int sum=0;
int os=0;
int js=0;
for(int b=0;b<=100;b++,sum+=b){
    if(b%2==0){
        os+=b;
    }else{
        js+=b;
    }
    //System.out.println(b);
}
System.out.println(""+sum);
System.out.println(""+os);
System.out.println(""+js);

请问老师为什么b=0求和输出结果是5151，而b=1结果输出是5050？

#with上下句管理
with open("d.txt","w",encoding="utf-8") as f:
    s="sxt"
    f.write(s)

"E:\xuan\新建文件夹 (2)\python.exe" E:/pycharm/pythonProject/11.py

SyntaxError: Non-UTF-8 code starting with '\xd3' in file E:\pycharm\pythonProject\11.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details

老师 这为什么会报错啊

老师病毒信息这一部分的数据都没有了，这个是下载下来的源码，我自己那边访问也没有这一部分数据

老师，我创建了非空约束，但是在输出的时候在建立非空约束哪里不输入内容保存时系统不会报错就直接保存了是怎么回事？

package Test;

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * 接收客户端消息的线程类
 */
class ChatReceive extends Thread{
    private Socket socket;

    public ChatReceive(Socket socket) {
        this.socket = socket;
    }


    @Override
    public void run() {
        this.receiveMsg();
    }
    /**
     * 实现接收客户端发送的消息
     */
    private void receiveMsg(){
        BufferedReader br=null;
        try{
            br=new BufferedReader(new InputStreamReader(this.socket.getInputStream()));
            while (true){
                String msg=br.readLine();
                synchronized ("abc"){
                    //把读取到的数据写入公共数据区
                    ChatRoomServer.buf="["+this.socket.getInetAddress()+"]"+msg;
                    "abc".notifyAll();

                }
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            if (br!=null){
                try {
                    br.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
         
            if (socket!=null){
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

        }
    }
}

/**
 * 向客户端发送消息的线程类
 */
class ChatSend extends Thread{
    private Socket socket;

    public ChatSend(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        this.SendMsg();
    }
    /**
     * 将公共数据区的数据发送到客户端
     */
    private void SendMsg(){
        PrintWriter pw=null;
        try{
            pw=new PrintWriter(this.socket.getOutputStream());
            while (true){
                synchronized("abc"){
                    //让线程处于等待状态
                    "abc".wait();
                    pw.println(ChatRoomServer.buf);
                    pw.flush();

                }
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            if (pw!=null){
                pw.close();
            }
            if (this.socket!=null){
                try {
                    socket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}
public class ChatRoomServer {
    public static  String buf;
    public static void main(String[] args) {
        System.out.println(" 1.0");
        System.out.println("Listen at 8888");
        ServerSocket serverSocket=null;
        try {
            serverSocket=new ServerSocket(8888);
            while (true){
                Socket socket=serverSocket.accept();
                System.out.println("连接到 "+socket.getInetAddress());
                new ChatReceive(socket).start();
                new ChatSend(socket).start();
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            if (serverSocket!=null){
                try {
                    serverSocket.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

老师我这里代码怎么报错，没找到问题

为啥 我这反射0,普通方法252- -,不是反射比普通方法运行慢吗

package com.a.cx;

import java.lang.reflect.Method;

/**
 * 测试反射机制的效率
 */
public class TestReflectDemo2 {
    public static void main(String[] args) {
        try {
            //反射耗时
            Class clazz = Class.forName("com.a.cx.Users");
            //newInstance() 返回的是Object类型的结果,这里强制类型转换为Users类型
            Users users = (Users)clazz.newInstance();
            long reflectStart = System.currentTimeMillis();
            Method method = clazz.getMethod("setUsername", String.class);
            for (int i=0;i<100000000;i++){
                method.invoke(users,"李二狗");
            }
            long refectEnd = System.currentTimeMillis();

            //非反射方式耗时
            long start = System.currentTimeMillis();
            Users u = new Users();
            for (int i=0;i<100000000;i++){
                u.setUsername("oldlu");
            }
            long end = System.currentTimeMillis();
            System.out.println("反射执行时时间:"+(refectEnd-refectEnd));
            System.out.println("非反射执行时时间:"+(end-start));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

老师，为什么添加不上去这个配置

int c = 5 << 2 ;    //5*2*2=20

int c = 6 << 3     //6*2*2*2还是6*3*3*3

这里为什么会报错呢

这个视频和上一个视频不连着，不知道中间落下了多少

老师我想问一下，Set不是单例集合么 为什么能存键值对呢

老师,图片怎么获取啊

环境配置是怎么弄得了

# coding=utf-8
"""开发画图软件测试
"""


from tkinter.filedialog import *
from tkinter.colorchooser import *
from tkinter import *

#窗口的宽度和高度
win_width=900
win_height=450

class Application(Frame):

    def __init__(self, master=None,bgcolor="#000000"):
        super().__init__(master)        # super()代表的是父类的定义，而不是父类对象
        self.master = master
        self.bgcolor=bgcolor
        self.x=0
        self.y=0
        self.fgcolor="#ff0000"
        self.lastDraw=0   # 表示最后绘制的图形
        self.startDrawFlag=False
        self.pack()
        self.createWidget()

    def createWidget(self):
        self.drawpad=Canvas(root,width=win_width,height=win_height*0.9,bg=self.bgcolor)
        self.drawpad.pack()


        #创建按钮
        btn_start=Button(root,text="开始",name="start")
        btn_start.pack(side="left",padx="10")
        btn_pen = Button(root, text="画笔", name="pen")
        btn_pen.pack(side="left", padx="10")
        btn_rect = Button(root, text="矩形", name="rect")
        btn_rect.pack(side="left", padx="10")
        bth_oval=Button(root, text="椭圆", name="oval")
        bth_oval.pack(side="left",padx="10")
        btn_clear = Button(root, text="清屏", name="clear")
        btn_clear.pack(side="left", padx="10")
        btn_erasor = Button(root, text="橡皮擦", name="erasor")
        btn_erasor.pack(side="left", padx="10")
        btn_line = Button(root, text="直线", name="line")
        btn_line.pack(side="left", padx="10")
        btn_lineArrow = Button(root, text="箭头直线", name="lineArrow")
        btn_lineArrow .pack(side="left", padx="10")
        btn_color = Button(root, text="颜色", name="color")
        btn_color.pack(side="left", padx="10")

        # 事件处理
        # bind_class 同类事件的统一绑定，这里是Button按键类的事件绑定-鼠标左键条件
        # self.drawpad.bind("")
        btn_pen.bind_class("Button","<1>",self.eventManager)
        self.drawpad.bind("<ButtonRelease-1>",self.stopDraw)

        # 增加颜色的快捷键处理
        root.bind("<KeyPress-r>",self.kuaijiejian)
        root.bind("<KeyPress-g>",self.kuaijiejian)
        root.bind("<KeyPress-y>",self.kuaijiejian)

        # 添加渐变颜色的处理
        root.bind("<KeyPress-j>",self.kuaijiejian)


    def eventManager(self,event):
        name=event.widget.winfo_name()
        print(name)
        if name=="line":
            self.drawpad.bind("<B1-Motion>",self.myline)

        elif name=="lineArrow":
            self.drawpad.bind("<B1-Motion>",self.mylineArrow)

        elif name=="pen":
            self.drawpad.bind("<B1-Motion>",self.myPen)

        elif name=="rect":
            self.drawpad.bind("<B1-Motion>",self.myRect)

        elif name=="oval":
            self.drawpad.bind("<B1-Motion>",self.myOval)

        elif name=="erasor":
            self.drawpad.bind("<B1-Motion>",self.myErasor)

        elif name=="clear":
            self.drawpad.delete("all")

        elif name=="color":
            c=askcolor(color=self.fgcolor,title="选择画笔颜色")
            # [(255,0,0),"#ff0000"]
            self.fgcolor=c[1]




    def stopDraw(self,event):
        self.startDrawFlag=False
        self.lastDraw=0


    def startDraw(self,event):
        self.drawpad.delete(self.lastDraw)

        if not self.startDrawFlag:
            self.startDrawFlag = True
            self.x = event.x
            self.y = event.y


    def myline(self,event):
        self.startDraw(event)
        self.lastDraw=self.drawpad.create_line(self.x,self.y,event.x,event.y,fill=self.fgcolor)


    def mylineArrow(self,event):
        self.startDraw(event)
        self.lastDraw=self.drawpad.create_line(self.x,self.y,event.x,event.y,arrow=LAST,fill=self.fgcolor)


    def myRect(self,event):
        self.startDraw(event)
        self.lastDraw=self.drawpad.create_rectangle(self.x,self.y,event.x,event.y,outline=self.fgcolor)


    def myOval(self,event):
        self.startDraw(event)
        self.lastDraw=self.drawpad.create_oval(self.x,self.y,event.x,event.y,outline=self.fgcolor)


    def myPen(self,event):
        self.startDraw(event)
        self.drawpad.create_line(self.x,self.y,event.x,event.y,fill=self.fgcolor)
        self.x=event.x
        self.y=event.y


    def myErasor(self,event):
        self.startDraw(event)
        self.drawpad.create_rectangle(event.x-10,self.y-10,event.x+10,event.y+10,outline=self.bgcolor)
        self.x = event.x
        self.y = event.y


    def kuaijiejian(self,event):
        if event.char=="r":
            self.fgcolor="#ff0000"
        elif event.char=="g":
            self.fgcolor="#00ff00"
        elif event.char=="y":
            self.fgcolor="#ffff00"
        elif event.char=="j":
            b=["#00ff00","#ff0000","#ffff00"]
            for i in range(3,333):
                l=i%3
                self.fgcolor=b[l]

if __name__ == '__main__':
    root = Tk()
    root.geometry(str(win_width)+"x"+str(win_height)+"+200+300")
    root.title("画图软件")
    app = Application(master=root)
    root.mainloop()

老师，渐变颜色怎么弄了。