedStatement.executeLargeUpdate(PreparedStatement.java:5104)
at com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:1998)
at com.jinhege.customer.dao.impl.JdbcCustomerDao.insert(JdbcCustomerDao.java:28)
... 1 more
Process finished with exit code 1
注释这句后，程序可以运行，估计插入数据部分有问题，明天再看看
//ps.executeUpdate();
package com.jinhege.customer.dao.impl;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.sql.DataSource;
import com.jinhege.customer.D.CustomerDAO;
import com.jinhege.customer.model.Customer;
public class JdbcCustomerDao implements CustomerDAO
{
private DataSource dataSource;
public void setDataSource(DataSource dataSource) {
this.dataSource = dataSource;
}
public void insert(Customer customer) {
String sql = "INSERT INTO CUSTOMER" + "(CUST_ID,NAME,AGE)VALUES(?,?,?)";
Connection conn = null;
try {
conn = dataSource.getConnection();
PreparedStatement ps = conn.prepareStatement(sql);
ps.setInt(1, customer.getCustId());
ps.setString(2, customer.getName());
ps.setInt(3, customer.getAge());
//ps.executeUpdate();
ps.close();
} catch (SQLException e) {
throw new RuntimeException(e);
} finally {
if (conn != null) {
try {
conn.close();
} catch (SQLException e) {
}
}
}
}
public Customer findByCustomerId(int custId) {
String sql = "SELECT *FROM CUSTOMER WHERE CUST_ID = ?";
Connection conn = null;
try {
conn = dataSource.getConnection();
PreparedStatement ps = conn.prepareStatement(sql);
ps.setInt(1, custId);
Customer customer = null;
ResultSet rs = ps.executeQuery();
if (rs.next()) customer = new Customer(
rs.getInt("CUST_ID"),
rs.getString("NAME"),
rs.getInt("Age")
);
rs.close();
ps.close();
return customer;
} catch (SQLException e) {
throw new RuntimeException(e);
} finally {
if (conn != null) {
try {
conn.close();
} catch (SQLException e) {
}
}
}
}
}
收获：
通常，变量用大写字母命名：PI 而非pi或Pi
使用常量的优点：
a,不必重复输入同一个值
b,如果必须修改常量值，只需在源代码中一个地方做改动
c,给常量一个描述性的名字会提高程序易读性
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