Transcript 02-JavaFundamentals

Java Fundamentals
Asserting Java
Chapter 2: Introduction to Computer Science
©Rick Mercer
Outline
 Distinguish the syntactical parts of a program
• Tokens: special symbols, literals, identifiers,
• Output with System.out.println
• An executable program as a Java class with a
main method
 Introduce two of Java's primitive types: int and
double
Preview: A Complete Java program
import java.util.Scanner;
// Read number from user and then display its squared value
public class ReadItAndSquareIt {
public static void main(String[] args) {
double x;
double result = 0.0;
Scanner keyboard = new Scanner(System.in);
// 1. Input
System.out.print("Enter a number: ");
x = keyboard.nextDouble();
// 2. Process
result = x * x;
// 3. Output
System.out.println(x + " squared = " + result);
}
}
Programs have 4 types of tokens
 The Java source code consists of
1)
2)
3)
4)
special symbols + < = >= && ||
identifiers customerName totalBill n
reserved identifiers int while if void
literals 123 "A String" true
 These tokens build bigger things like variables,
expressions, statements, methods, and classes.
 Also, comments exist for humans to read
// Document your code if it is unreadable :-(
Overloaded Symbols
 Some special symbols are operators and have
different things in different contexts
• with two integers, + sums integers
2 + 5 evaluates to the integer 7
• with two floating point literals, + sums to floating
point (types make a difference)
2.0 + 5.0 evaluates to 7.0
• with two strings, + concatenates
"2" + "5" evaluates to the new string "25"
Identifiers
 An identifier is a collection of certain characters
that could mean a variety of things
 There are some identifiers that are Java defines:
sqrt
String
Integer
System
in
out
 We can make up our own new identifiers
test1
lastName
dailyNumber
MAXIMUM
$A_1
Valid identifiers
 Identifiers have from 1 to many characters:
'a'..'z', 'A'..'Z', '0'..'9', '_', $
• Identifiers start with letter a1 is legal, 1a is not
• can also start with underscore or dollar sign: _
$
• Java is case sensitive. A and a are different
 Which letters represent valid identifiers?
a) abc
b) m/h
c) main
d) $$$
e) 25or6to4
f) 1_time
i) a_1
j) student Number
k) String
Reserved Identifiers (keywords)
 A keyword is an identifier with a pre-defined
meaning that can't be changed it's reserved
double
int
 Other Java reserved identifiers not a complete list
boolean
break
case
catch
char
class
default
do
double
else
extends
float
for
new
if
private
import
public
instanceOfreturn
int
void
long
while
Literals -- Java has 6
Floating-point literals
1.234
-12.5
1.2
3.
.4
1e10
0.1e-5
String literals
"characters between double quotes"
"'10"
"_"
Integer literals (Integer.MIN_VALUE and Integer.MIN_VALUE)
-1
0
1
-2147483648
2147483647
Boolean literals (there are only two)
true
false
Null (there is only this one value)
null
Character literals
'A'
'b'
'\n'
'1'
' '
Comments
• Provide internal documentation to explain program
• Provide external documentation with javadoc
• Helps programmers understand code--including
their own
//
on one line,
or
• There
are three
type of comments
/*
between slash star and star slash
you can mash lines down real far, or
*/
/**
* javadoc comments for external documentation
* @return The square root of x
*/
public static double sqrt(double x)
General Forms
 The book uses general forms to introduce parts of
the Java programming language
 General forms provide information to create
syntactically correct programs
• Anything in yellow boldface must be written exactly
as shown (println for example)
• Anything in italic represents something that must be
supplied by the user
• The italicized portions are defined elsewhere
Output Statements
 A statement, made up of tokens, is code that causes
something to happen while the program runs
 General Forms for three output statements
System.out.print( expression );
System.out.println();
System.out.println( expression );
 Example Java code that writes text to the console
System.out.print("hello world.");
System.out.println(); // print a blank line
System.out.println("Add new line after this");
General Form: A Java program
// This Java code must be in a file named class-name.java
public class class-name {
public static void main(String[] args) {
statement(s)
}
}
// Example Program stored in the file HelloWorld.java
import java.util.Scanner;
public class HelloWorld {
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter your name: ");
String myName = keyboard.next(); // keyboard input
System.out.println("Hi Rick");
System.out.println("This is " + myName);
}
}
Primitive Numeric Types
 Type: A set of values with associated operations
 Java has many types, a few for storing numbers
• Stores integers in int variables
• Store floating-point numbers in double variables
 A few operations for numeric types
• Assignment Store a new value into a variable
• Arithmetic
+, -, * (multiplication), /
• Methods
Math.sqrt(4.0)
See class Math for others
Math.max(3, -9)
Variables to store numbers
 To declare and give initial value:
type identifier = initial-value;
 Examples
int creditsA = 4;
double gradeA = 3.67;
String name = "Chris";
int hours = 10;
boolean ready = hours >= 8;
Assignment
 We change the values of variables with assignment
operations of this general form
variable-name = expression;
 Examples:
double x;
int j;
// Undefined variables
// can not be evaluated
j = 1;
x = j + 0.23;
Memory before and after
 The primitive variables x and j are undefined at first
Variable
Name
Initial
Value
j
?
Assigned
Value
1
? means undefined
x
?
1.23
 The expression to the right of = must be a value that
the variable can store assignment compatible
x = "oooooh nooooo, you can't do that"; // <-Error
j = x; // <-Error, can't assign a float to an int
Assignment
double bill;
What is value for bill now? _________
bill = 10.00;
bill = bill + (0.06 * bill);
What is value for bill now? ________
Which letters represent valid assignments given these 3
variable initializations?
String s = "abc";
int n = 0;
double x = 0.0;
a)
b)
c)
d)
s
n
x
s
=
=
=
=
n;
x;
n;
1;
e)
f)
g)
h)
n
x
s
n
=
=
=
=
1.0;
999;
"abc" + 1;
1 + 1.5;
Arithmetic Expressions
Arithmetic expressions consist of operators
such as + - / * and operands such as 40, 1.5,
payRate and hoursWorked
Example expression used in an assignment:
grossPay = payRate * hoursWorked;
Another example expression:
5 / 9 * (fahrenheit - 32);
For the previous expression,
Which are the operators?_____
Which are the operands?_____
Arithmetic Expressions
Arithmetic expressions take many forms
or
or
or
or
or
or
a numeric variable
a numeric constant
expression + expression
expression - expression
expression * expression
expression / expression
(expression )
double x = 1.2;
100 or 99.5
1.0 + x
2.5 - x
2*x
x / 2.0
(1 + 2.0)
Precedence of Arithmetic Operators
 Expressions with more than one operator require
some sort of precedence rules:
*
-
/
+
evaluated in a left to right order
evaluated in a left to right order in the absence of parentheses
Evaluate 2.0 + 4.0 - 6.0 * 8.0 / 6.0
 Use (parentheses) for readability or to intentionally
alter an expression:
double C, F;
F = 212.0;
C = 5.0 / 9.0 * (F - 32);
What is the current value of C ____?
Math functions
 Java’s Math class provides a collection of
mathematical and trigonometric functions
returns 4.0
Math.min(-3, -9) returns -0
Math.max(-3.0, -9.0) returns -3.0
Math.abs(4 - 8) returns 4
Math.floor(1.9) returns 1.0
Math.pow(-2.0, 4.0) returns 16.0
Math.sqrt(16.0)
int Arithmetic
variables are similar to double, except they can
only store whole numbers (integers)
 int
int anInt = 0;
int another = 123;
int noCanDo = 1.99;
// ERROR
 Division with integers is also different
• Performs quotient remainder whole numbers only
anInt = 9 / 2;
// anInt = 4, not 4.5
anInt = anInt / 5;
What is anInt now? ___
What is anInt now? ___
anInt = 5 / 2;
The integer % operation
 The Java % operator returns the remainder
anInt = 9 % 2;
// anInt ___1___
anInt = 101 % 2;
What is anInt now? ___
anInt = 5 % 11;
What is anInt now? ___
anInt = 361 % 60;
What is anInt now? ___
int quarter;
quarter = 79 % 50 / 25; What is quarter? ___
quarter = 57 % 50 / 25; What is quarter now? ___
Integer Division, watch out …
int celcius, fahrenheit;
fahrenheit = 212;
celcius = 5 / 9 * (fahrenheit - 32);
What is the current value of celcius _____?