SE-1010 Dr. Mark L. Hornick 1 Variables & Datatypes

Java (like virtually all programming languages) supports the concept of variables You probably are familiar with the concept of variables from algebra, such as x = 3 Where the variable x in this case represents the integer 3 (initially), but can be reassigned to represent another value at any time. SE Focus Dr. Mark L. Hornick 2

Brainstorm v = 3.14 w = 1/3 u = 1 c = sin(w) + cos(z) y 2 = -1 z = 1 + y SE Focus Dr. Mark L. Hornick 3 Suppose these are algebraic equalities. What type of numbers do the variables represent?

Variables in Java are a similar concept, but different If we want to use a variable x to represent an integer value of 3, we write int x = 3; In Java, we have to declare the datatype (i.e. the kind of value) that the variable will represent. In this case, “int” means that x can represent only integer values [of a specific range]. We say that x is the identifier (i.e. the name) of the variable SE Focus Dr. Mark L. Hornick 4 Note the semicolon; This is required in Java!

In Java, we can arbitrarily make up [nearly] any name we like to use as a variable identifier, provided we follow a few rules: 1. An identifier may consist of a sequence of one or more letters, digits, underscores, and dollar signs ( e.g. my_1st_$ ) 2. The first character in a Java identifier is generally a letter, rarely an underscore or dollar sign, and may not be a digit:  x_1 (ok)  $y2(ok, but unconventional)  _z3(ok, but unconventional)  1w(illegal) 3. Uppercase and lowercase letters are distinguished; the following are treated as 3 different identifiers:  myValue  Myvalue  MyValue 4. No spaces are allowed in an identifier. 5. A Java Reserved Word may not be used as an identifier. SE Focus Dr. Mark L. Hornick 5

6 Java’s reserved words cannot be used as identifiers abstract default if private this boolean do implements protected throw break double import public throws byte else instanceof return transient case extends int short try catch final interface static void char finally long strictfp volatile class float native super while const for new switch continue goto package synchronized

7 Java supports various “built-in” primitive datatypes There are six numeric datatypes in Java: byte short int long float double byte, short, int, and long represent integer values float and double represent real values

SE-1010 Dr. Mark L. Hornick 8 Numeric values have a limited range of allowed values Why do you think there are four different datatypes just to represent integer values?

SE-1010 Dr. Mark L. Hornick 9 There are generally accepted rules for naming Java variables that will represent primitive datatypes Use a lowercase letter for the first letter of a variable identifier, uppercase for subsequent words Use nouns or noun phrases to represent things that have values x areaCircle sumOfValues Identifiers that begin with uppercase letters (or are all uppercase letters) are used for other Java elements we’ll see later on… Good variable names convey a logical meaning as to the type of data the variables represent.

SE-1010 Dr. Mark L. Hornick 10 Don’t use verbs, adverbs, adjectives or meaningless names for variable identifiers getIt Quickly happy FredFlintstone iDontLikeBeets Names like these don’t convey any meaning to the reader of a program.

In Java, a variable can be also represent many non-primitive datatypes… String s; SE 1011 Dr. Mark L. Hornick 11 This declares a variable s of datatype String. String is another predefined, non-primitive Java datatype. What kind of values can be held by this type of variable???

SE-1010 Dr. Mark L. Hornick 12 The String datatype is a built-in class (from the java.lang package) that represents a sequence of characters String s1 = “a”; String s2 =“SE-1011”; String s3 =“123”; String s4 =“This is a 6 word string.”; String s5 = “”; There is no fundamental restriction on the maximum length of a Java String How does this differ from a int that represents 123? This represents an empty String String sequences are surrounded by double-quotes

SE-1010 Dr. Mark L. Hornick 13 The char primitive datatype represents only a single character char c1 =‘1’; char c2 =‘a’; char c3 =‘?’; char c4 =‘B’; char c5 =‘%’; How does this differ from a int that represents 1? Characters are surrounded by single-quotes

SE-1010 Dr. Mark L. Hornick 14 char c1 = ‘\t’; // tab char c2 =‘\n’; // newline char c3 =‘\r’; // carriage return char c4 =‘\”’; // double-quote char c5 =‘\’’; // single quote char c5 =‘\\’; // backslash Use escape sequences to represent special characters Escape sequences are two-characters consisting of the backslash followed by an escape code

SE-1010 Dr. Mark L. Hornick 15 String hello = “Hello \n World”; System.out.println( hello ); Escape sequences can be used in Strings too NOTE: PowerPoint uses distinct single- and double-quote characters that do not appear on your keyboard!

SE-1010 Dr. Mark L. Hornick 16 The boolean datatype represents only two logical values: true and false boolean isOff = true; boolean isCold = false; The boolean datatype is named after George Boole, an English mathematician/logician who developed Boolean algebra true and false are both Java reserved words

Repeat: You must declare a variable’s datatype in Java before you can use that variable int x; String name; char c1; boolean isDone; float someValue; double areaCircle; SE-1010 Dr. Mark L. Hornick 17 x, name, c1, isDone, someValue, and areaCircle are all identifiers that represent only specific types of data. So, you cannot assign a numeric value to name, nor can you assign a character string value to x.

Variables can be initialized at the same time as they are declared int x = 0; String name = “Mark”; char c1 = ‘a’; boolean isDone= false; float someValue = 3.14F; double areaCircle = 2.6; SE-1010 Dr. Mark L. Hornick 18 The value of a variable can be reassigned at any time, even when it’s initialized.

A constant is a value that cannot be changed after initialization final int ZERO = 0; final String YES = “Yeah”; final char QUESTION = ‘?’; final boolean OFF = false; final float E = 2.17F; final double PI= ; SE-1010 Dr. Mark L. Hornick 19 The value of a constant must be assigned at initialization, and cannot be reassigned afterwards.

SE-1010 Dr. Mark L. Hornick 20 Arithmetic operators in Java are used on numeric datatypes (byte, short, int, long, float, and double)

Examples of arithmetic expressions and assignment using the binary arithmetic operators *, +, -, / and % int x = 1; int y = x+1; x = x+3-y; int z = x*y*2; z = (x+y)*2+4; // note use of () int w = 4/2+5-3; w = 5 % 2; SE-1010 Dr. Mark L. Hornick 21

Increment and Decrement arithmetic operators The ++ and -- unary operators are used to increment or decrement the value of a variable by 1. x++; // increments x by 1 ++x; // ditto --x; // decrements x by 1 x--; // ditto SE-1010 Dr. Mark L. Hornick 22

SE-1010 Dr. Mark L. Hornick 23 Arithmetic Expressions Precedence rules for arithmetic operators and parentheses

Integer Overflow The range of an int is to So what happens when you add 1 to an int holding a value of 32767?? SE-1010 Dr. Mark L. Hornick 24

Integer Division Integers variables can only hold whole values, so what happens if you do the following? int x = 2; int y = 3; int z = x/y; // what is the result? SE-1010 Dr. Mark L. Hornick 25

Variable values can be assigned (and re-assigned) as the result of various operations int radius = 3; float areaCircle = radius * ; radius = 4; areaCircle = radius * ; SE-1010 Dr. Mark L. Hornick 26 Declaration and assignment of radius Declaration and assignment of areaCircle Reassignment of previously declared radius Reassignment of previously-declared areaCircle

Q: Would it make sense to declare areaCircle to be an int? int radius = 3; int areaCircle = radius * ; radius = 4; areaCircle = radius * ; SE-1010 Dr. Mark L. Hornick 27 Declaration and assignment of radius Declaration and assignment of areaCircle Reassignment of previously declared radius Reassignment of previously-declared areaCircle

SE-1010 Dr. Mark L. Hornick 28 Details: Primitive variables vs. non-primitive (object) variables The only difference between a variable for a primitive and a variable for objects is the contents in the memory locations. For primitives, a variable contains the numerical value itself. For objects (like Strings), a variable contains an address where the object is stored. 10 str1 x String object

SE-1010 Dr. Mark L. Hornick 29 Can we use arithmetic operators (like + or -) on object variables? String s1 = “se”; String s2 = “1011”; String output = s1 + s2; // does this work??

SE-1010 Dr. Mark L. Hornick 30 The only operator defined by Java for a String variable is “+” The “+” operator, when used to “add” Strings, actually causes the Strings to be concatenated String s1 = “se”; String s2 = “1010”; String output = s1 + s2; // output is “se1010” The only operator defined by Java for an object variable is “+”, and it only works on String objects