Java: Increment/Decrement Operators & While Loop

Increment and Decrement Operators

• There are times when a variable must simply be incremented or decremented
• Java provides shortened ways to increment and decrement a variable's value
• Use the ++ or -- unary operators to quickly increment or decrement
• When only operations in a statement, there is no difference between prefix and postfix notation

Differences Between Prefix and Postfix

• Prefix notation indicates that the variable will be incremented or decremented prior to the rest of the equation being evaluated
• Postfix notation indicates that the variable will be incremented or decremented after the rest of the equation has been evaluated

While Loop

• Java provides three different looping structures like the while loop, do-while loop and for loop
• While loop will execute as long as the condition is true, the statements will be executed repeatedly
• The while loop is a pretest loop, which tests the value of the condition prior to executing the loop
• Care must be taken to set the condition to false somewhere in the loop so the loop will end
• Loops that do not end are called infinite loops
• A while loop executes 0 or more times and if the condition is false the loop will not execute
• Curly braces are required to enclose block statement while loops, like block if statements are

Infinite loop

• For a while loop to end, the condition must become false
• If the variable 'x' never gets decremented it will always be greater than 0

Input Validation

• Input validation is the process of ensuring that user input is valid

Do-While Loop

• The do-while loop is a post-test loop, which executes the loop prior to testing the condition
• This is sometimes called a do loop
• Use the do-while loop when the loop executes at least once

The For Loop

• The for loop is a pre-test loop
• The for Loop allows the programmer to: initialize a control variable, test a condition, and modify and update the control variable all in one line of code
• The for loop takes the form for(initialization ; test ; update)
• First the program performs steps to initialization expression, then evaluates the test expression and if true the body of the loop executes. Finally the update expression performs, then returns to step 2

For Loop Sections

• The initialization section of the for loop allows the loop to initialize its own control variable
• The test section (BooleanExpression) of the for statement acts the same as the condition section of a while loop
• The update section of the for loop is the last thing to execute at the end of each loop
• The initialization section of a for loop is optional, but is usually provided
• Initialization section can initialize multiple variables separated with commas
• Variables declared in this section have scope only for the for loop
• The update expression is usually used to increment or decrement the counter variable(s) declared in the initialization section of the for loop
• Avoid updating the control variable of a for loop within the body of the loop and instead, use the update section
• The for Loop can initialize and update multiple variables
• The only parts of a for loop that are mandatory are the semicolons

Running Totals and sentinel Values

• Loops allow the program to keep running totals while evaluating data
• A running total is a sum of numbers that accumulates with each iteration of a loop
• The variable used to keep the running total is called an accumulator
• A sentinel value can be used to notify the program to stop acquiring input
• User could add data that is not normally in the input data range to signal to end of input, like entering -1 when positive numbers are usually entered

Nested Loops

• Just like if statements, loops can be nested
• If a loop is nested, the inner loop executes all of its iterations for each time the outer loop executes once

Break and Continue Statement

• The break statement can be used to abnormally terminate a loop which bypasses the normal mechanisms and makes the code hard to read and maintain
• The continue statement causes the currently executing iteration of a loop to terminate and the next iteration to begin causing evaluation of the condition in while and for loops
• The continue statement should be avoided because it makes the code hard to read and debug

Deciding Which Loop to Use

• The while loop is a pretest loop and should be used when you do not want the statements to execute if the condition is false in the beginning
• The do-while loop is a post-test loop and should be used when you want the statements to execute at least one time
• The for loop is a pretest loop and should be used when there is some type of counting variable that can be evaluated

File Input and Output

• Data can be saved to a file
• Files can be input files or output files
• Files are opened, written to, then closed prior to program termination
• There are two types of files, binary and text
• To open a file for text output you create an instance of the PrintWriter class
• Pass the name of the file that you wish to open as an argument to the PrintWriter constructor, erasing and replacing it of there is a file with the same name

PrintWriter Class

• The PrintWriter class allows you to write data to a file using the print and println methods, as you have been using to display data on the screen
• Just as with the System.out object, the println method of the PrintWriter class places a newline character after the written data
• The print method writes data without writing the newline character
• Use the PrintWriter class with an import statement and the top of the source file
• To avoid erasing a file that already exists, create a FileWriter object and then create a PrintWriter object
• Paths on Windows contain backslash () characters but it is the escape character (like"\n") so you must use two of them
• Java allows Unix style filenames using the forward slash (/) to separate directories

Exceptions

• When something unexpected happens in a Java program, an exception is thrown and the method that is executing when the exception is thrown must either handle the exception or pass it up the line
• To pass it up the line, the method needs a throws clause in the method header
• To insert a throws clause in a method header, simply add the word throws and the name of the expected exception like and IOException
• PrintWriter objects can throw an IOException

Input Files

• You use the File class and the Scanner class to read data from a file, passing the name of the file as an argument to the File class constructor
• The Scanner class reads from the keyboard, prompts the user for a filename, and creates an instance of the File class to represent the file and read from it
• Once an instance of Scanner is created, data can be read using the same methods that you have used to read keyboard input (nextLine, nextInt, nextDouble, etc).
• A Scanner class can throw an IOException when a File object is passed to its constructor and the Scanner class's hasNext() method returns true if another item can be read from the file

Random Class

• The need to use randomly generated numbers can be accomplished with the Java Random class
• To use the Random class, use the following import statement and create an instance of the class
• import java.util.Random;
• Random randomNumbers = new Random();

Random Class Methods

• nextDouble() - Returns the next random number as a double which will be within the range of 0.0 and 1.0
• nextFloat() - Returns the next random number as a float and will be within the range of 0.0 and 1.0
• nextInt() - Returns the next random number as an int and will be within the range of an int, which is -2,147,483,648 to +2,147,483,648
• nextInt(int n) - accepts an integer argument, n, and returns a random number as an integer. The number will be within the range of 0 to n, which includes 0 but excludes n