Logical operators and comparing Strings

In this exercise you will get experience using "and", "or", and "not" in Java using the logical operators &&, ||, and !. You will also learn how to compare String objects, which operates differently than the comparison of primitives.

Logical operators

There are three logical operators in Java, summarized in the following table.

logical operator	meaning	example	in English
&&	and	a < 3 && b > 10	a is less than 3 and b is greater than 10
||	or	a < 3 || b > 10	a is less than 3 or b is greater than 10
!	not	!(a < 3)	it is not the case that a is less than 3

Both && and || require some expression that evaluates to true or false on either side of the && or || operator. The && operator evaluates to true if both expressions it is operating on evaluate to true. Otherwise it evaluates to false. The || operator evaluates to true if either (or both) of the boolean expressions it is operating on evaluates to true. Otherwise it evaluates to false. Finally, the ! operator reverses the truth value of the expression it is acting on; it is true if the rest of the expression is false and false if the rest of the expression is true.

String comparison

There are two categories of data types in Java: primitives (such as int, double, boolean, and long), and objects (such as Scanner and String). Comparing primitives for equality can be done with the == operator. For objects, however, the == has a different meaning (it determines if two objects are the exact same object in memory). Usually, this is not what we want. Instead, we use methods to compare objects. These methods return boolean values, so we can use them in if statements and while loops just as we would when comparing primitives. Here is how to determine if two String objects contain exactly the same characters:

String method	meaning	example	in English
equals	do the two Strings have exactly the same characters? (case sensitive)	while(z.equals("Hello")) { }	Loop executes as long as z consists of exactly the characters Hello
equalsIgnoreCase	do the two Strings have exactly the same characters? (ignoring case)	while(z.equalsIgnoreCase("Hello")){ }	Loop executes as long as z consists of the characters Hello, in any combination of uppercase and lowercase letters

Shop smart. Shop KowMart.

Create a new Java project Lab6 in the usual way. Create a new class inside called Checkout

This will be a point-of-sale program for a self-checkout counter. Here's the final product in action:

KowMart express self-checkout (10 item limit) How many items (1-10)? 0 How many items (1-10)? 11 How many items (1-10)? 3 Cost of next item? 1.20 Cost of next item? .95 Cost of next item? 3.30 Do you want a bag (costs .05 extra if you have 3 or less items)? yes Your total with 6% MI tax is 5.829999999999999

The program first asks how many items there are, then adds up the cost of all the items, asks if they want a bag, and finally give the total amount due including tax. Start with the following code.

 //Create Scanner object and print greeting. Scanner keyboard = new Scanner(System.in); System.out.println("KowMart express self-checkout (10 item limit)"); //Ask how many items int items; do { System.out.println("How many items (1-10)?"); items = keyboard.nextInt(); }while( TODO ); //Add up the cost of each item (while loop, repeat items times) //Ask if a bag is needed //Add the cost of bag, if necessary. //Add in tax. //print out the total.

I've left a TODO where a condition is needed for whether or not to loop back again. Using our new logical operators, fill in some code that makes it loop as long as they give a number that is less than 1 or greater than 10. Your program should now act like this:

KowMart express self-checkout (10 item limit) How many items (1-10)? 0 How many items (1-10)? 11 How many items (1-10)? 3

Next, make a double variable called cost, for storing the cost of an item entered by the user, and another double variable called total, which will store the total cost of all the goods they are buying (initially this should be set to 0).

Also, create a counter variable, for instance, k, and set it to 0. Then write a second loop, a while loop that repeats as long as k is less than items, the number of items entered above. Inside this loop, 1) ask for the cost of each item, 2) read the cost using your Scanner already defined, and 3) add the current cost to the total (don't worry about checking for negative dollar amounts - we trust our customers and maybe they are entering a coupon). Now you are this far:

KowMart express self-checkout (10 item limit) How many items (1-10)? 0 How many items (1-10)? 11 How many items (1-10)? 3 Cost of next item? 1.20 Cost of next item? .95 Cost of next item? 3.30

Now ask if they want a bag. Get the user's input and store it into a String variable. If they have 3 or less items and they said anything other than "no", then charge them an extra 5 cents (use one of the equality methods of the String class, from the table at the start of this lab). There isn't a String method called notEqual, but we can make do with a logical operator. For example, if you have a String variable called s and you want an expression that tests to see if it is not equal to "hello", then the following code does it: !s.equals("hello")

Finally, add sales tax. That's just a simple matter of multiplying the total by 1.06, and then printing it out.

KowMart express self-checkout (10 item limit) How many items (1-10)? 0 How many items (1-10)? 11 How many items (1-10)? 3 Cost of next item? 1.20 Cost of next item? .95 Cost of next item? 3.30 Do you want a bag (costs .05 extra if you have 3 or less items)? yes Your total with 6 % MI tax is 5.829999999999999

To avoid printing out all those 9's above, you can change the last print statement to use a format command. It looks like this: System.out.printf("Your total with 6 %% sales tax is $%1.2f ",total);

and what it does is print out the value of total using the format instruction "%1.2f" which means floating point output with 2 digits after the decimal point. The " " symbol advances to the next line after the print.

In the printf command, the % symbol is used to mark formatting instructions (%1.2f) in the string you are printing. The $ is included to print out $ so it looks like currency.

To actually print a % sign, as in 6 %, you have to "escape" the normal meaning of % (as a formatting flag) by doubling the percent symbol (as in 6 %%)

If you are looking for an extra challenge: write a loop so that it keeps asking them if they want a bag until they either say "yes" or "no".