Beginning JAVA (Final)

Transcript

1. Java AN INTRODUCTION
2. What is Java u Is an object oriented programing language u Developed by Sun Microsystems which is now owned Oracle now since 2010 u Modeled after C++ an older programing language
3. What is Java cont. u It was developed as part of a research project to develop software for electronic devices like TV, VCRS, toasters, etc u Also wanted a programming language that was small, fast, efficient, and portable to a range of hardware devices
4. Applet u An interactive program that can run instead a web page u You write it in java language, compile using a Java compiler, and then insert it to an applet file in HTML (web page language)
5. Compiled vs. Interpreted u A compiler figures out everything a program will do, turns it into “machine code” (a format the computer can run really fast), then saves that to be executed later. u An interpreter steps through the source code line by line, figuring out what it’s doing as it goes. u Technically any language could be compiled or interpreted, but one or the other usually makes more sense for a specific language. Generally, interpreting tends to be more flexible, while compiling tends to have higher performance.
6. Compiled and Interpreted u Java has a Java compiler and Java interpreter. u Compiler takes Java program and generate bytecodes from it u Bytecodes are set of instruction processed by a program called the virtual machine, rather than the “real” computer u Processor is the circuit in a computer that processes the basic instructions and make the computer work u Java interpreter translate .class file (contains java bytecodes) into a code (raw machine language code) that works on your computer
7. Java compiler
8. Object-Oriented u Java is Object Oriented u Object oriented programing is a type of technique to organize programs by objects or a.k.a data u Rather than by actions or functions
9. If you want to try java on your computer u You should download Java onto your computer (most likely already has it) u A program to compile your code u Eclipse (advance) u Notepad++ (PC) u BlueJ
10. https://www.jdoodle.com/online-java- compiler Java online compiler
11. First Java application Public class HelloWorld { public static void main (String args [] ) { System.out.println(“Hello World!”) ; } }
12. Public class HelloWorld { public static void main (String args [] ) { System.out.println(“Hello World!”) ; } } TWO MAIN PARTS: - PROGRAM IS ENCLOSED IN A CLASS DEFINITION (HELLOWORLD) - THE BODY OF THE PROGRAM IS CONTAINED IN A ROUTINE CALLED MAIN() - MAIN() IS THE FIRST THING THAT RUNS WHEN A PROGRAM EXECUTES
14. Syntax cont u Still have strings in “ “ u After every method/function, you must end with ; a semicolon u Use { } u Indentation is not important, it’s only for making your code neat u To print, you need to type out u System.out.println( “ “ ); u For variables, it’s the same but you need to declare the variable type u Ex, string phrase = “hello world”; u int number = 4;
15. Errors u You still get Syntax errors like you would in Python u Run time error or logic error is when syntax is correct but the program does something it isn’t supposed to do u You made an logic error u like misspelling in a string, a wrong math formula, etc
16. Algorithms
17. Algorithm Concepts u A sequence of steps for solving a problem u MUST BE: u Unambiguous : Precise instructions for what to do at each step and where to go next. No guess work. u Executable : It can be carried out, no syntax errors and etc u Terminating : the sequence of steps must eventually come to a stop u if it doesn’t your program will continue to run forever and forever and it most likely will break your program/computer u Will usually show up as an error
18. Algorithm example
19. Algorithm example cont.
20. Comments u In python it was # u In Java, you use // (double slashes) u Or /* and */ // this is a comment for only one line /* this is also a comment but I can write multiple lines of comments */
21. Activity
22. Activity
23. Objects and Classes
24. Objects u Like lego building blocks of code u Objects are made up of many kinds of smaller object u Is an instance of a class u Objects have state and behaviors u Example: dogs has states: color, name, breed, and behaviors like barking, eating, and etc
25. Class u A template or blueprint that describes the behavior and state of an object u Embody all the features of a particular set of objects u When you write a program in object-orientated language, you don’t define the objects. You define the classes of objects
26. Constructors u VERY VERY VERY IMPORTANT. ALSO VERY CONFUSING (and that’s okay if you don’t 100% understand them now) u Every class has a constructor u If you don’t create one yourself, Java will make a default one for you that may not be correct or not u Each time a new object is created, one constructor will be made and used. u Can have more than one constructor for one class u Rule: constructors have the same name as the class.
27. Constructor cont.
28. Methods u In a class, you have methods that you write and make u Methods are similar to functions u Java has some premade functions like print
29. Variables
30. Variables u Classes can contain 3 of these variable types u Local variable u Instance variables u Class variables
31. Local Variable u Variables defined inside a method u Made in the method and then destroyed with the method/function is finished
32. Instance Variables u Variables that within the class but not in any particular method/function u They can be reused by any method, will not be destroyed u Can be changed by methods
33. Class variables u Variables declared within a class u Cannot be changed
34. How to name variables, methods, classes u Must start with a letter, or underscore, u No starting with numbers u No other symbols other than underscore u CASE SENSITIVE u milespergallon and milesPerGallon are two different variables u Never start with a capital letter for variables u Don’t use Java reserved worlds like class, double, and etc (you’ll learn more soon)
35. Correct or incorrect? u distance_1 u x u CanVolume u 6pack u can volume u milesPurGalleon
36. Assigning variables
37. Types of variables
38. Methods
39. Methods u We learned methods before (in Python we called them functions) u Methods often require values that give them details about the work that the methods need to do u Like Println method, you must supply the string or value to be printed u Argument is the value that is supplied in a method call u System.out.println(“greetings”)
40. Return Values u There are some methods compute and return a value u Ex. length method returns a value string greeting = “Hello”; int numberOfCharacters = greeting.length(); System.out.println(numberOfCharacters);
41. ( ) u So yes, most methods you seen has the () u Reminder, you usually put arguments or parameters in () u Or the code will give you something (return a value) that you might need to store in a variable if you want to save it
42. String variable u String name = “Vina” ; u System.out.println(name);
43. Numbers u double num1 = 3.14159 ; u int numb2 = 12345;
44. Char u char letter = ‘A’ ;
45. Boolean u Stores true or false u boolean value = true ; u boolean bob = false;
46. Try these out
47. Operators
48. Operators
49. Order of Operations - Basic Math - PEMDAS - Parentheses first, exponents, multiply, divide, add, subtract
50. Modulo % u Gives you the remainder Activity: Finish the code so that will print out the remainder of 2 public class Modulo { public static void main(String[] args) { int myRemainder = System.out.println(myRemainder); } }
51. Relational Operators u Relational operators compare data types (like numbers) u < Less than u <= less than or equal to u > greater than, u >= greater than or equal to
52. Try this out u System.out.println( 5 < 7) ; u What do you get?
53. Equality operators u == is equal to u != is not equal to public class EqualityOperators { public static void main(String[] args) { int myInt = -1; double myDouble = -1.0; System.out.println(myInt == myDouble); } }
54. && and || or (boolean operators) u && means and u Returns true only if both sides of the equation are true u System.out.println ( true && false ); u || means or u Returns true if at least one expression is true u System.out.println ( true || false);
55. ! u Means not u !true = false u !false = true u != u ! <= u etc
56. Precedence u public class Precedence { public static void main(String[] args) { boolean riddle = ??( 1 < 8 && (5 > 2 ??? 3 < 5)); System.out.println(riddle); } } You can order Boolean operators
57. Answer public class Precedence { public static void main(String[] args) { boolean riddle = !( 1 < 8 && (5 > 2 || 3 < 5)); System.out.println(riddle); } }
58. API Documentation
59. API Documentation u The classes and methods of the Java Library are listed in the API documentation u Application Programing Interface u If you need help finding a code or figuring out what a particular code does, you go there u You can also create your own methods (System programming) u Programmers who uses the already created standard Java classes are Application programmers
60. Activity u http://docs.oracle.com/javase/8/docs/api/index.html. u Go to this website for the API documentation. See if you can figure out how to use some of the new code and methods you find
61. Format u Click on a method, gives you the name u Description of what it does u The type and name of the parameter variables u The value that it returns
62. If Statements
63. Conditionals u If is the first part of a conditional expression u Followed by a Boolean expression and then a block of code u If Boolean expression is true, then block of code that folls will be run
64. Example if (9 > 2) { System.out.println("Control flow rocks!"); } ______________________________________________ - No semicolon after if, need { }
65. Try it out public class If { public static void main(String[] args) { if (???????????) { System.out.println("Access granted."); } } }
66. If-Else Statement u So you only need If, if keyword is true u Sometimes we want to execute a different block of code when Boolean expression is false u That’s why we have else
67. if (1 < 3 && 5 < 4) { System.out.println(“I defy Boolean laws!”); } else { System.out.println(“You can thank George Boole!”); }
68. If / else if / else u Sometimes you just need more conditionals if (shoeSize > 12) Print “sorry, your shoe size is out of stock” else if (shoeSize >= 6) Print “you shoes size is in stock” else Print “We don’t carry sizes smaller than a size 6”
69. Set int round = ????; so that else if runs int round; if (round > 12) System.out.println("The match is over!"); } else if (round > 0) { System.out.println("The match is underway!"); } else { System.out.println("The boxing match hasn't started yet."); }
70. Switch Statements u Another type of conditionals u Switch statements keep code organized and less wordy u You don’t need to use them but useful (I don’t like them) u Switch u Case u Break
72. Constructing Objects
73. Constructing Objects u You need to define the properties of objects with classes u Objects of type Rectangle describes rectangular shapes u Etc
74. Constructing Rectangles (x,y) is the top left corner
75. Unfortunately… u You can’t do this with the online Java compilers you guys are using, you would need the actually compiler program for this to work properly
76. Activity Write a program as a class to create an image - snowman - dog - or etc.
77. Class Syntax class Dog { } Created a class Dog and we will define the behavior of dog
78. Constructors u A class constructor will allow us to create instances of Dog and set some information about the Dog u If you don’t create one, Java provides one for you class Dog { public Dog () { } }
79. Instance Variables u Specific details about the class u Ex, we might want to describe the age of the Dog
80. Constructor Parameters u Parameters allow data types to be created with specific attributes
81. Finally creating an Object of the class you created
82. Methods u Pre-defined set of instruction u Teach you have to create a new method for the class you made
83. Accessor and Mutators u An accessor method does not change the data of the object u A mutator method changes the data
84. Which of these are Accessor and Mutator Methods? u .getWidth() u .getX() u .translate(12, 30) u .toUpperCase()
86. Calling the method you made u In your same program you’ve been writing public static void main(String[] args) { Dog spike = new Dog(2); spike.bark(); }
87. Creating Methods with parameters u Same thing as writing a method without one (like bark) u Except you have to write variable type and variable name in the ( ) public void run (int feet) --------------------------------------------- Spike.run(30);
89. void u Note how the previous methods all had void in it u It means that the method returns no value u It’s performing another duty that does not require it to give you an answer
90. Making a method that returns a value u It’s pretty much the same as methods that return nothing u Use data type keywords (String, int, char, and etc.) to specify what that method should return
91. Method returning an int u In between the run and main method, add a method called getAge that returns the age of your dog
92. Now in the main u Set an int variable spikeAge to a value returned by spike.getAge(); u Then print it out
93. If you needed help, here’s the answer
94. YAY you created a fully functional class!!!!
95. Inheritance
96. Inheritance u You can share or inherit behavior from one class to another u It’s so that we can reuse and maintain code more efficiently
97. For example class Dog extends Animal { …. …. public static void main… { spike.checkStatus(); } }
98. Review • - Class: a blueprint for how a data structure should function • - Constructor: instructs the class to set up the initial state of an object • - Object: instance of a class that stores the state of a class • - Method: set of instructions that can be called on an object • - Parameter: values that can be specified when creating an object or calling a method • - Return value: specifies the data type that a method will return after it runs • - Inheritance: allows one class to use functionality defined in another class
99. Data Structure
100. Data Structure u They are much more advance topic in programming u Most professional code requires heavy use of different types of data structures u As long as you understand how data structure works, you’re in good shape
101. Data Structures u It’s a particular way of storing and organizing data in a computer so that it can be used efficiently u The are the key to designing and programming efficient algorithms u All of these affects the quality of your program u Which affects your electronic products and user’s experiences with them
102. In this class u We can’t go over all of them, but here are some: u Interfaces u List, Queue, Deque, etc. u Classes u ArrayList, Vector, LinkedList, HashSets, TreeSets, etc
103. Loop BASICS OF DATA STRUCTURE
104. While Loop and For Loop u Loop Statements execute instructions repeatedly while a condition is true u Uses boolean expressions like if statements u So if true, loop continues u If false, loop stops or terminates
105. While loops while (condition) { statements } int num = 5; while (num < 10) { int equ= num + 2; num++; System.out.println(equ); }
106. Variables inside a loop u When you declare a variable inside the loop body, the variable only exists for that iteration u As your code keeps going through different iteration, the variable is created again and again and then forgotten again and again
107. u That means the variable equ is constantly being made and forgotten u This loop will keep printing as long as num < 10 u ++ is a short cut for +1 (adding 1 to the variable value) int num = 5; while (num < 10) { equ= num + 2; num++; System.out.println(equ); }
108. Examples: what do they print
109. Errors to look out for u Infinite loop: a loop that runs forever and can only be stopped by killing the program or restarting the computer. u You forgot to update the variable the controls the loop (in boolean expression) u Off by one error: You end the program maybe 1 times off after you wanted to. u Keep in mind when you want to us < or <=
110. Pseudocode u Psuedocode is “fake” programming. You use english and diagrams to help you figure out what you need to write in real coding language. u When you’re having trouble writing a loop, always go back to pseudocode and hand trace the loop you want to write u Then go back to the programming language
111. For Loops u For loops is used when a value runs from a starting point to an ending point with a constant increment or decrement. u In other words, it is controlled by a counter u There isn’t much difference between while and for loops except for loops are cleaner and neater to use
112. What does this print? for (int counter =1 ; counter <= 10; counter++) { System.out.println(counter); }
114. For loops cont. u Groups initialization, conditions, and update expressions together u Can count down instead of up u Instead of ++ you can use --
116. i and j u So people use “ i ” and “ j “ more commonly to use as the counter variable u So they aren’t confused with x, y, z, or n which are common names for other number variables
117. Common Loop Algorithms u Sum and average values u Counting and finding matches of number or strings (like ctrl F) u Maximum and Minimum (storing data of the largest or smallest numbers)
118. Nested Loops u You can have a loop contain another loop
119. Something fun… u Scanner class u Allow users to input info into the terminal (where code stuff is previewed like your print statements) https://bit.ly/2u3rOt2
120. Scanner class syntax
121. Arrays
122. Array u An array collects a sequence of values of the same type u You could technically store 10 values into variables like this: value1, value2, value3, and etc… but that’s not practical
123. Declaring Arrays u How to create an array that holds 10 double (data type) double [] value = new double [10] u double[] values = we are creating the variable of the type array u [10] is the length of the array u new operator constructs the array
124. Declaring Arrays cont. double [] moreValues = { 0, 3, 5, 7, 9 }; u Here you can declare an array with specific variables
125. How to access your array double [] value = new double [10] u So your array hold elements of the type double here u Position number of the elements (or index number) starts from 0 u To access an element and store a number in it value [0] = 1.0;
126. Accessing elements u value[0] is the first element u value[1] is the second element u And etc. u System.out.println(value[2]); u value[3] = anotherVariable
127. Try making an array of type String data_Type variable_Name = new data_Type [length#] Replace with the appropriate items If you still need help, here’s another example int [ ] string = new int [2];
128. Using Arrays with Methods u You can have a for loop and store variables into an array
129. Two-Dimensional Array
130. Other Java Object Oriented Terms
131. Packages u Classes in the standard library are organized into packages u Package is a collection of classes with a related purpose u Ex. Rectangle class belong to package java.awt which contains many classes for drawing window, graphics, and shapes u You have to import these classes u System and string classes don’t need to be imported by it’s already automatically imported