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https://github.com/jlchn/language-x

language x cheatsheet
https://github.com/jlchn/language-x

basics c cheatsheet go java javascript language shell

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language x cheatsheet

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README 

        

# basic types, operators, expressions



# control flow



## if, if else, else if



## loops (for, foreach, while, break, continue)



### for



```

- go

```go

for i := 0; i < len(os.Args); i++ {

  fmt.Println(os.Args[i])

}

fmt.Println(s)

```



## switch



- python 

    - no switch-case in python



# object or struct



# pointers



# functions



### scope and external variables



- shell



``` bash

f1(){

local xxx=1

echo $xxx;

echo "all variables in a function are global like in js, set it with local to make them local variables"

echo "local can only be used in a function"

}

```



## pass by value and pass by reference



## dynamic length variable



# array



### array to stream



- java



``` java  

String[] array = {"a", "b", "c", "d", "e"};

Stream stream1 = Arrays.stream(array);

stream1.forEach(x -> System.out.println(x));

```



## array to map



# list 



### initialization



### access via index



### add, upate the list



### delete from the list



### traverse the list



### slice the list



### sort the list



### copy the list



- python



- python tuple



### reverse the list



### list to map 



- java



``` java

Map map = list.stream().filter(

                vo -> !vo.getEnvType().equals("commercial"))

                .collect(

                        Collectors.toMap(vo -> String.join("@", vo.name(), vo.region()),

                                vo -> vo));

```

### numeric list operations



# map



### initialization



### add, update map



### delete from map



### traverse the map



# enum



- java

``` java

// declare enum

public enum Day {

    MONDAY,TUESDAY,WEDNESDAY,THURSDAY,FRIDAY,SATURDAY,SUNDAY

}



// print enum

System.out.println(Day.MONDAY);//MONDAY

System.out.println(Day.TUESDAY);//TUESDAY

System.out.println(Day.WEDNESDAY);//WEDNESDAY

System.out.println(Day.TUESDAY);//TUESDAY

System.out.println(Day.FRIDAY);//FRIDAY

System.out.println(Day.SATURDAY);//SATURDAY

System.out.println(Day.SUNDAY);//SUNDAY



// get enum by string



Day day = Day.valueOf("FRIDAY");

System.out.println(day);//FRIDAY

day = Day.valueOf("WRONG");

System.out.println(day);//Exception in thread "main" java.lang.IllegalArgumentException: No enum constant com.company.Day.WRONG



// loop over enum

for (Day day : Day.values()){

    System.out.println(day);

}



// switch by enum

Day day = Day.FRIDAY;

switch (day){

    case MONDAY:

        System.out.println(0);

        break;

    case TUESDAY:

        System.out.println(1);

        break;

    case WEDNESDAY:

        System.out.println(2);

        break;

    case THURSDAY:

        System.out.println(3);

        break;

    case FRIDAY:

        System.out.println(4);

        break;

    case SATURDAY:

        System.out.println(5);

        break;

    case SUNDAY:

        System.out.println(6);

        break;

}



// compare enum

Day day = Day.FRIDAY;

System.out.println(day==Day.FRIDAY);

```



# string



### string initialization



### length 



### to lower case and upper case



### trim



### start with and end with

### contains and index of

### join and split



- java



``` java



// using String.join

String result = String.join("-", "2015", "10", "31" );// 2015-10-31

String result = String.join(", ", list);



// using StringJoiner

StringJoiner sj = new StringJoiner(",");

sj.add("aaa");

sj.add("bbb");

String result = sj.toString(); // aaa,bbb



StringJoiner sj = new StringJoiner("/", "prefix-", "-suffix");

sj.add("2016");

sj.add("02");

String result = sj.toString(); // prefix-2016/02-suffix



// using lambda

String result = list.stream().map(x -> x).collect(Collectors.joining(" | "));



```



### slice the string



### compare

### title

# type conversion



# date and time



# json



- java gson



``` xml

/* To use Gson, add following to pom.xml  */



    	com.google.code.gson

    	gson

    	2.6.2



```



``` java

// Convert java object to json string

User user = new User();

String userInString = new Gson().toJson(user);



// Convert json string to java object

User user = new Gson().from(userInString,User.class);



// Convert json array to java List

String jsonInstring = "[{\"tenant\":\"Google\"}, {\"tenant\":\"Facebook\"}]";

List list = gson.fromJson(jsonInString, new TypeToken>(){}.getType());

list.forEach(x -> System.out.println(x));



// Convert json to java Map

String jsonInstring = "{\"tenant\":\"Google\", \"landscape\":"develop"}";

Map map = gson.fromJson(jsonInString, new TypeToken>(){}.getType());

map.forEach((x,y)-> System.out.println( x + "-" + y));

```



# module



- python



# class

# errors and exceptions



# file IO

- java



``` java

// construct a file path

Paths.get(part1,part2,part3,.....)

// create file as well as dirs

Files.createDirectories(scriptPath.getParent());

Files.createFile(scriptPath);

// get a file or dir, create if not exists

File dir = FileUtils.getFile(path);

// delete a directory

FileUtils.deleteDirectory(dir);

// inputstream to file

FileUtils.copyInputStreamToFile(x.getInputStream(), targetFile);

```

# network IO



# object oriented programming



## object lifecycle



## object initialization



## memory nodel



## reflect 



## garbage collection



# process 



# thread



# format 



# read from stdin



# test



# dependency/package management



## go



``` bash

go get -u -v -f all # download all dependencies



```



## maven for java



``` shell

mvn dependency:tree # check dependency

```



## npm for node.js



```bash

npm search db # locate the modules by keywords db

npm install commander@">1.0.0" # greater than version 1.0.0

npm install [email protected] # exactly matches version 1.0.0

npm install commander@"=1.0.0" # exactly matches version 1.0.0

npm install commander@"~1.0.0" # greater than or equal to version 1.0.0 , but less than the mexe major version



npm outdated [module-name] [-g] # display outdated packages [in golbal folder]

npm update [module-name] [-g]  # update outdated packages [in golbal folder]



npm adduser

npm publish



npm ls # view all the m-odules installed in current directory

npm ls -g # view all the modules installed in global directory

npm root -g # view the path that the global modules been installed



npm rm module-name # remove a module



# linking packages is very useful if you are developing a module and want another project to reference your local copy of the module . 

# a linked package can be refered as if it were a global package , 

# which is very similar to module installation . 

#1. create global link

cd my-project & npm link # now your 'my-project' is a global module , you can verify using npm ls -g 

#2. reference an existing link 

cd other-project & npm link my-project # link my-project as a module in your other-project 

#3.unlink package 

cd other-project & npm unlink my-project 

cd my-project & npm unlink 



```