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Variables, Types, Values
14 January 2019 OSU CSE 1
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Variables• A variable is the name of a “location” that
“stores” a value of a particular type– We might say the variable “has” that value– We might say the variable “has” that type or
“is of” that type
14 January 2019 OSU CSE 2
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Examples
14 January 2019 OSU CSE 3
true 'y' 13 3.14 "Go"
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Examples
14 January 2019 OSU CSE 4
This is a boolean variableb
whose value is true, i.e.,b = true
or, more simply, justb
true 'y' 13 3.14 "Go"
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Examples
14 January 2019 OSU CSE 5
This is a char variablec
whose value is 'y', i.e.,c = 'y'
true 'y' 13 3.14 "Go"
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Examples
14 January 2019 OSU CSE 6
This is a int variablei
whose value is 13, i.e.,i = 13
true 'y' 13 3.14 "Go"
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Examples
14 January 2019 OSU CSE 7
This is a double variabled
whose value is 3.14, i.e.,d = 3.14
true 'y' 13 3.14 "Go"
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Examples
14 January 2019 OSU CSE 8
This is a String variables
whose value is "Go", i.e.,s = "Go"
(or s = <'G', 'o'>)
true 'y' 13 3.14 "Go"
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Types
• A type is the name of the set of all possible values that a variable might have
• Examples:– A variable of type String might have values
like "foo", "Hello World", etc.– A variable of type int might have values like –1, 18, etc.
– A variable of type double might have values like 3.1416, 10.0, etc.
14 January 2019 OSU CSE 9
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Program vs. Mathematical Variables
• A program variable has a particular value at any one time during program execution, and that value (generally) may change at other times
• A mathematical variable stands for an arbitrary but fixed value
14 January 2019 OSU CSE 10
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Program vs. Mathematical Types
• A program type has a corresponding mathematical type that models it
14 January 2019 OSU CSE 11
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Program vs. Mathematical Types
• A program type has a corresponding mathematical type that models it
14 January 2019 OSU CSE 12
When reasoning about a program variable of a given program type, treat its value at any given time as if it were a mathematical variable of the corresponding mathematical type.
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Mathematical ModelsProgram type Mathematical type
String string of characterboolean booleanchar characterint integer
(-2147483648 through 2147483647)
double real(about ±10±308, 15 significant digits)
14 January 2019 OSU CSE 13
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Mathematical ModelsProgram type Mathematical type
String string of characterboolean booleanchar characterint integer
(-2147483648 through 2147483647)
double real(about ±10±308, 15 significant digits)
14 January 2019 OSU CSE 14
String is built-in to Java;boolean, char,
int, and double are among the 8 primitive (and also built-in) types of Java;
differences later.
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Mathematical ModelsProgram type Mathematical type
String string of characterboolean booleanchar characterint integer
(-2147483648 through 2147483647)
double real(about ±10±308, 15 significant digits)
14 January 2019 OSU CSE 15
All these mathematical types are “built-in” to mathematics!
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Mathematical ModelsProgram type Mathematical type
String string of characterboolean booleanchar characterint integer
(-2147483648 through 2147483647)
double real(about ±10±308, 15 significant digits)
14 January 2019 OSU CSE 16
Program code is shown in a blue fixed-width font, with
keywords in bold.
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Mathematical ModelsProgram type Mathematical type
String string of characterboolean booleanchar characterint integer
(-2147483648 through 2147483647)
double real(about ±10±308, 15 significant digits)
14 January 2019 OSU CSE 17
Mathematics is shown in a green fixed-width italic
font, with keywords in bold.
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Declaring a Variable• When you declare a program variable,
you both provide a name for a location to store its value, and indicate its program type– Recall: the program type determines the
mathematical type, which in turn determines the possible values the variable can haveint j;
14 January 2019 OSU CSE 18
?
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Declaring a Variable• When you declare a program variable,
you both provide a name for a location to store its value, and indicate its program type– Recall: the program type determines the
mathematical type, which in turn determines the possible values the variable can haveint j;
14 January 2019 OSU CSE 19
?
The standard Java convention for naming variables is to use camel case: start with a lower case letter and only capitalize the first letter of
each following word, e.g., myLuckyNumber
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Declaring a Variable• When you declare a program variable,
you both provide a name for a location to store its value, and indicate its program type– Recall: the program type determines the
mathematical type, which in turn determines the possible values the variable can haveint j;
14 January 2019 OSU CSE 20
This is an intvariable j whose
value isundefined.
?
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Initializing a Variable• To initialize a variable, you assign it a
value– Recall: the program type determines the
mathematical type, which in turn determines the possible values the variable can haveint j = 13;
14 January 2019 OSU CSE 21
13
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Initializing a Variable• To initialize a variable, you assign it a
value– Recall: the program type determines the
mathematical type, which in turn determines the possible values the variable can haveint j = 13;
14 January 2019 OSU CSE 22
This is an intvariable j
whose value is 13, i.e.,j = 13
13
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 23
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 24
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
Every other row in the left column contains
some programstatement(s).
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 25
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
Every other row in the right column contains some mathematicalsentences ("facts").
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 26
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
This equal sign, in code, means
assignment of a value to a program
variable.
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 27
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
This equal sign, in mathematics, means
equality of two values.
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 28
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
There is no value for mathematical variable j
in this state because program variable j hasn’t
been declared yet.
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Reasoning: Tracing Tables
14 January 2019 OSU CSE 29
Code State
x = 1.414
int j = 13;
x = 1.414j = 13
There is a value for jin this state because j has been declared
before this state.
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 30
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 31
This is a String literal;written as characters
between double-quote marks: "…"
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 32
This is a boolean literal;must be eithertrue or false.
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 33
This is a char literal;normally written as a
single character between single-quote marks: '…'
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 34
This is an int literal;normally written (as in
mathematics) as a decimal constant.
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Literals• A data value appearing, literally, in a
program is called a literalString fileName = "foo.txt";boolean found = false;char win = 'W';int j = 13;double ht = 9.27;
14 January 2019 OSU CSE 35
This is a double literal;normally written (as in
mathematics) as a decimal constant with a
decimal point.
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 36
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 37
escaped specialcharacter:
single-quote
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 38
non-printing character:
tab
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 39
Unicode character:
small Greek π
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 40
octal integer (base-8):
29 in decimal
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 41
hexadecimal integer (base-16):
26 in decimal
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Forms of LiteralsProgram type Literal examples
String "I\'m" "at OSU"
boolean true false
char 'A' '\t' '\"''\u03c0'
int 29 -13035 0x1a
double 18. 18.08E-4 6.022E23
14 January 2019 OSU CSE 42
scientific notation:8 x 10–4
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Constants• A variable whose value is initialized and
never changed is called a constantint myLuckyNumber = 13;double avogadro = 6.022E23;
14 January 2019 OSU CSE 43
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Constants• A variable whose value is initialized and
never changed is called a constantfinal int myLuckyNumber = 13;final double avogadro = 6.022E23;
14 January 2019 OSU CSE 44
The keyword final indicates to the compiler your intent that a variable
is actually a constant.
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Constants• A variable whose value is initialized and
never changed is called a constantfinal int myLuckyNumber = 13;final double avogadro = 6.022E23;
14 January 2019 OSU CSE 45
When constants are declared inside a
method, the standard Java convention is to use
camel case just like for variables.
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Constants• A variable whose value is initialized and
never changed is called a constantfinal int MY_LUCKY_NUMBER = 13;final double AVOGADRO = 6.022E23;
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When constants are declared at the class level, the
standard Java convention is to use all upper case letters and to separate the words
with '_'.
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Resources• Java for Everyone, Chapter 2
– http://osu.worldcat.org/title/java-for-everyone-late-objects/oclc/808511232
14 January 2019 OSU CSE 47