Another Example. Other Constructs

Transcription

1 Another Example Re-do DemoSentinel02 using a sentinel of 'q' or 'Q' (for quit). Use a do-statement to setup the loop. DemoSentinel03.java Instructor Note: Demo from Eclipse 36 Other Constructs switch break continue Note: The switch statement is only included in JBA (4 th ed.) as a sidebar (p. 186). And the text pronounces "not recommended" for the continue and break statements (p. 201). In fact, these statements are widely used in Java, and they are relatively each to understand and learn. First, though, let's go to the horse's mouth and look at the "Language Basics" section in Oracle's Java Tutorials (link below)

2 Example (switch, break) Write a Java program that s presents the following menu to the user: What is your annual income: 1. Less than $25K 2. $25K to $50K 3. More than $50K Enter: Get the user's response then output either "Pay no taxes", "Pay 20% taxes", or "Pay 30% taxes", for responses 1, 2, or 3, respectively. Solution: See DemoTaxRate.java 38 Example (break) Write a Java program to process a line of input from the user. Print each character in the line, one by one. However, stop if a period ('.') is detected. Example input: Yes! No. Maybe? Output Yes! No Solution: See DemoBreak.java 39 2

3 Example (continue) Write a Java program to process a line of input from the user. Process the line one character at a time. Print each character, but only if it is a digit (0 9). Example input: Office: LAS 3045, Postal code: M3J 1P3 Output Solution: See DemoContinue.java 40 Infinite loops are Infinite Loops A side effect of incorrectly setting up a loop A simple why to repeatedly do something until a certain condition occurs My advice If it suits your mental model, do it Write a Java program that inputs and prints lines of text entered by the user. Use a line beginning with 'Q' to quit. Solution: See DemoInfiniteLoop.java 41 3

4 Selection Iteration Applications Topics 42 Input Validation Three ways to handle bad input (review): 1. Crash Primitive (but better than no validation) 2. Print a message then end Better. Requires an else statement to skip or continue the rest of the program 3. Print a message and allow retries Best. Requires a test inside a loop Generates an exception Exception free 43 4

5 1. Crash (review) read the input ToolBox.crash(condition, message); rest of program Print a Message, Then End (aka Message-based Validation) Print a message and end the program No exception is thrown Code on previous slide is replaced with read the input if (condition) { output message } else { rest of program } 45 5

6 3. Print a Message and Allow Retries (aka Friendly Validation) Allows the user to retry if invalid input Example Write a program that reads a positive integer from a user and outputs the integer on the console. If the integer is not positive, ask the user to re-enter. DemoFriendlyValidation01.java Instructor Note: Demo from Eclipse 46 for vs. while If you find the for-statement in DemoFriendlyValidation01.java a bit convoluted, you're not alone The flow using a while-statement is, arguably, a closer match to the problem specification What do you think? DemoFriendlyValidation02.java Instructor Note: Demo from Eclipse 47 6

7 File I/O Key points to remember: Use JFileChooser to read file names (maybe!) File input is done through Scanner File output is done through PrintStream Always close the file after using it Suffix the main method with throws java.io.ioexception Handle end-of-file by using: for (; fileinput.hasnextp();) or while (fileinput.hasnextp()) 48 The code Using JFileChooser JFileChooser chooser = new JFileChooser(); chooser.showopendialog(null); displays a file chooser dialog and waits until the user takes action. Follow with chooser.getselectedfile(); to examine the value of the reference. If it is null, the user cancelled. If not, use tostring to get the filename (fully qualified with path) Example (next slide) 49 7

8 Example Write a program that gets the name of a file from the user and then prints that name on the console. DemoFileChooser.java Instructor Note: Demo from Eclipse 50 8