尝试初建学生管理系统时，遇到一个问题Un reac hable Code ，查了很多都说是循环体中有问题让后续代码无法执行，正好我也是循环体出现这个问题，但在后续代码检查很久都没找出哪里有问题。 今天一早重新来看发现了问题所在：是循环前面的if()分支语句在if结束后忘了删除开始的return 0；导致后续的for循环无法执行，于是得到这类问题的解决思路就是——查看该问题标红代码整体的前面代码 public int delete(String classId, String id) { //3.删除操作

单机搭建Android(解决 fatal: Cannot get https://gerrit.googlesource.com/git-repo/clone.bundle fatal: error [Errno 101] Network is un reac hable ),具体操作请阅readme.txt

报错：Un reac hable code Un reac hable ，在执行它们前已跳出循环，所以不可到达。， 在循环的return、break后的代码无法执行，属于不可达代。 可以先执行这段代码，再return、break。

大多出错的原因都与 return 有关，可能是带有返回值的方法之中调用另一个方法的时候 把方法写在了 return 后面 代码的意思就是没有执行到这个方法；还有可能就是在遍历判断中使用了 return，符合条件 return 后面的判断没有执行到。我出错的原因是第二种 switch (val) { case 0: return '待审核' break case 1: return '审核通过' break case 2: match x { Some (n) if n >= 0 => println! ( "Some(Non-negative)" ), Some (n) if n < 0> println! ( "Some(Negative)" ), Some (_) => xkcd_un reac hable ! (), None => println! ( "None" ) 如果达到则输出 :warning: ERROR If you're seei

go语言系统编程，是英文版， What this book covers Chapter 1, Getting started with Go and Unix Systems Programming, starts by defining what systems programming is before talking about the advantages and the disadvantages of Go, the features of Go version 1.8, two handy Go tools named gofmt and godoc, as well as the various states of Unix processes. Chapter 2, Writing Programs in Go, helps you learn how to compile Go code and how to use the environment variables that Go supports, and understand how Go reads the command line arguments of a program. Then, we will talk about getting user input and output, which are fundamental tasks, show you how to define functions in Go, where the defer keyword is mentioned for the first time in this book and continue by discussing the data structures that Go offers using handy code examples. In the remaining sections of the chapter, we will discuss Go interfaces and random number generation. I am sure that you are going to enjoy this chapter! Chapter 3, Advanced Go Features, goes deeper and starts talking about some advanced Go features, including error handling, which is critical when developing systems software and error logging. Then it introduces you to pattern matching and regular expressions, Go Reflection, and talks about unsafe code . After that, it compares Go to other programming languages and presents two utilities, named dtrace(1) and strace(1), that allow you to see what happens behind the scenes when you execute a program. Lastly, it talks about how you can use the go tool to detect un reac hable code and how to avoid some common Go mistakes. Chapter 4, Go Packages, Algorithms, and Data Structures, talks about algorithms and sorting in Go and about the sort.Slice() function, which requires Go version 1.8 or newer. Then it shows Go implementations of a linked list, a binary tree and a hash table. After that, it discusses Go packages and teaches you how to create and use your own Go packages. The last part of the chapter discusses Garbage collection in Go. Chapter 5, Files and Directories, is the first chapter of this book that deals with a systems programming topic, which is the handling of files, symbolic links, and directories. In this chapter, you will find Go implementations of the core functionality of Unix tools such as which(1), pwd(1), and find(1), but first you will learn how to use the flag package in order to parse the command-line arguments and options of a Go program. Additionally, you will learn how to delete, rename, and move files as well as how to traverse directory structures the Go way. The last part of this chapter implements a utility that creates a copy of all the directories of a directory structure! Chapter 6, File Input and Output, shows you how to read the contents of a file, how to change them, and how to write your own data to files! In this chapter, you will learn about the io package, the io.Writer and io.Reader interfaces, and the bufio package that is used for buffered input and output. You will also create Go versions of the cp(1), wc(1), and dd(1) utilities. Lastly, you will learn about sparse files, how to create sparse files in Go, how to read and write records from files, and how to lock files in Go. Chapter 7, Working with System Files, teaches you how to deal with Unix system files, which includes writing data to Unix log files, appending data to existing files, and altering the data of text files. In this chapter, you will also learn about the log and log/syslog standard Go packages, about Unix file permissions, and take your pattern matching and regular expressions knowledge even further using practical examples. You will also learn about finding the user ID of a user as well as the Unix groups a user belongs to. Lastly, you will discover how to work with dates and times in Go using the time package and how to create and rotate log files on your own. Chapter 8, Processes and Signals, begins by discussing the handling of Unix signals in Go with the help of the os/signal package by presenting three Go programs. Then it shows a Go program that can rotate its log files using signals and signal handling and another Go program that uses signals to present the progress of a file copy operation. This chapter will also teach you how to plot data in Go and how to implement Unix pipes in Go. Then it will implement the cat(1) utility in Go before briefly presenting the Go code of a Unix socket client. The last section of the chapter quickly discusses how you can program a Unix shell in Chapter 9, Goroutines – Basic Features, discusses a very important Go topic, which is goroutines, by talking about how you can create goroutines and how you can synchronize them and wait for them to finish before ending a program. Then it talks about channels and pipelines, which help goroutines communicate and exchange data in a safe way. The last part of the chapter presents a version of the wc(1) utility that is implemented using goroutines. However, as goroutines is a big subject, the next chapter will continue talking about them. Chapter 10, Goroutines – Advanced Features, talks about more advanced topics related to goroutines and channels, including buffered channels, signal channels, nil channels, channels of channels, timeouts, and the select keyword. Then it discusses issues related to shared memory and mutexes before presenting two more Go versions of the wc(1) utility that use channels and shared memory. Lastly, this chapter will talk about race conditions and the GOMAXPROCS environment variable. Chapter 11, Writing Web Applications in Go, talks about developing web applications and web servers and clients in Go. Additionally, it talks about communicating with MongoDB and MySQL databases using Go code . Then, it illustrates how to use the html/template package, which is part of the Go standard library and allows you to generate HTML output using Go HTML template files. Lastly, it talks about reading and writing JSON data before presenting a utility that reads a number of web pages and returns the number of times a given keyword was found in those web pages. Chapter 12, Network Programming, discusses topics related to TCP/IP and its protocols using the net Go standard package. It shows you how to create TCP and UDP clients and servers, how to perform various types of DNS lookups, and how to use Wireshark to inspect network traffic. Additionally, it talks about developing RPC clients and servers in Go as well as developing a Unix socket server and a Unix socket client. As you will see, at the end of each chapter there are some exercises for you to do in order to gain more information about important Go packages and write your own Go programs. Please, try to do all the exercises of this book