342 |
343 | *Not these loops*
344 |
345 | Loops can execute code multiple times. So instead if writing a certain statement 5 times, you can use one nice loop. There are a few different types of loops.
346 |
347 | **While loop:** These loops repeat a certain block of code, *while* the expression is true. Before each execution the statement will be checked; if the expression is true the loop will execute, otherwise it will break and continue normal execution. As with any other loops inifite loops are something you have to watch out for, but while loops have a higher 'infinite loop potential' than other loops. Example:
348 | ```java
349 | int i = 0;
350 |
351 | while (i < 5)
352 | {
353 | System.out.println("Hi.");
354 | i++; // This line is important, without it this loop would become infinite!
355 | }
356 | ```
357 |
358 | **Do..while loop:** This loop is the same as a while loop with one big difference: Instead of checking the loop expression before execution, this loop will check the expression *after* execution.
359 |
360 | **For loop:** A for loop is a compact method for iterating over a range of values. The standard form is:
361 | ```java
362 | for (initialization of variable; continuation case; increment)
363 | {
364 | // code goes here
365 | }
366 | ```
367 | The first part is where you initialize a variable to be looped upon. The continuation case is an expression which must be true in order for the code to be executed. Increment is where you increment your initialized variable towards the termination. An example:
368 | ```java
369 | for (int i = 0; i < 5; i++)
370 | {
371 | System.out.println("Hi, for the " + i + "th time!");
372 | }
373 |
374 | // This will print "Hi, for the xst time!" 5 times
375 | // Much better than:
376 | System.out.println("Hi, for the 0th time!");
377 | System.out.println("Hi, for the 1th time!");
378 | System.out.println("Hi, for the 2th time!");
379 | System.out.println("Hi, for the 3th time!");
380 | System.out.println("Hi, for the 4th time!");
381 | ```
382 | *Notice how I used 'i' instead of some descriptive name as I [recommended earlier](#naming-conventions). A quick answer can be found [here](https://softwareengineering.stackexchange.com/questions/86904/why-do-most-of-us-use-i-as-a-loop-counter-variable)*
383 |
384 | **Enhanced for loop:** A few years ago (Java 5) the enhanced for loop was added. This loop is very useful for iterating over a collection of elements such as arrays, lists, and sets. The syntax is simple:
385 | ```java
386 | for (initialization of variable : iterable)
387 | {
388 | // code goes here
389 | }
390 | ```
391 | Like in the regular for loop, initialization of variable is simply a variable that can be used inside the loop. It may sound more difficult than it actually is:
392 | ```java
393 | int[] numbers = new int[]{1, 3, 3, 7};
394 |
395 | for (int i : numbers)
396 | {
397 | System.out.println("Current number: " + i);
398 | }
399 |
400 | /*
401 | This would print:
402 | Current number: 1
403 | Current number: 3
404 | Current number: 3
405 | Current number: 7
406 | */
407 | ```
408 |
409 | ### Flow control
410 | Flow control statements allow you to specifically control the flow of execution of certain code. In places where code could potentially cause issues if run under certain conditions, these statements are particulary helpful and sometimes even necessary.
411 |
412 | **Return statements:** The return statement has a few different purposes.
413 | In a **void** method (a method with the modifier **void**/does not return a value), this statement can be used to simply exit the method. For example:
414 | ```java
415 | private void runLoop()
416 | {
417 | while (true)
418 | {
419 | if (!shouldBeLooping())
420 | {
421 | return;
422 | }
423 |
424 | // do something
425 | }
426 | }
427 | ```
428 | Any value that is not declared **void** has to return something, like this:
429 | ```java
430 | private boolean shouldBeLooping()
431 | {
432 | return running == null ? false : true;
433 | }
434 | ```
435 |
436 | **Continue statement:** This statement can be used within the body of a loop to skip execution of the following code and move to the next loop iteration. Example:
437 | ```java
438 | for (int i = 0; i < 5; i++)
439 | {
440 | if (i == 2)
441 | {
442 | continue; // If i == 2, then skip to the next iteration
443 | }
444 |
445 | System.out.println(i);
446 | }
447 |
448 | /*
449 | The result would be:
450 | 0
451 | 1
452 | 3
453 | 4
454 | */
455 | ```
456 |
457 | **Break statement:** This statement can be used to immediately exit out of a loop. Example:
458 | ```java
459 | for (int i = 0; i < 5; i++)
460 | {
461 | if (i == 3)
462 | {
463 | break; // If i == 3, then break out of loop
464 | }
465 |
466 | System.out.println(i);
467 | }
468 |
469 | /*
470 | The result would be:
471 | 0
472 | 1
473 | 2
474 | */
475 | ```
476 |
477 | ## Objects
478 | ***
479 | *Work in progress*
480 | *This is just a baseline, not sure if all these things will be added. If you want a certain topic explained/discussed, you can create an issue on github or send a message on [discord](https://discord.gg/DxH42Ca).*
481 | ### Instantiation
482 |
483 | ### Instance Variables vs Class Variables
484 |
485 | ### Inheritance
486 |
487 | ### Abstraction
488 |
489 | ### Interface
490 |
491 | ## I/O
492 | ***
493 | *Work in progress*
494 | *This is just a baseline, not sure if all these things will be added. If you want a certain topic explained/discussed, you can create an issue on github or send a message on [discord](https://discord.gg/DxH42Ca).*
495 | ### Files
496 |
497 | ### Console
498 |
499 | ## Tests
500 | ***
501 | *Work in progress*
502 | *This is just a baseline, not sure if all these things will be added. If you want a certain topic explained/discussed, you can create an issue on github or send a message on [discord](https://discord.gg/DxH42Ca).*
503 |
504 | ## Running everything
505 | ***
506 | *Work in progress*
507 | *This is just a baseline, not sure if all these things will be added. If you want a certain topic explained/discussed, you can create an issue on github or send a message on [discord](https://discord.gg/DxH42Ca).*
508 |
509 | ## Helpful links and credits
510 | ***
511 | *Work in progess*
512 |
513 | ### Links
514 | You can find some links here, which may be of use to use to you.
515 | #### Codecademy
516 | Codecademy is a website that teaches you to code in an interactive way (for free!)
517 | * [codecademy](https://www.codecademy.com/learn/learn-java)
518 |
519 | #### thenewboston
520 | thenewboston is a youtube channel that has many great programming and science-related tutorials
521 | * [thenewboston - beginner](https://m.youtube.com/playlist?list=PLFE2CE09D83EE3E28)
522 | * [thenewboston - intermediate](https://m.youtube.com/playlist?list=PL27BCE863B6A864E3)
523 | * [thenewboston - java game development](https://m.youtube.com/playlist?list=PLA331A6709F40B79D)
524 |
525 | ### Credits
526 | Since we've signed the Code of Honour I might as well add this too:
527 | * [stackoverflow (several links throughout the guide)](https://www.stackoverflow.com)
528 | * [tutorialspoint - java (tables + definitions)](https://www.tutorialspoint.com/java)
529 | * [SpigotMC (structure + tables + definitions + information)](https://www.spigotmc.org/wiki/spigot-plugin-development/)
530 |
531 |
532 |
533 |
534 |
535 |
536 |
537 | # WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP - WIP
538 | ***
539 | *(Work in progress)*
540 |
541 | **BASICS WITH HELLO WORLD**
542 |
543 | First of all, I will give you a simple overview of how a Java program looks like.
544 | In the below code, I have created a class – MyFirstJavaProgram and printed “Hello World”.
545 | Go ahead and try to execute the below example in your Eclipse IDE. Do not worry, we will discuss about Java class in a while.
546 | ```
547 | public class MyFirstJavaProgram {
548 | public static void main(String[] args)
549 | {
550 | System.out.println("Hello World");
551 | }
552 | }
553 | ```
554 | Next, let us understand different member variables in Java.
555 |
556 | **MEMBER VARIABLES**
557 |
558 | A member variable plays a major role in a class as it is used to store a data value. When we define a class, we can declare a member variable. These variables are members of a class.
559 | Member variables are further classified into three types:
560 |
561 | Local variable
562 | Instance variable
563 | Class/Static variable
564 | Let me discuss about each one of them:
565 |
566 | Local variable: These are the variables which are declared within the method of a class. Let’s understand this with a programmatic example:
567 | ```
568 | public class Car {
569 | public void display(int m){ // Method
570 | int model=m; // Created a local variable model
571 | System.out.println("Model of the car is" +model);
572 | }
573 | ```
574 | In the above code, my local variable is ‘model’ which I have declared inside a method ‘display’ which has a parameter ‘m’.
575 |
576 | Instance variable: Instance variable is declared in a class but outside a method, constructor or any block. Let’s understand this with a programmatic example.
577 |
578 | ```
579 | public class Car {
580 | public String color; // Created an instance variable color
581 |
582 | Car(String c)
583 | {
584 | color=c;
585 | }
586 |
587 | public void display() { // Method
588 | System.out.println("color of the car is"+color);
589 | }
590 |
591 | public static void main(String args[]){
592 | Car obj=new Car("black");
593 | obj.display();
594 | }
595 | }
596 | ```
597 | In the above code, ‘color’ is my instance variable which has a value “black” associated to it.
598 |
599 | Class variable: Class variables are also called as static variables. These variables have only one copy that is shared by all the different objects in a class. Let’s understand this with a programmatic example.
600 |
601 | ```
602 | public class Car {
603 | public static int tyres; // Created a class variable tyres
604 | public static void main(String args[]){
605 | tyres=4;
606 | System.out.println("Number of tyres are"+tyres);
607 | }
608 | }
609 | ```
610 | All cars must be having 4 tyres, right? So in my above code, I have declared a static variable as ‘tyre’ whose value remains same throughout the class.
611 |
612 | **DATA TYPES**
613 |
614 | A data type is used to represent different values which are stored in a variable. They are mainly classified into 4 different aspects – Integer, Float, Character and Boolean. You can refer to the below image to understand the different data types with respect to the memory allocated to them.
615 |
616 | 
617 |
618 | As you can see in the above image, data types are of 4 major types.
619 |
620 | The first data type is an Integer which stores a numerical value.
621 | Now, if a numerical value contains decimal part, it will be referred as float.
622 | Next, if you wish to store a character, then the third data type i.e char is used. In char, you can store any alphabetical character as well as a special character.
623 | The last data type is Boolean which stores only ‘true’ or ‘false’ value.
624 |
625 | Let’s move forward and look at the various data operations which you can perform in Java.
626 |
627 | **DATA OPERATORS**
628 |
629 | There are mainly 4 different types of operators, which are listed below:
630 |
631 | Arithmetic Operator: Perform arithmetic operations such as addition, subtraction, multiplication, division and modulus.
632 |
633 | Unary Operator: Unary operators are used to increment or decrement a particular value. For example: ++ stands for increment, – – stands for decrement.
634 |
635 | Relational Operator: It defines some kind of relation between two entities. For example: <, >, <=, >=, !=, ==.
636 |
637 | Logical Operator: Logical operators are typically used with boolean (logical) values.
638 |
639 | **CONTROL STATEMENTS**
640 |
641 | Control statements are the statements that define the flow of your program. There are 3 types of control statements in Java: Selection, iteration and jump statements.
642 |
643 | 
644 |
645 | Selection Statements:
646 |
647 | Selection statements allow you to control the flow of the program during run time on the basis of the outcome of an expression or state of a variable. For example: you want to eat pizza, but then where can you get that pizza in best price. You can select between various popular options like Domino’s, Pizza Hut or any other outlet. So here you are following a selection process from the various options available.
648 |
649 | Now these statements can be further classified into the following:
650 |
651 | If-else Statements
652 | Switch Statements
653 |
654 | Refer to the following flowchart to get a better understanding of if-else statements:
655 |
656 | 
657 |
658 | In this flowchart, the code will respond in the following way:
659 |
660 | First of all, it will enter the loop where it checks the condition.
661 | If the condition is true, the set of statements in ‘if’ part will be executed.
662 | If the condition is false, the set of statements in the ‘else’ part will be executed.
663 | Here you must have got an idea of how these if-else statements work. Now, how can we use these statements in Eclipse IDE? Let’s have a look at the code:
664 |
665 | ```
666 | public class Compare {
667 | int a=10,
668 | int b=5;
669 |
670 | if(a>b)
671 | { // if condition
672 | System.out.println(" A is greater than B");
673 | }
674 | else
675 | { // else condition
676 | System.out.println(" B is greater");
677 | }
678 | }
679 | ```
680 | In the above code, I have created a class Compare where I have compared two numbers ‘a’ and ‘b’. First of all, it will go in ‘if’ condition where it checks whether the value of ‘a’ is greater than ‘b’ or not. If the condition is true, it will print “A is greater than B” else it will execute “B is greater”.
681 |
682 | Moving on, we have Switch case statement. The switch statement defines multiple paths for execution of a set of statements. It is a better alternative than using a large set of if-else statements as it is a multi-way branch statement.
683 |
684 | Refer to the following flowchart to get a better understanding of switch statements:
685 |
686 | 
687 |
688 | In this Switch case flowchart, the code will respond in the following steps:
689 |
690 | First of all it will enter the switch case which has an expression.
691 | Next it will go to Case 1 condition, checks the value passed to the condition. If it is true, Statement block will execute. After that, it will break from that switch case.
692 | In case it is false, then it will switch to the next case. If Case 2 condition is true, it will execute the statement and break from that case, else it will again jump to the next case.
693 | Now let’s say you have not specified any case or there is some wrong input from the user, then it will go to the default case where it will print your default statement.
694 | Again, if we look at the code for switch statements in IDE, here it is:
695 |
696 | ```
697 | public class SwitchExample {
698 | int week=7;
699 | String weeknumber;
700 |
701 | switch(week){ // switch case
702 | case 1:
703 | weeknumber="Monday";
704 | break;
705 |
706 | case2:
707 | weeknumber="tuesday";
708 | break;
709 |
710 | case3:
711 | weeknumber="wednesday";
712 | break;
713 |
714 | default: // default case
715 | weeknumber="invalid week";
716 | break;
717 | }
718 | System.out.println(weeknumber);
719 | }
720 | }
721 | ```
722 | In the above code, I have created a class SwitchExample which has 3 cases that print days of a week. It also has a default case which is executed whenever a user doesn’t specify a case.
723 |
724 | Concluding both of the selection statements, we understood that if we are comparing two statements, we are using if-else, but let’s say if you are checking a specific value against a particular statement, then we are going for the Switch statement.
725 |
726 | Next, there is another set of control statements, i.e Iteration Statements.
727 |
728 | Iteration Statements: In Java, these statements are commonly called as loops, as they are used to iterate through small pieces of code. Iteration statements provide the following types of loop to handle looping requirements.
729 |
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/object-oriented-programming/Java.md:
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1 | # Java
2 | Links and FAQ about java.
3 |
4 | ## Table of contents
5 | 1. [Free Java Courses](free-java-courses)
6 | 1. [Codecademy](codecademy)
7 | 2. [thenewboston](thenewboston)
8 |
9 | ## Free Java Courses
10 |
11 | ### Codecademy
12 | Codecademy is a website that teaches you to code in an interactive way (for free!)
13 |
14 | * [codecademy](https://www.codecademy.com/learn/learn-java)
15 |
16 | ### thenewboston
17 | thenewboston is a youtube channel that has many great programming and science-related tutorials
18 |
19 | * [thenewboston - beginner](https://m.youtube.com/playlist?list=PLFE2CE09D83EE3E28)
20 |
21 | * [thenewboston - intermediate](https://m.youtube.com/playlist?list=PL27BCE863B6A864E3)
22 |
23 | * [thenewboston - java game development](https://m.youtube.com/playlist?list=PLA331A6709F40B79D)
24 |
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/reasoning-and-logic/README.md:
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1 | # Reasoning and Logic
2 | FAQ For Reasoning and Logic
3 | ## Table of contents
4 | 1. [Disjunctive Normal Form](disjunctive-normal-form)
5 | 2. [De Morgan's Law](de-morgans-law)
6 | 3. [Arguments](arguments)
7 |
8 | ## Disjunctive Normal Form
9 | Disjunctive Normal Form or DNF for short is the disjunction of conjunctions of literals. Every formula has an equivalent in DNF.
10 |
11 | ### Examples of DNF
12 | ```
13 | A
14 |
15 | A ∨ ¬A
16 |
17 | A ∨ (B ∧ ¬C) ∨ D
18 | ```
19 |
20 | ### Transformation Algorithm
21 | There are several different methods for transforming an arbitrary formula into DNF. The following is one of the simplest with three steps:
22 |
23 | 1. Eliminate the connectives for implication (⇒) and equivalence (⇔) by rewriting using the following equivalences:
24 | * A ⇒ B is equivalent to ¬A ∨ B
25 | * A ⇔ B is equivalent to (¬A ∨ B) ∧ (A ∨ ¬B)
26 | 2. Push negations (¬) inside subformulas as far as possible, applying [De Morgan's Law](de-morgans-law) where possible, and eliminate double negations. We also handle the negation of the propositional constants. We do this by rewriting with the following equivalences:
27 | * ¬(¬A) is equivalent to A
28 | * ¬(A ∧ B) is equivalent to ¬A ∨ ¬B
29 | * ¬(A ∨ B) is equivalent to ¬A ∧ ¬B
30 | * ¬t is equivalent to f
31 | * ¬f is equivalent to t
32 | 3. Distribute conjunctions (∧) over disjunctions (∨). We rewrite all applicable subterms of the formula using one of the following two equivalences:
33 | * A ∧ (B ∨ C) is equivalent to (A ∧ B) ∨ (A ∧ C)
34 | * (A ∨ B) ∧ C is equivalent to (A ∧ C) ∨ (B ∧ C)
35 |
36 | ## De Morgan's law
37 | De Morgan's law states the following equivalences:
38 | * ¬(A ∧ B) ⇔ ¬A ∨ ¬B
39 | * ¬(A ∨ B) ⇔ ¬A ∧ ¬B
40 |
41 | ## Arguments
42 | An argument consists of a conclusion and a number of premesis. There can be one, more then one or even zero premesis in an argument.
43 |
44 | An argument is valid if and only if the conclusion is true for all cases (i.e. rows in the truth table) where all propositions are true. For propositional logic it is relatively easy to prove that an argument is valid. You draw out the truth table. You look at the conclusion. If and only if the conclusion is true, you look at the colomn of all the premesis. All premesis have to be true. If this isn't the case, the argument is invalid. Repeat this process for all rows where the conclusion is true.
45 |
46 | For predicate logic, this process is more difficult. You need to use a mathematical proof. I can't explain all mathematical proofs here, because they are different for every argument. Look at the text book, that explains it pretty well.
47 |
48 | ### Special cases
49 |
50 | If an argument has no premesis, the conclusion must be a tautology (i.e. always true) for the argument to be valid. A example of a tautology is (p or not p).
51 |
52 | Principal of explosion: if one of the premesis is a contradiction (i.e. always false), the argument is always valid. Look at the definition of a valid argument. The conclusion has to be true for all cases where all propositions are true. If there are no cases where the propositions are true, this definition technically holds. It won't win you a debate, but the argument is logically valid.
53 |
54 | ## Sources
55 | * Hugtenburg, S. & Yorke-Smith, N. (2018) Delftse Foundations of Computation. Available for download [here](https://textbooks.open.tudelft.nl/index.php/textbooks/catalog/book/13)
56 | * [DNF](http://www.barrywatson.se/cl/cl_dnf.html)
57 |
58 |
59 | ## Useful Links
60 | * [Logical Equivalence](https://en.wikipedia.org/wiki/Logical_equivalence)
61 | * [Recursion](https://en.wikipedia.org/wiki/Recursion#In_mathematics)
62 |
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/web-and-database-technology/Demo1.md:
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1 | ```
2 |
3 | jonathan@DESKTOP-B1K4GDE:~$ telnet microsoft.com 80
4 | Trying 40.76.4.15...
5 | Connected to microsoft.com.
6 | Escape character is '^]'.
7 | HEAD / HTTP/1.1
8 | host:microsoft.com
9 |
10 | HTTP/1.1 301 Moved Permanently
11 | Date: Mon, 12 Nov 2018 11:22:47 GMT
12 | Server: Kestrel
13 | Location: https://www.microsoft.com/
14 |
15 | HEAD / HTTP/1.1
16 | host:www.microsoft.com
17 |
18 | HTTP/1.1 404 Not Found
19 | Date: Mon, 12 Nov 2018 11:23:18 GMT
20 | Server: Kestrel
21 |
22 | ^]
23 |
24 | telnet> ^C^Z
25 | [1]+ Stopped telnet microsoft.com 80
26 | jonathan@DESKTOP-B1K4GDE:~$ telnet www.microsoft.com 80
27 | Trying 2.20.27.153...
28 | Connected to e13678.dspb.akamaiedge.net.
29 | Escape character is '^]'.
30 | HEAD / HTTP/1.1
31 | host:www.microsoft.com
32 |
33 | HTTP/1.1 200 OK
34 | Server: Apache
35 | ETag: "6082151bd56ea922e1357f5896a90d0a:1425454794"
36 | Last-Modified: Wed, 04 Mar 2015 07:39:54 GMT
37 | Accept-Ranges: bytes
38 | Content-Length: 1020
39 | Content-Type: text/html
40 | Date: Mon, 12 Nov 2018 11:24:11 GMT
41 | Connection: keep-alive
42 |
43 | GET / HTTP/1.1
44 | host:www.microsoft.com
45 |
46 | HTTP/1.1 200 OK
47 | Server: Apache
48 | ETag: "6082151bd56ea922e1357f5896a90d0a:1425454794"
49 | Last-Modified: Wed, 04 Mar 2015 07:39:54 GMT
50 | Accept-Ranges: bytes
51 | Content-Length: 1020
52 | Content-Type: text/html
53 | Date: Mon, 12 Nov 2018 11:24:28 GMT
54 | Connection: keep-alive
55 |
56 | Your current User-Agent string appears to be from an automated process, if this is incorrect, please click this link:United States English Microsoft Homepage
57 | 58 | ``` 59 | -------------------------------------------------------------------------------- /web-and-database-technology/README.md: -------------------------------------------------------------------------------- 1 | # Web & Database Technology 2 | 3 | FAQ For Web & Database Technology 4 | 5 | ## Content 6 | 7 | This course is split into two parts. [web](./web.md) and [database](./database.md). 8 | Check them out to learn more about that section. 9 | 10 | ## Usefull links 11 | 12 | | Link | Type | Description | 13 | | --------------------------------------------------------------- | ---- | ---------------------------------- | 14 | | [You Don't Know JS](https://github.com/getify/You-Dont-Know-JS) | Book | A free introduction to JavaScript. | 15 | -------------------------------------------------------------------------------- /web-and-database-technology/database.md: -------------------------------------------------------------------------------- 1 | # Database 2 | 3 | This document contains information about the database lectures. 4 | 5 | This should be filled with information, 6 | But it is not right now. 7 | You can submit a PR to fix this! 8 | -------------------------------------------------------------------------------- /web-and-database-technology/web.md: -------------------------------------------------------------------------------- 1 | # Web 2 | 3 | This page contains information about the web part of the course. 4 | 5 | This should be filled with information, 6 | But it is not right now. 7 | You can submit a PR to fix this! 8 | 9 | 10 | ## lecture 1: 11 | 12 | * 20% lab 13 | * 40% web 14 | * 40% database 15 | 16 | 17 | ### organisations: 18 | 19 | W3C --> negotiator between browsers so implementations are standardised between browsers 20 | IETF --> Request for Comment 21 | 22 | ### HTTP 23 | 24 | #### HTTP/1.1, RFC 2086 25 | 26 | more support 27 | 28 | server waits 29 | client initiates conversation (request) 30 | server handles requests (response) 31 | 32 | 33 | 34 | #### HTTP/2, RFC 7540 35 | 36 | less support 37 | 38 | 39 | #### Content type 40 | MIME: 41 | 42 | pattern: 43 | [primary object type]/[subtype] 44 | examples: 45 | 46 | * text/plain 47 | * text/html 48 | * image/jpg 49 | * unknown/unknown 50 | * application/xml 51 | * model/mesh 52 | 53 | #### Content length 54 | 55 | if size of body doesnt match header length --> reject 56 | 57 | also important for persistent connections. if multiple http requests are sent this field can help the browser 58 | find the end of the stream of requests 59 | 60 | #### Content encoding 61 | 62 | tells what kind of compression the server or client understands and thus can send to eachother. 63 | 64 | advantage: less bandwidth 65 | disadvantage: more cpu time 66 | 67 | examples: 68 | 69 | * gzip 70 | * deflate 71 | * identity 72 | 73 | #### Content MD5 74 | 75 | removed in HTTP 2.0 though it's still used. 76 | used to verify content. 77 | 78 | md5 is a hash computed of the entire body. if a bit is changed the md5 is different. so if the client recalculates md5 and it doesnt match the server knows there is something wrong 79 | 80 | #### Expires 81 | 82 | lets the client know when the content is not valid anymore and the client has to re-request it. this however has in history rarely actually been used by clients and is mostly used by webcaches. when a webcache has a recource, and it is not expired it will serve it, else it is forwarded to the origin server (Eventually, it might go through multiple webcaches) 83 | 84 | web browsers also cache webpages. Therefore nowadays the expires field is more useful for clients too. 85 | 86 | #### Last modified 87 | 88 | again mostly used by webcaches to know when it has to re-request data to cache. used to be used by search engines to give higher rankings to recently modified pages. due to abuse searchengines dont do this anymore. 89 | 90 | #### Connection & Upgrade 91 | 92 | Used so the server can push to clients (instead of client initiating requests) 93 | 94 | simulating server --> client requests: 95 | * polling 96 | * long polling (server keeps conenction open until new data arrived) 97 | 98 | HTTP/1.1 doesnt allow more than this 99 | 100 | ##### Websocket protocol: 101 | 102 | enables bidirectional communcation between client and server 103 | 104 | 105 | 106 | client initiates bidirectional communication 107 | client sends 2 headers: 108 | 109 | if server allows connection upgrade to websockets 110 | then bidirectional communication is established 111 | 112 | 113 | 114 | 115 | #### Status code 116 | 117 | 1xx 118 | informational 119 | 120 | 2xx 121 | request ok 122 | 123 | 3xx 124 | redirected, resource moved 125 | 126 | 4xx 127 | client error 128 | 404 resource not found 129 | 130 | 402: reserved for *payment requred* 131 | 132 | 5xx 133 | server error 134 | should never be seen. server errors shouldnt be seen by clients! 135 | 136 | 137 | 138 | ### HTTP Methods 139 | 140 | GET 141 | sends resource, header and body 142 | 143 | HEAD 144 | only send the header, no body 145 | 146 | POST 147 | WIP 148 | 149 | PUT 150 | WIP 151 | 152 | TRACE 153 | WIP 154 | 155 | OPTIONS 156 | WIP 157 | 158 | DELETE 159 | WIP 160 | 161 | ### IP addresses 162 | 163 | DNS server converts domain name to ip address 164 | 165 | ipv4: 166 | 167 | xx.xx.xx.xx (hex numbers) 168 | max 2^32 ip adresses 169 | push to convert to ipv6 170 | 171 | ipv6: 172 | 173 | xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx (hex numbers) 174 | 2^128 possibilities 175 | 176 | 177 | 178 | 179 | 180 | 181 | 182 | --------------------------------------------------------------------------------