DPCompSci.Java.1718.notebook April 29, 2018

Transcription

1 Option D OOP 1

2 if Test a condition to decide whether to execute a block of code. Java Tutorials int x = 10; if (x < 20 { System.out.println(x + " < 20"); Prints 10 < 20 if else Test a condition to decide which of two blocks of code to execute. int x = 10; if (x < 20 { System.out.println(x + " < 20"); else { System.out.println(x + " is not < 20"); Prints Yes, 10 < 20 Can be extended with else if to test multiple conditions. if( x == 10 ) { System.out.print("Value of X is 10"); else if( x == 20 ) { System.out.print("Value of X is 20"); else if( x == 30 ) { System.out.print("Value of X is 30"); else { System.out.print("Something else"); else if's must come before the final else (if there is one) Once an else if succeeds, no further tests are performed. See also: switch Prints the value of x if it's 10, 20, or 30; otherwise prints "Something else". switch Execute a block of code that depends on the value of a variable. Variable must be an integer, string, or enum Can execute multiple cases Use break to exit the switch Default gets executed if no case is matched char grade = 'C'; switch(grade) { case 'A' : System.out.println("Excellent!"); break; case 'B' : case 'C' : System.out.println("Well done"); break; case 'D' : System.out.println("You passed"); case 'F' : System.out.println("Better try again"); break; default : System.out.println("Invalid grade"); System.out.println("Your grade is " + grade); Conditionals 2

3 For loop Java Tutorials When you know how many times you need to execute the code block Increment happens after the code block Code block needs to cause the condition to be met if you ever want to exit for (int x = 10; x < 20; x++ ) { Prints value of x : 10 value of x : 11 value of x : 12 value of x : 13 value of x : 14 value of x : 15 value of x : 16 value of x : 17 value of x : 18 value of x : 19 While loop When you don't know how many times you need to execute the code block May never execute Code block needs to cause the condition to be met if you ever want to exit int x = 10; while( x < 20 ) { x++; Prints value of x : 10 value of x : 11 value of x : 12 value of x : 13 value of x : 14 value of x : 15 value of x : 16 value of x : 17 value of x : 18 value of x : 19 Do While loop When you don't know how many times you need to execute the code block Executes the code, then tests the condition. Code block needs to cause the condition to be met if you ever want to exit int x = 10; do { x++; while( x < 20 ) Prints value of x : 10 value of x : 11 value of x : 12 value of x : 13 value of x : 14 value of x : 15 value of x : 16 value of x : 17 value of x : 18 value of x : 19 break Exits the innermost for, while, do while, or switch statement Execution continues after the innermost code block for (int x = 10; x < 20; x++ ) { if (x > 12) break; int x = 10; while( x < 20 ) { if (x > 12) break; x++; All these print value of x : 10 value of x : 11 value of x : 12 int x = 10; do { if (x > 12) break; x++; while( x < 20 ) continue Skips the current iteration of the innermost for, while, do while, or switch statement Execution continues at the start of the code block for (int x = 10; x < 20; x++ ) { if (x > 12 && x < 16) continue; int x = 10; while( x < 20 ) { if (x > 12 && x < 16){ x++; continue; x++; All these print value of x : 10 value of x : 11 value of x : 12 value of x : 16 value of x : 17 value of x : 19 value of x : 20 int x = 10; do { if (x > 12 && x < 16){ x++; continue; x++; while( x < 20 ) Loops 3

4 Keyboard Input using Scanner Java Tutorials The Scanner class handles keyboard input The class is in java.util so you need to import java.util.scanner to use it Start by creating an instance of a Scanner object. Scanner objects have lots of methods to read and interpret input from the keyboard. You can connect the Scanner class to various input sources the console, files, strings, etc. import java.util.scanner; import java.io.*; //optional if you want to declare System.in... variable keyboard points to an object of type Scanner void echome() { Scanner keyboard = new Scanner(new InputStreamReader(System.in)); System.out.println("Enter some text: "); String instr = keyboard.nextline(); System.out.println("You answered:" + instr + ":"); Some of the more useful methods of the Scanner class are: System.in identifies the "standard system input" as the source to be scanned. This is the console. Scanner objects have a nextline() method that waits for the user to type something then returns the line entered. boolean hasnext(string pattern) This method returns true if the next token matches the pattern constructed from the specified string. boolean hasnextint() and boolean hasnext<type>() This method returns true if the next token in this scanner's input can be interpreted as an int value in the default radix using the nextint() method. String next(string pattern) This method returns the next token if it matches the pattern constructed from the specified string. int nextint() and <type> next<type>() This method scans the next token of the input as an int. String nextline() This method advances this scanner past the current line and returns the input that was skipped. Keyboard Input using BufferedReader The BufferedReader class is another way to handle keyboard input The class inherits from java.io.reader and java.io.inputstreamreader so you need to import them both Use this class when you do not need to parse the input but want to read complete lines. See more at Oracle's BufferedReader page Keyboard Input 4

5 File Input using Scanner Java Tutorials The Scanner class also handles file input Connect your Scanner object to a file when you instantiate it. import java.util.scanner; import java.io.*;... //optional if you want to declare System.in void ReadFile() { try { System.out.print("Enter the file name with extension : "); Scanner input = new Scanner(new InputStreamReader(System.in)); File infile = new File(input.nextLine()); input = new Scanner(inFile); while (input.hasnextline()) { String line = input.nextline(); System.out.println(line); input.close(); catch (Exception ex) { ex.printstacktrace(); Keyboard Input using BufferedReader The BufferedReader class is another way to handle keyboard input The class inherits from java.io.reader and java.io.inputstreamreader so you need to import them both Use this class when you do not need to parse the input but want to read complete lines. See more at Oracle's BufferedReader page File Input 5

6 Java Documentation File Operations FileChooser Graphics Create a Frame GUIs in NetBeans Arrays Linked Lists Overview of Q4 Concepts OOP ideas Algorithms Start on IA Main Assignments Mini Project IA Client & Solution IA Conceptual Prototype Some resources: Kjell online book CodingBat Programming by Doing 3/26/18: Q4 Start 6

7 3/26/18 Where are we? You should have a TestFileScanner class in your Exercises project similar to the one below. How did I get this documentation? Let's look. From now on, you will be expected to document your code, with a minimum of a header for each class and method. You may do this after the code is debugged and working, but anything that you turn in must be documented. Today: Start a new BlueJ project called GUITools. In it, create a class called TestGUIFileIO Our first method will be choosefile which uses Java's JFileChooser class. public File choosefile(string title, String defname){ JFileChooser fc = new JFileChooser(); fc.setdialogtitle(title); fc.setcurrentdirectory(new File(defName)); fc.setselectedfile(new File(defName)); int result = fc.showopendialog(null); if (result == JFileChooser.APPROVE_OPTION) { System.out.println("Chose " + fc.getselectedfile()); return fc.getselectedfile(); System.out.println("User cancelled"); //debug return null; //debug JFileChooser is very powerful. Look at some of its other features to, for example, show only directories, manage the look and feel, filter to only certain file types, etc. HW: Read Kjell, Ch 23 Review & Exercise 5 or 6 3/26 File IO 7

8 3/26/18 Here's the code we used on Monday to select a file: public File choosefile(string title, String defname){ JFileChooser fc = new JFileChooser(); fc.setdialogtitle(title); fc.setcurrentdirectory(new File(defName)); fc.setselectedfile(new File(defName)); int result = fc.showopendialog(null); if (result == JFileChooser.APPROVE_OPTION) { System.out.println("Chose " + fc.getselectedfile()); return fc.getselectedfile(); System.out.println("User cancelled"); //debug return null; //debug We saw that, on a Mac, the showopendialog method was not allowing us to type in a file name. The same code works differently in Windows! Option to type in filename To avoid this inconsistency, I found that we can use showdialog instead. It is more customizable than showopendialog and works the same on OSX or Windows. Whew! public File choosefile(string title, String buttontext, String defname){ JFileChooser fc = new JFileChooser(); fc.setdialogtitle(title); fc.setcurrentdirectory(new File(defName)); fc.setselectedfile(new File(defName)); int result = fc.showdialog(null, buttontext); //not showopendialog if (result == JFileChooser.APPROVE_OPTION) { System.out.println("Chose " + fc.getselectedfile()); //debug return fc.getselectedfile(); System.out.println("User cancelled"); //debug return null; JFileChooser is very powerful. Look at some of its other features to, for example, show only directories, manage the look and feel, filter to only certain file types, etc. 3/28/18 Monday 3/26 HW: Read Kjell, Ch 23 Review & Exercise 5 or 6 echofile(): Use choosefile to create a method that opens a text file and echoes the contents to the console. Call it echofile(). Hint: See your readfilewerror method. keyboardtofile(): Use choosefile to open an output file and enter lines from the keyboard that get appended to that file. Call it keyboardtofile(). Be careful not to overwrite a file that you want. And be sure to close the file! appendtofile(): Use choosefile to open an input file and an output file. Append the lines from the input file to the end of the output file. Call it appendtofile(). Be careful not to overwrite a file that you want. And be sure to close both files! loopthroughfile(): Use choosefile to open an input file and an output file. Loop through the lines from the input file and write them to the output file, overwriting it if it exists! Call it LoopThroughFile(). Be careful not to overwrite a file that you want. And be sure to close both files! Notice that loopthroughfile() this could be done directly with Java's Files.copy() method. Why write our own loop? We now have a template for looping through the lines of a file, processing them in some way, and then writing the processed line to the output file. Very powerful! Wednesday 3/28 HW: Read Kjell, Ch 24 3/28 File IO Cont 8

9 (later) DPCompSci.Java.1718.notebook 4/2/18 As we get into more complicated territory, Let's take a time out to go back and review some of the basic constructs that we've seen: Program Organization Projects Packages Classes Objects Methods Functions Variables > "primitive" types > Advanced?, Composite?, Civilized? types Program creation (building) Source Code Compiled Code "Build" or "Make" instruction files. Run Projects File Content Organization imports main() (... later...) Classe(s) Class methods local functions Program flow control (statements) Syntax If then If then else while loops do while loops for loops try catch Packages...in Java...in BlueJ...in NetBeans Classes a "Source Code" file (text;.java) a "Compiled" file (binary;.class) > Made by "compiling" the "source code" into "machine code" (1's & 0's) a "Class Context" file (text;.ctxt) > A "Build instruction file" Method (defining) Methods (calling) <obj>.<method>...later in the same file... Returns a Value of type "File" Methods vs. Functions Basically the same Methods are implicitly called by an object, and Method (defining) so, have access to information about the parent object (this). Java doesn't really have "Calling" Parameters "functions" since all processes are defined in a Class and hence, called methods. Return Value Note: Type must be File...later. still in the same file... To "run" appendfile(), for example Compile TestGUIFileIO (BlueJ creates.class file) > Compilers "import" supporting classes Instantiate a new TestGUIFileIO object > Can be done in the code with a "constructor" method: new <Class> Execute a method from the Object > If needed, BlueJ will prompt you for parameters From another point of view: Variables, run time, & flow control Variables of type File Try Catch construction while loop...later in the same file... "Instantiate" a new object from the class JFileChooser using new "fc is a JfileChooser object" conditional block if...later. still in the same file... primitive type int for loop At "Run time" Events that occur during the time that the program is running. Objects...only exist at run time....are specific "instances" of a Class..."carry with them" the fields and methods of the parent Class...are generally created with a "constructor" method of the Class using new <Class>. Let's look at your Kjell Exercise 23.5 or 23.6 Which of these do you have finished? echofile(): Use choosefile to create a method that opens a text file and echoes the contents to the console. Call it echofile(). Hint: See your readfilewerror method. PrintWriter keyboardtofile(): Use choosefile to open an output file and enter lines from the keyboard that get appended to that file. Call it keyboardtofile(). Be careful not to overwrite a file that you want. And be sure to close the file! Pseudo code for keyboardtofile Choose a file (the output file) Create an input Scanner object from the keyboard Write some general instructions to the user on the console Try to Create a Printwriter connected to the output file Print a prompt, such as ">", to the console Get the next line of input from the console into a string As long as the string is not of zero length Print the next line Print another prompt (">") Get the next line from the console Close the PrintWriter when finished (.close() method) Catch any IO exceptions with a stack trace Close the console 4/2 Questions 9

10 4/4/18 HW Quiz: 1. Generally speaking, what does the code at the right do? 2. Fill in the values that belong in line: 18: 0 [1] 24: sizea [1] 27: suma = suma + value [1] 28: 1. count++ [1] 32: 2. suma/sizea [1] 3. In the code shown, how does the user enter the input file name? In what line? [2] 4. Name at least 2 problems with this technique. [2] 5. Name a better technique. [1] 6. What line(s) of code could you change to make it easier for the user to enter a valid filename? [2] 7. Describe how you would do that. Be as specific as you can. [4] appendtofile(): Use choosefile to open an input file and an output file. Append the lines from the input file to the end of the output file. Call it appendtofile(). Be careful not to overwrite a file that you want. And be sure to close both files! loopthroughfile(): Use choosefile to open an input file and an output file. Loop through the lines from the input file and write them to the output file, overwriting it if it exists! Call it LoopThroughFile(). Be careful not to overwrite a file that you want. And be sure to close both files! Notice that loopthroughfile() this could be done directly with Java's Files.copy() method. Why write our own loop? We now have a template for looping through the lines of a file, processing them in some way, and then writing the processed line to the output file. Very powerful! HW: Write yesno() 4/4 More FileIO 10

11 Download, install NetBeans Creating a graphical user interface with NetBeans 4/9/18 Kjell: Read Ch 46 HW: PBD 136: A little puzzle Create a NetBeans Projects folder somewhere on your computer where you can find it (like in.. \Desert\1718\IBCompSci\NetBeans Projects) Start a new project called NBExercises in your NetBeans Projects folder. Identify the Main class as nbexercises.nbexercises. Create a main method The main() method All java programs that run in a stand alone environment need a main method. The main method can be in any class that is part of the program. The Java virtual machine looks for the main method when a program is first run to "get started". The main method can be in any class that is part of the program. A typical main method might look like: public static void main(string[] args){ try { // Set System L&F UIManager.setLookAndFeel( This instantiates UIManager.getSystemLookAndFeelClassName()); the class <parentclass> program = new <parentclass()>; //the code to run (test?) goes below program.method1totest(); //program.method2totest(); catch (Exception e) { e.printstacktrace(); System.exit(0); // necessary Task: Get your loopthroughfile method running in NetBeans Include choosefile (with 3 parameters and using a parent dialog) Include your working yesno() method How did I get the default name of the file to copy to? Next, work on PBD 136 and finish it for Wed. PBD 136: Open a file specified by the user. This file will contain a bunch of characters. You should read in each character from the file, one character at a time. Display every third character on the screen. Throw the other characters away. There is a sample input file called puzzle.txt, containing a little message you can use to test your program. For fun, the "thrown away" characters might say something, too, in case you care to try to view them somehow. Pseudo code for PBD136 Choose the input file (puzzle.txt or puzzle2.txt) Try to Connect the input file to a Scanner object (so we can read it) As long as there is another line in the input file Get the next line Connect the line to a Scanner so we can loop through characters Set the delimiter to "" so we can get one character at a time As long as the line has another character if the position is a multiple of 3 print the character to the console Close the input file when finished (.close() method) Catch any IO exceptions with a stack trace Hint: On PBD136 you need to do a little trick to use Scanner to read one character at a time. Kjell: Read Ch 46 HW: PBD 136: A little puzzle All further programming work should be done in NetBeans 4/9 Netbeans 11

12 4/11/18 Arrays Java Tutorials HW: PBD Basic Arrays An ordered group of elements Arrays are objects from an Arrays class, with associated methods Elements can be primitive types or objects Need to be declared and created (can be done in one statement) Need to know the size when it's created int[] myintarray; myintarray = new int[10]; //declare an array of integers //create it with 10 elements char[] mychararray = new char[10]; //a 10 element array of chars //you can initialize to values String[] mystringarray = {"Pine", "Fred", "Avacado", "granite"; for (int s: ) { Do the series of exercises PBD , Basic Arrays. Please follow the instructions exactly. me the.java file that contains your code. The method names should be PBD138, PBD139, PBD140, and PBD141 PBD138: Create an array that can hold ten integers. Fill up each slot of the array with the number 113. Then display the contents of the array on the screen, each value on a separate row. Do not use a loop. Also, do not use any variable for the index; you must use literal numbers to refer to each slot. PBD139: Create an array that can hold ten integers. Fill up each slot of the array with the number 113. Then display the contents of the array on the screen, each value on a separate row. This time, you must use a loop, to put the values in the array and also to display them. Also, in the condition of your loop, you should not count up to a literal number. Instead you should use the length field of your array. PBD140: Create an array that can hold ten integers. Fill up each slot of the array with a random number from 1 to 100. Then display the contents of the array on the screen. You must use a loop to fill the array. And, like last time, you must use the length field of your array and not a literal number (like 10) in the condition of the loop. PBD141: Create an array that can hold 1000 integers. Fill the array with random numbers in the range Then display the contents of the array on the screen in rows of 20. You must use a loop to fill the array. If you're careful to only pick random numbers from 10 to 99 and you put two spaces after each number, then your output will line up as below. And if you don't know how to pick random numbers in a certain range, see the nextint() method of the Random class in Javadocs. HW: PBD Basic Arrays 4/11 Arrays 12

13 4/13/18 In class today, you will work on Kjell: Ch 47 Java Tutorials Once you have read it, work on Exercises 4 6 Finish for Monday HW: Kjell Ch 47 Ex 4 6 4/16/18 4/20/18 In class today, and Wed we will work on Kjell: Ch 49C (2D Arrays) Java Tutorials Once you have read it, work on Exercises 1,2,4,5,6 & 7 (optional) Finish for Monday 4/23 HW: Kjell Ch 49C Exercises 1,2,4,5,6 & 7 (optional) 4/23/18 4/27/18 More with arrays and sorting 4/13 Array algorithms 13

14 4/27/18 Sorting Algorithms Exchange sort For each element in the array, starting with the first one: Compare each of the remaining elements to the first one If any element is larger than the first one, swap the two. 3 < 7? i j < 3? i j 20 < 3? i j < 3? i j a[0] = min 12 < 7? i j 20 < 7? i j < 7? i j a[1] = 2nd smallest < 12? i j 16 < 12? i j 8 < 12? i j < 7? i j a[2] = 3rd smallest Bubble sort Selection sort 4/27 Array algorithms 14

15 4/30/18 PBD160: Bubble sort PBD161: Selection sort 5/2/18 Kjell Ch 36: Graphics HW: Kjell Ch 36 Exercises 1 6 5/4/18 Kjell Ch 37: More graphics ideas HW: Kjell Ch 37 Mini Project (Due Wed, 5/9) 5/2 Graphics Intro 15

16 Attachments JavaCheatSheet.pdf DPCS.MiniProject.pdf