Transcript chp4VariablesConstantArithmetic

Variables, Data Types,
&
Constants
Topics & Objectives
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Declaring Variables
Assignment Statement
Reserve Words
Data Types
Constants
Packages & Libraries
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Scanner Class
Math Class
Arithmetic
Compound
Assignment
• Flow Charting
Variable
• Is a name for a value stored in memory
• Containers for values
• Is a name for a location in memory used to
hold a data value.
Variable Declaration
• Tells the compiler to reserve a portion of
main memory space large enough to hold the
value.
• All variables must be declared with a name
and a type before they can be used.
– int myValue = 3; //Declared & Initialized myValue to 3
• Variable can only store one value of its type
at a time.
Assignment Statement
• (=) symbol is used as the assignment statement.
– (=) - is known as the assignment operator.
• All assignments occurs from right to left.
Meaning the right side of the equal sign is
evaluated first and then stored to the variable on
the left side.
– Identifier variable = expression ;
– myVariable = myValue + 3;
• myValue + 3 is evaluated first
• The result is then stored in myVariable
Assignment Statement
An assignment statement gives a value to a
variable.
Assignment can take several forms:
x = 5;
a literal (5) is assigned to x
x = y + 2;
the value of an expression (y + 2)
is assigned to x
x = z;
the value of another variable (z)
is assigned to x
Variable Assignment
A variable can store only one value at any time.
int x;
x = 5;
x = 10;
x
10
5
Naming Variables
• 1st letter of a variable name should be lowercase.
• Name should consist of letters and numbers and
underscore.
• No spaces
• No symbols
• If using two words join together, the 1st letter of
the second word should be capitalized.
– myHouse, totalSum
• Do not use reserve or keywords
Reserve
• Reserve Words – words that have a predefine
meaning
– boolean, char, byte, int, short, long, float, double,
void, true, false, public, private, protected, static, final,
import, class, interface, extends, implements, this,
super, abstract, new, if, else, for, while, do, switch,
case, default, break, return, try, catch, finally, throw,
throws, continue, package, native, volatile, transient,
synchronized, instanceof
• All reserved words use only lowercase letter
Java Keywords
abstract
double
int
strictfp
boolean
else
interface
super
break
extends
long
switch
byte
final
native
synchronized
case
finally
new
this
catch
float
package
throw
char
for
private
throws
class
goto
protected
transient
const
if
public
try
continue
implements
return
void
default
import
short
volatile
do
instanceof
static
while
Primitive Data Types or built-in
data types
Data type – determines the type of data the
variable will store
They are called primitive because data types
have no properties.
Eight Primitive Data Types
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4 kinds of Integers data types
2 kinds of floating point data types
1 character data type
1 boolean data type
Primitive Data Types
Type
int
double
char
boolean
Storage Required
4 bytes //AP EXAM
8 bytes //AP EXAM
2 bytes //Not on AP Exam
1 bit //AP EXAM
Integer Data Types
• Use for whole numbers
• byte – 1 byte range –27 to 27-1 or –128 to 127
• short – 2 bytes range –215 to 215 – 1 or 32,768
to 32,767
• int – 4 bytes range –231 to 231 - 1
• long – 8 bytes range –263 to 263 – 1
Floating Points Data Types
• Use for real numbers (numbers with
decimal).
• float – 4 bytes –3.4 x 1038 to 3.4 x 1038
• double – 8 bytes –1.8 x 10308 to 1.8 x 10308
Character Data Type
• Used for single letters in single quotes
• char – 2 bytes Unicode character set (with
ASCII subset)
Boolean Data Type
• Used for true or false
• boolean – 1 byte true or false
Choosing a data type
• It is important to choose the most
appropriate type for the quantity being
represented.
Constants
• Literal – A primitive value used in a program .
o Numeric literal – 8 , 9, 427
o String literal – “hello”
• Symbolic - uses the keyword final when
declaring variables
– final int SUM = 8;
– Use all caps the distinguish constant variables from
other variables.
• Constants are like variables, but they have the
same value throughout the program.
• Constant can’t change their value once they are
assigned a value.
Named Constants
 A named memory location that cannot be changed
from its initial value.
 The keyword final is used in a constant declaration.
 Constant identifiers are typically all uppercase with an
underscore (_) separating words within the identifier
name.
 Example:
final double TAX = 0.08;
Constants Advantages
• Prevent accidental changing during the
program run by another source.
• Maintenance - by changing the assignment
where it is assigned. It will change
throughout the program.
ASCII
• American Standard Code for Information
Interchange
– Represents the numeric code for each
character
– One Byte per character
– 0 to 127
• Extended ASCII
– 128 to 255
Unicode
• Two bytes per character
– ASCII is a subset of Unicode
• Represents 65,000 characters for most
world languages
Import, Arithmetic, Casting,
Logical Operators, Relational
Operators
Java Packages
 Numerous packages are included with JDK
 Packages contain classes
 Packages can be added to an application with an
import statement. For example, the statement
import java.util.Scanner;
makes the Scanner class and its methods accessible to
the application.
Packages
• Group of related classes by one name.
• Eamples: java.lang – java.util
Class Libraries
• Class Library – is a set of classes that
supports the development of programs.
• The Java standard class Library is a useful
set of classes that anyone can use when
writing Java programs.
• Java APIs – Application Programmer
Interfaces. Class library made up of
several sets of related classes.
Import Declaration
• import – keyword used to identify
packages and classes that will be used by
the program.
• import java.util.Random;
– Gains access to the Random Class
• import java.util.*;
– Gains access to the package that contains class
Random
java.lang.*;
• Automatically imported
• Classes in java.lang package
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String
System
Double
Integer
Comparable
Math
Object
Etc.
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Random
ArrayList
HashMap
HashSet
Iterator
LinkedList
List
ListIterator
Map
Set
TreeMap
TreeSet
java.util.*;
Inputting
Scanner Class
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next()
nextLine()
nextInt()
nextDouble()
nextBoolean()
nextFloat()
nextLong()
nextShort()
Creating a Scanner object
• Scanner console = new
Scanner(System.in);
• console is the object of Scanner
• console has access to all the methods using
the dot operator.
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Scanner Class Methods
next()
– Returns a string
nextLine()
– Returns a string of the entire sentence
nextInt()
– Returns an integer
nextInt()
– Returns an integer
nextDouble()
– Returns a double value
nextBoolean()
– Returns a Boolean value
nextFloat()
– Returns a float value
nextLong()
– Returns a long value
nextShort()
– Returns a short value
Inputting Char
• No methods for inputting chars
• Scanner console = new console
Scanner(System.in);
• char dude = console.next().charAt(0);
//return the char at the 0 index
Program
• import java.util.*;
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public static void main(String args[]){
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int x=0,y=0, z=0;
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String temp = new String();
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Scanner console = new Scanner(System.in);
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System.out.println("Input 3 values");
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x = console.nextInt();
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y=console.nextInt();
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z=console.nextInt();
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System.out.println(x);
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System.out.println(y);
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System.out.println(z);
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}
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When inputting values, do not
use the enter keys between the
values.
The Math Class
 Part of the java.lang package
 The random() methods generates a double between 0
and 1.0. For example,
double rNum;
rNum = Math.random();
 A random integer in a range is generated by using the
expression:
(highNum – lowNum + 1) * Math.random() + lowNum
Math Class
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abs(int) or abs(double) - absolute value
acos(double) – arc cos
asin(double) – arc sin
atan(double) – arc tan
cos(double) – cosine
sin(double) - sine
tan(double) - tangent
ceil(double) – smallest whole number greater than or equal to
num
exp(double) - power
floor(double) – largest whole number less than or equal to num.
pow(double, double) – return num raised to a power
random() – 0.0 to < 1.0
sqrt(double) – square root
Built-in arithmetic operators
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* - multiplication
% - modulus division
/ - division
+ - addition
- - subtraction
Modulus Division
Modulus division (%) returns the remainder of a
division operation:
Compound Assignment Operators
Operator
+=
-=
*=
/=
%=
Operation
addition and then assignment
subtraction and then assignment
multiplication and then assignment
division and then assignment
modulus division and then assignment
Order of Precedence
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!, (unary) -, Cast, ++, -*, /, %
+, <, >, <=, >=, ==, !=
&&
||
In the absence of parentheses, binary operators of
the same rank are performed left to right, and
unary operators right to left. If in doubt use
parentheses!
Parentheses
• Operations inside of parentheses are evaluated first.
• The order in which operators are evaluated can be
changed by using parentheses. For example,
addition is performed first, then multiplication, and
finally division:
• Example:
– 6 + 4 * 2 – 1 // 13
– (6 + 4) * (2 – 1) // 10
– (5 + 6) * 4 / 2 // 22
Type promotion (implicit
conversion)
• Occurs when using a math expression of different
data types.
• Example: int + double will return a double and
the int will be promoted to a double by the
compiler automatically.
• You will need to have a double data type to store
the result.
• If you try to store it in an int memory location,
you will get a loss of precision error.
+, -, *, %
• Will return the data type of the operand if
they of same data type.
• Mixed mode operands will use type
promotion to determine the data type that
will be return.
• Can’t % by a 0
– 5 % 0;
• Can 0%8 = 0
Mixed Mode Operands
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int + int  int
int + double  double
double + double  double
int * int  int
int *double  double
double * double  double
int / int  int
int / double  double
double / double  double
Just as long as one of the operand is a double, the
other operand will be promoted up and result will
be a double.
/ division
• Must be careful when working with
division.
• int/int will return an int and the remainder
is dropped.
• One or both of the sides of the division
must be a double to return a double
• Can’t divide by 0 example 5/0;
• Can 0/5 = 0;
Real Division
Real division (/) is performed when one or both
operands are type double. The entire quotient,
including the decimal portion is returned:
double result;
result = 20.0/7.0;
//result is 2.857
Integer Division
Integer division (/) is performed when both operands
are integers. Only the integer portion of the quotient is
returned:
Division & Modulus
• Can use if statement to eliminate errors
if(x != 0)
8/x
• Eliminates any errors if x = 0;
Casting or Type Casting
• Converts a value Data Type temporary to another
type.
• Examples:
int x, i = 2;
double d = 3.7;
x = i * (int) d; // d is explicitly cast
• Type casting is necessary in this case because one
of the operands has less precision than the variable
that will store the result.
• Explicit casting also makes it clear that the
programmer intended for the calculation result to
be an int.
Type Casting
Type Casting converts a number of one type to a
number of a different, but compatible type. Type
casting is used to:
1. make the operand types in an expression match.
For example, wholeNum = (int)y * 2
2. truncate the decimal portion of a double. For
example, wholeNum = (int)z
3. change the way in which a division (/) operation
will be performed. For example,
realDivision = (double)a / (double)b
Casting real division
• Casting is useful when real division with integers
is preferred
• result = (double) 8/ (double) 5; //explicitly
• result = (double) 8/ 5; // explicitly & implicitly
• Java will implicitly type cast operands in a mixed
expression to match the precision of the variable
storing the value. Although explicitly type
casting is not necessary, it is better programming
style to include casts.
• Casting makes the programmer’s intentions clear
and makes bugs easier to find.
Truncate
• Casting a double to an int truncates the
decimal portion of the number.
Rounding
• Rounding can be simulated when casting a
double to an int by adding .5 to the number
before the casting.
• x = i * (int)(d + 0.5);
Abstract Data Types
A variable declared with a class is called an object. For
example, the object spot is type Circle:
Circle spot = new Circle(4);
spot
getRadius()
area()
Programming Errors
 Syntax errors violate the rules of Java.
 Logic errors, also called semantic errors, occur in
statements that are syntactically correct, but produce
undesired or unexpected results.
 Run-time errors, also called exceptions, halt program
execution at the statement that cannot be executed.
One type of exception is called
InputMismatchException.
Flowchart Symbols
process
The BirthdayGame Flowchart