Java Crash Course

1. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. Java Crash Course Learn how to do stuff in 2 hours! A fun, slightly organized, but easy to follow guide by bitinvert.com

2. Before we begin: • Install the JRE and the JDK • http://www.oracle.com/technetwork/java/javase/downloads/index.html • Install jGrasp • http://www.jgrasp.org/

3. The key to learning • Understand what you are doing • If you don't, interrupt me and ask about it • This guide won’t teach you everything, but you will learn enough to teach yourselves or understand new concepts easily. • There is no assumed knowledge. What you need to understand will be explained. (I will say things that aren't on the slides)

4. Computer basics - storage(useful information) • 1 bit is either a 1 or a 0 • 1 byte is 8 bits • 1 kilobyte is 1024 bits • Why? 210 = 1024 • 1 megabyte is 1024 kilobytes • 1 gigabyte is 1024 megabytes • Terabytes, petabytes, exabytes, and yottabytes follow

5. Computer basics - storage(useful information) • To put this information in perspective: • A typical short novel is 1 megabyte • All of Shakespeare's work is 5 megabytes • A minute of high quality sound is 10 megabytes • A CD holds 700 MB • A DVD holds 4.7 GB • A Blu-Ray disc holds 25 GB source: http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci944596,00.html

6. Java basics - data types • boolean: either true or false • byte: min. value of -128 and max. of 127 • short: min. -32,768 and max. 32,767 • int (most widely used): • min. -2,147,483,648 and max. 2,147,483,647 • long: • min. -9,223,372,036,854,775,808and max. 9,223,372,036,854,775,807

7. Java basics - data types • float: number with a decimal • double: number with decimal, holds more significant figures (more accurate) • char: a single character. Represented inside single quotes. (like 'A') Note: this is a very simplified explanation

8. What makes a program run? • A programmer enters in source code • The source code is compiled to byte code • Byte code is source code converted to be far more efficient • Byte code is interpreted by Java to machine code • Computers can understand and execute machine code

9. Important structural parts of source code • Type this: • The { and } control scope. • Everything inside can see everything else inside • This will make more sense later • Now save and compile it (but don't run it yet). Were there any errors?

10. There were errors! • What does this mean for us and our blah? • We'll have to examine the error to find out.

11. Open a web browser, and googlejava.lang • Do notuse any other search engines. • The first link is from oracle.com, the owners • The first link's title is java.lang. This looks relevant. Open this search result.

12. The error is fundamental. Because the error is a part of java.lang, we can find a description of it on this java.lang page. • What is a method?

13. What is a method? • A method is a set of instructions. • An example of a real-world method for brushing hair is: • To brush your hair, grab a brush by the handle with the bristles pointing down, place on your head, and stroke downward. • Computer programs need methods to be typed in a very specific way

14. The main methodmain(String[] args) • Every runnable Java program needs something called a main method. • Let's add a main method to our code. • You might not know what all these words mean, but you will later today. • Save, compile, and run. What happens?

15. Nothing happened. • (but you don't get an error!) • Why? Because your method doesn't contain any instructions for the computer. • Let's add some instruction: • System.out.println("stuff"); • Save, compile, and run your program.

16. What do these words mean? • The purple words are keywords. They have specific uses for program structure. • A class is a file containing an object. • The object name here is IntroductionProgram • How about the stuff on the line creating the main method?

17. What do these words mean? • How about the stuff on the line creating the main method? • public: everything with permission to the class has access to whatever is declared public • Let's explore a demonstration.

18. Create a new Java file (File > New > Java) • Type this in, then compile it: • Modify your original class, IntroductionProgram, like this: (save/compile/run)

19. Inside the main method, you call the sayHello method DemoOfPublic object (aka class) • (call means run)

20. Now change the sayHello() method to private instead of public: • Run the InstructionProgram class again. • What happens?

21. You get an error! • It's another java.lang. error. Let's find out what it means. • The error name is descriptive enough that we didn't have to look it up. • The sayHello method is private, so only the class it's in can use it. Make it public again, and save/compile • (run the main method again if you want)

22. You know what public means, now • Everything with permission to the class has access to whatever is declared public

23. Explaining static • Create a new class, called DemoOfStatic • Change the main class (IntroductionProgram): • Run the main class

24. Remove the static modifier from sayGoodbye and compile. Run the main class again. • What happens?

25. What happens? • You get an error. “Non-static method cannot be referenced from static context” • To run the method, you have to create an instance of the DemoOfStatic class. • This means that you have to create a DemoOfStatic object in order to use its methods • Let’s fix this

26. Creating an instance of a class • Modify your main program. • Line 4 means: • We are creating space in the computer’s memory for a DemoOfStatic • The space will be called object • new DemoOfStatic() creates a default DemoOfStatic

27. Creating an instance of a class • Now that an object is created in line 4, you can call methods from the object using dot notation. • Type the object’s variable name, then a dot, then the method that the object contains.

28. Creating an instance of a class • Here is a way to think about an applied real-life situation: • Human is a class (object) • Human has methods to do things, like brushing hair or eating. • But you need a Human to brush hair or eat, you can't do that without having one. • The Human methods would be non-static

29. Creating an instance of a class • Here is a way to think about an applied real-life situation: • Math is a class • With Math, you can take the absolute value of something without having to remember everything you learned about Math, ever. • The absolute value method would be static.

30. You know what static means, now • static means that you don’t need to create an instance of the object to run the method.

31. Explaining void • Modify your main program: • When you run the program, the main method is called automatically. The main method calls the answerToLife method.

32. Explaining void • Remove void from line 6. Compile. What happens?

33. Explaining void • You get an error. • When you have a method that is not void, it has to return a variable. • In Java, variables can hold various data types. • To return a variable from a method you have to say what type of variable you will return.

34. Explaining void • Insert int to where void was. • Compile again. You will get another error: • This means that you have to specify what variable to return in the method.

35. Explaining void • Change the answerToLife method like this: • Compile and run. What happens?

36. It looks like nothing happens, but actually line 3 is being replaced by what is returned to it in line 8 • In this example it’s the same as writing • Neat, but what can we do with this?

37. Modify your main method: • Line 3 creates a new int, called something, with the value that answerToLife returns minus two, all divided by ten. • Line 4 prints the int something.

38. What if we want a method to take input and change it? • That’s what the parenthesis are for. Modify your main program like this:

39. The calculateProfit method takes input. • You can pass an int to it • The input is named objectsSold • The initial cost to sell objects is 95 • The profit per object is 35, but we have to subtract the initial cost • The profit is then returned

40. The main method creates an int called employeeProfit1 with the value calculated by passing 2 to the employeeProfit method • The int is then printed

41. Modify your main method: • Methods can be reused any number of times.

42. Real-life example of returning • A factory production method would take in raw materials:public int produceCan(int amountOfAluminum) • It would tell the size of the biggest can it can make back to the supplier of the resources:return sizeOfCan;

43. You should now understand passing values • Input can be passed to a method inside parenthesis to modify output • Output is returned and can be used after the method is called

44. Methods • You should now understand the basics of methods. • You've already been using one for a while:System.out.println("hello"); • println is the method, located in the out object of the System package.

45. Creating variables • Type the variable type, then what to call the variable, then an = sign, and then what you want the variable to contain. • Variable names should start with a lowercase letter. After that, they can contain any letter or number. • theConventionIsToUseCamelCaseToSeparateWords • = is called the assignment operator

46. Manipulating basic data types • Except for boolean, all basic data types can be changed using these operators: • + adds • - subtracts • * multiplies • / divides • % returns the remainder

47. Manipulating basic data types • Handy tip: Usea <operator>= binstead ofa = a <operator> b • Example: a *= 4 is the same as a = a * 4 • Another handy tip:++ increments by 1-- decrements by 1 • Example: b++ is the same as b = b + 1

48. Manipulating basic data types • Examples:1 + 1 (returns 2)3.0 - 2 (returns 1.0)5 * 10 (returns 50)5.0 / 10 (returns 0.5)10 % 3 (returns 1)

49. Manipulating basic data types • The / operator is tricky, though • If you divide an integer by an integer, an integer is returned! • This means that the decimal is truncated (chopped off) • Example: 10 / 3 returns 3 (because it is 3 remainder 1, the 1 gets chopped off) • Moral of the story: be careful

50. Order of operations • Java executes assignment from right to left • Everything else is executed left to right. • Math is executed in this order: • Parenthesis • Multiplication and division • Addition and subtraction • Assignments and math executions always return the result