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
• Also, constants should be written in uppercase
• To declare a constant, add the word final:
final double PI= 3.1415;
final int HOURS_IN_A_DAY= 24;
PI = 3.7; // this would cause an error
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
Double precision problem
• In Java, doubles are slightly imprecise.
• For example, sometimes if you multiply 6.0
by 3.0, the answer is 17.999999999, or
18.00000000001.
• So, if your program requires precision,
either do not use doubles, or, use them but
round them off to a certain amount of
decimal places.
• Demo: DecimalRound
Working with Strings
• Characters in a String can be identified by their index #.
• The first character in a String has index # 0.
• You need to know the following String methods:
Length( )
returns the number of characters in the String
charAt(index)
returns the char at the given index
indexOf(char)
reverse of charAt. returns the first index # of
the given char
returns the chars from index Start to index
(End – 1)
substring(Start, End)
substring(Start)
when only the starting index is given, returns
the chars from start to the end of the String
toUpperCase, toLowerCase converts the entire String to upper or lowercase
Examples…
String coin = “Nickel”;
code
result
coin.length( )
6 (even though the last letter
is at index #5)
coin.toUpperCase( )
NICKEL
coin.charAt(1)
i
coin.indexOf(“k”)
3
coin.indexOf(“g”)
–1 (since g was not found)
coin.substring(1,3)
“ic” (notice that letters 1 and
2 are the result, but not 3)
coin.substring(2)
“ckel” (starts at 2 and goes to
the end)
Demo: StringMethods
Assignments
1. (Strings1)
 Prompt the user to enter a word.
 If the word is “apple,” then ask the user to enter a letter. If that
letter is a or b, display “banana.” If it’s another letter, display
“canteloupe.”
 If the word is “kiwi,” then ask the user to enter a number.
 If the number is odd and divisible by 9, display “orange.”
 If the number is negative or equals 2112, display “pineapple.”
 If the word is something other than apple or kiwi, display the 3rd
letter of the word.
 Display how many letters are in the word.
(see next slide)
2. (Middle)

Prompt the user to enter their middle name.
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If the 1st letter is a vowel, display “Vowel”, otherwise, “consonant”
If there are more than 8 letters in the name, display “long”, if between 5
and 8 (inclusive), display “medium”, otherwise, display “short.”
Display whether the # of letters in the name is even or odd. Also, if it’s
even, display whether or not it is divisible by 4. If it’s odd, display
whether or not it is divisible by 5.
If the letter ‘a’ is in the name, display the first index # it was found at
(example: “The letter a was first found at index #3”) If it is not found,
then say “The letter a was not found.”
If the first and last letters in the name are the same, display “same,”
otherwise “different”
If the 2nd letter and the 2nd-to-last letter are both k, display “YES K”
Display the 1st through the 3rd letters.
Display all letters from the 2nd to the end of the name.
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