We input data from whoever is using the program by using the SavitchIn class

1. We input data from whoever is using the program by using the SavitchIn class • A class is another Java program that you can use without having to rewrite it each time • To ask the user for an integer, we use: SavitchIn.readLineInt( ) • Example: int testScore = 0; System.out.println(“Enter test score: “); testScore = SavitchIn.readLineInt( );

2. The SavitchIn command changes, depending on what kind of variable you are asking for • Int: SavitchIn.readLineInt( ) • Double: SavitchIn.readLineDouble( ) • Char: SavitchIn.readLineNonwhiteChar( ) (why nonWhite?? This way if the user types spaces -- “white space” -- before whatever they type, Java ignores the spaces) • String: SavitchIn.readLine( ) • Demo

3. Type Casting • Let’s say you have a double that you need to convert into an integer • This can be accomplished by type casting: int x = 0; double y = 3.7; x = (int)y; // the (int) is where the type casting occurs

4. Integer vs. Double division • Guess the result of the following: int x = 5; int y = 2; int z = x/y; • Dividing with integers: the decimal part of the answer is truncated (erased). • To be accurate, use at least one double, or one decimal in your division.

5. int x = 5; int y = 2; double z = x/y; what does z equal now? 2.0

6. int x = 5; double y = 2; double z = x/y; what does z equal now? 2.5

7. int x = 5; int y = 2; double z = double(x/y); what does z equal now? 2.0 why? because x/y = 2, then 2 is type-casted (converted) into a double.

8. int x = 5; int y = 2; double z = double(x)/y; what does z equal now? 2.5 why? Notice the slight change in parentheses from the previous slide. Now x is type-casted from 5 to 5.0. Next, 5.0/2 = 2.5. Remember, as long as one of the two numbers being divided is a double, then the result is a double.

9. Variables do not have to be used in order to work with doubles and integers. For example: System.out.println(9/4); (this would display 2) System.out.println(9.0/4); (this would display 2.25) System.out.println(9/4.0); (this would display 2.25) System.out.println(9.0/4.0); (this would display 2.25) Notice that as long as one of the two numbers being divided is a double, then the answer will be a double.

10. Modulus Division • Using the modulus symbol, %, determines the remainder 10 % 6 = 4 13 % 5 = ? = 3 283 % 100 = ? = 83 26 % 2 = ? = 0 27 % 2 = ? = 1 (any even #) % 2 = ? = 0 (any odd #) % 2 = ? = 1

11. Constants • A constant is a variable whose value is expected to remain the same throughout a program • It is considered “good programming style” to use constants for certain variables – for instance, interest rate on a loan, pi, etc • To declare a constant, add the word final: final double RATE = 4.25; • Constants are always written in uppercase • See TempConverter, p.77

12. Class Libraries and Packages • A class library is a set of packages, each of which contains classes (files) of code that can be used by programmers • You can use this code simply by importing a package into your program, with a single line of code • Top line of code: import library.package.class • This is why Object-Oriented languages like Java are so popular – you can use code someone else has written, over and over again

13. Formatting Numbers • Java provides an easy way to format #s into percents, currency, and decimals • For percent and currency, you must import java.text.NumberFormat • For decimals, you must import java.text.DecimalFormat • Example: p97 & 99

14. Assignments (files should be named with this format: P117num4) • P. 117-118 • #4 (Ignore “Create… application to”; look up formula online) • #5 (remember: “floating point value” is another name for a double) • #7 (hints: use both modulus division and integer division; how many seconds are in an hour?) • #11 (start by asking user for # of gallons used, starting odometer #, and ending odometer #) • #12 (ask the user how many of each coin; also, see how the variable subtotal is formatted on p.97)