Transcript Input, output, variable, assignment, calculation

Basic I/O - CIS 1068
Program Design and Abstraction
Zhen Jiang
CIS Dept.
Temple University
1008 Wachman Hall, Main Campus
Email: [email protected]
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Table of Contents
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Your first Java program
Simple Output/Input
Variable
Data types
Arithmetic (use of variables)
String and its use in I/O statement
Summary of programs
Summary of concepts
Other materials
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Welcome – Your first program
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http://www.cis.temple.edu/~jiang/Welcome.java
http://www.cis.temple.edu/~jiang/Welcome1.java
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Environment
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JDK (Java Development Kit)
IDE (Integrated Development Environment)
Installment guide
 http://www.cis.temple.edu/~jiang/Installment1068.pdf
Test run
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Before you run a program, you must compile it.
compiler: Translates a computer program written in one
language (i.e., Java) to another language (i.e., byte code)
Compile (javac)
source code
(Hello.java)
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Execute (java)
byte code
(Hello.class)
output
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Details
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Names
Main
{ } and ( )
Println and print (p93)
;
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Textbook
Highlighted by content in ppt slides
Indexed by page number in ppt slides
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Discussion
Summary
Exercises (in ppt slides and class), quiz,
project, test, final
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Take a lesson from the learning in a
previous section
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Learning target
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http://www.cis.temple.edu/~jiang/CIS1068_07_summary.pdf
www.cis.temple.edu/~jiang/ShowButtonDemo.pdf
www.cis.temple.edu/~jiang/ButtonDemo.pdf
www.cis.temple.edu/~jiang/WindowDestroyer.pdf
Three keys
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Loop, (instance) method, and text/string
processing
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Simple Output
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Hello.java,
syntax error: A problem in the structure of a program.
1 public class Hello {
2
pooblic static void main(String[] args) {
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System.owt.println("Hello, world!")
4
}
5 }
compiler output:
2 errors found:
File: Hello.java [line: 2]
Error: Hello.java:2: <identifier> expected
File: Hello.java [line: 3]
Error: Hello.java:3: ';' expected
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Error messages do not always help us
understand what is wrong:
File: Hello.java [line: 2]
Error: Hello.java:2: <identifier> expected
pooblic static void main(String[] args) {
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Why can’t the computer just say “You
misspelled ‘public’”?
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First lesson
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Computers can’t read minds.
Computers don’t make mistakes.
If the computer is not doing what you want, it’s because
YOU made a mistake.
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Java is case-sensitive
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Public and public are not the same
1 Public class Hello {
2 public static void main(String[] args) {
3 System.out.println("Hello, world!");
4 }
5}
1 error found:
compiler output:
File: Hello.java [line: 1]
Error: Hello.java:1: class, interface, or enum expected
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System.out.println: A statement to print a line of output.
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pronounced “print-linn”
The use of System.out.println (P93) :
System.out.println("<message>");
 Prints the given message as a line of text.
System.out.println(<number>);
 Prints the given number as a line of text.
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System.out.println(<output1> + <output2>+… +<output_Last>);
 Performs a output string concatenation and prints all as a line of
text.
System.out.println();
 Prints a blank line.
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String: A sequence of text characters, also
called message.
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Start and end with quotation mark characters
Examples:
"hello"
"This is a string"
"This, too, is a string. It can be very long!"
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A string may not span across multiple lines.
"This is not
a legal string."
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A string may not contain a “ character.
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The ‘ character is okay.
"This is not a "legal" string either."
"This is 'okay' though."
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This begs the question…
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A string can represent certain special characters by
preceding them with a backslash \ (this is called an
escape sequence, p89).
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\t
\n
\"
\\
tab character
newline character
quotation mark character
backslash character
Example:
System.out.println("Hello!\nHow are \"you\"?");
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What is the output of each of the following println
statements?
System.out.println("\ta\tb\tc");
System.out.println("\\\\");
System.out.println("'");
System.out.println("\"\"\"");
System.out.println("C:\nin\the downward spiral");
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Write a println statement to produce the following line of
output (space counted):
/ \ // \\ /// \\\
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What a println statement will generate the following output (one statement
only)?
This program prints a
quote from the Gettysburg Address.
"Four score and seven years ago,
our 'fore fathers' brought forth on this continent
a new nation."
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What a println statement will generate the following output?
A "quoted" String is
'much' better if you learn
the rules of "escape sequences."
Also, "" represents an empty String.
Don't forget to use \" instead of " !
'' is not the same as "
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comment: A note written in the source code to make the code easier
to understand (p104).
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Comments are not executed when your program runs.
Most Java editors show your comments with a special color.
Comment, general syntax:
/* <comment text; may span multiple lines> */
or,
// <comment text, on one line>
Examples:
/* A comment goes here. */
/* It can even span
multiple lines. */
// This is a one-line comment.
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… at the top of each file (also called a "comment header"), naming
the author and explaining what the program does.
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… at the start of every method, describing its behavior or function.
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… inside methods, to explain the complex pieces of code.
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Comments provide important documentation.
Comments provide a simple description of what each class, method,
etc. is doing.
Later programs will span hundreds or thousands of lines, split into
many classes and methods.
When multiple programmers work together, comments help one
programmer understand the other's code.
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Simple Input
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Sample Program, P15
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http://www.cis.temple.edu/~jiang/1068FirstProgram.pdf
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Variable
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A piece of your computer's memory that is given
a name and type and can store a value.
Usage
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compute an expression's result
store that result into a variable
use that variable later in the program
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To use a variable, it must be declared.
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Variable declaration syntax (P51):
<type> <name>;
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Convention: Variable identifiers follow the same rules as
method names.
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Examples:
int x;
double myGPA;
int varName;
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Declaring a variable sets aside a piece of
memory in which you can store a value.
int x;
int y;
 Inside the computer:
x: ?
y: ?
(The memory still has no value yet.)
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identifier: A name given to an entity in a program such as a class or
method.
 Identifiers allow us to refer to the entities.
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Examples (in bold):
 public class Hello
 public static void main
 double salary
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Conventions for naming in Java (which we will follow):
 classes: capitalize each word (ClassName)
 everything else: capitalize each word after the first (myLastName)
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Name, p103
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Begin with [a]-[Z], _, or $
Contain only [a]-[Z], [0]-[9], _, and $
No keyword
Case distinct
“punctuate”, page 104
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Examples:
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legal:susan
TheCure
myMethod
second_place _myName
ANSWER_IS_42 $variable
name2
illegal: me+u
side-swipe
jim's
49er
question?
hi there ph.d
2%milk [email protected]
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method1
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keyword: An identifier that you cannot use, because it already has a
reserved meaning in the Java language.
Complete list of Java keywords:
abstract
boolean
break
byte
case
catch
char
class
const
continue
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default
do
double
else
extends
final
finally
float
for
goto
if
implements
import
instanceof
int
interface
long
native
new
package
private
protected
public
return
short
static
strictfp
super
switch
synchronized
this
throw
throws
transient
try
void
volatile
while
NB: Because Java is case-sensitive, you could technically use Class or cLaSs
as identifiers, but this is very confusing and thus strongly discouraged.
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Data types
data type: A category of data values.
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Example: integer, real number, string
Data types are divided into two classes:
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primitive types: Java's built-in simple data types
for numbers, text characters, and logic.
class types: type of objects, coming soon!
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Java has eight primitive types. Here are two examples:
Name Description
int
integers
double
real numbers
Examples
42, -3, 0, 926394
3.4, -2.53, 91.4e3
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Numbers with a decimal point are treated as real numbers.
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Question: Isn’t every integer a real number? Why bother?
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Temperature
Sum of a group of integers
Average of a group of integers
Number of seconds left in a game
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Lesson two:
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Are you able to handle the data that is out
of the original plan?
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What a kind (type) of data is in use?
Should it be changed to new type?
What is the new type?
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Discrete Types
byte
short
int
long
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Continuous Types
float
double
Non-numeric Types
boolean
char
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Type
Representatio
n
Bits
Bytes
#Values
boolean
True or False
1
N/A
2
char
‘a’ or ‘7’ or ‘\n’
16
2
216 = 65,536
byte
…,-2,-1,0,1,2,…
8
1
28 = 256
short
…,-2,-1,0,1,2,…
16
2
216 = 65,536
int
…,-2,-1,0,1,2,…
4
> 4.29 million
long
…,-2,-1,0,1,2,…
8
> 18
quintillion
float
0.0, 10.5, -100.7
32
double
0.0, 10.5, -100.7
64
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Arithmetic (Use of variables)
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17/3=?
http://www.cis.temple.edu/~jiang/Variable.
pdf
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Assignment statement: A Java statement that stores a value into a
variable.
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Variables must be declared before they can be assigned a value.
Assignment statement syntax:
<variable> = <expression>;
Examples:
x = 2 * 4;
x: 8
myGPA = 3.25;
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myGPA: 3.25
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A variable can be assigned a value more than once.
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Example:
int x;
x = 3;
System.out.println(x);
// 3
x = 4 + 7;
System.out.println(x);
// 11
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Once a variable is assigned a value, it can be used in any expression.
int x;
x = 2 * 4;
y = x +3;
System.out.println(x * 5 - 1);
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The above has output equivalent to:
System.out.println(8 * 5 - 1);
What happens when a variable is used on both sides of an assignment
statement ?
int x;
x = 3;
x = x + 2; // what happens?
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ERROR: Declaring two variables with the same name
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Example:
int x;
int x;
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// ERROR: x already exists
ERROR: Reading a variable’s value before it has been
assigned
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Example:
int x;
System.out.println(x); // ERROR: x has no value
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The assignment statement is not an algebraic equation!
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<variable> = <expression>; means:
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Some people read x = 3 * 4; as
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"store the value of <expression> into <variable>"
"x gets the value of 3 * 4"
ERROR: 3 = 1 + 2; is an illegal statement, because 3
is not a variable.
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A variable can only store a value of its own type.
 Example:
int x;
x = 2.5;
// ERROR: x can only store int
An int value can be stored in a double variable. Why?
 Type compatibility: The value is converted into the equivalent
real number (p64).
 Example:
double myGPA;
myGPA: 2.0
myGPA = 2;
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double
float
long
int
char
boolean
short
byte
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Manipulating data via expressions
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Expression: A data value or a set of operations that
produces a value.
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Examples:
1+4*3
3
"CSE142"
(1 + 2) % 3 * 4
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Arithmetic operators we will use:
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+
*
/
%
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addition
subtraction or negation
multiplication
division
modulus, a.k.a. remainder
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When Java executes a program and encounters an
expression, the expression is evaluated (i.e., computed).
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Example:
3 * 4 evaluates to 12
System.out.println(3 * 4) prints 12 (after evaluating 3 * 4)
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How could we print the text 3 * 4?
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When dividing integers, the result is also an integer: the quotient.
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Example: 14 / 4 evaluates to 3, not 3.5 (truncate the decimal part)
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Examples:
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1425 / 27 is 52
35 / 5 is 7
84 / 10 is 8
156 / 100 is 1
24 / 0 is illegal (what do you think happens?)
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The modulus computes the remainder from a division of
integers.
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Example: 14 % 4 is 2
1425 % 27 is 21
3
4 ) 14
12
2
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52
27 ) 1425
135
75
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What are the results of the following expressions?
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45 % 6
4 % 2
8 % 20
11 % 0
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What expression obtains (ChangeMaker.java, P77)
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the last digit (unit’s place) of a number?
 Example: From 230857, obtain the 7.
the last 4 digits of a Social Security Number?
 Example: From 658236489, obtain 6489.
the second-to-last digit (ten’s place) of a number?
 Example: From 7342, obtain the 4.
the part of a number rounded off to the nearest hundredth?
 Example: From 73.424, obtain the 73.42.
From 73.425, obtain the 73.42.
the part of a number rounded up to the nearest hundredth?
 Example: From 73.424, obtain the 73.42.
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From 73.425, obtain the 73.43.
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Precedence: Order in which operations are computed in an
expression.
 Operators on the same level are evaluated from left to right.
Example:
1 - 2 + 3 is 2 (not -4)
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Spacing does not affect order of evaluation.
Example:
1+3 * 4-2 is 11
Parentheses
Multiplication, Division, Mod
Addition, Subtraction
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()
* / %
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1 * 2 + 3 * 5 / 4
\_/
|
2
+ 3 * 5 / 4
\_/
|
2
+ 15
/ 4
\___/
|
2
+
3
\________/
|
5
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1 + 2 / 3 * 5 - 4
\_/
|
1 +
0
* 5 - 4
\___/
|
1 +
0
- 4
\______/
|
1
- 4
\_________/
|
-3
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When an operator is used on
an integer and a real number,
the result is a real number
(Type compatibility, p64).
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Examples:
4.2 * 3 is 12.6
1 / 2.0 is 0.5
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Type cast (p65)
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Examples:
(int)4.2 is 4
(double)17 is 17.0
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7 / 3 * 1.2 + 3 / 2
\_/
|
2
* 1.2 + 3 / 2
\___/
|
2.4
+ 3 / 2
\_/
|
2.4
+
1
\________/
|
3.4
Notice how 3 / 2 is still
1 above, not 1.5.
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String concatenation: Using the + operator between a string
and another value to make a longer string.
Examples:
"hello" + 42 is
1 + "abc" + 2is
"abc" + 1 + 2is
1 + 2 + "abc"is
"abc" + 9 * 3is
"1" + 1is "11"
4 - 1 + "abc"is
"hello42"
"1abc2"
"abc12"
"3abc"
"abc27" (what happened here?)
"3abc"
"abc" + 4 - 1causes a compiler error. Why?
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Write a program to print out the following output.
Use math expressions to calculate the last two numbers.
Your grade on test 1 was 95.1
Your grade on test 2 was 71.9
Your grade on test 3 was 82.6
Your total points:
Your average:
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249.6
83.2
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The computer internally represents real
numbers in an imprecise way.
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Example:
System.out.println(0.1 + 0.2);
 The output is 0.30000000000000004!
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ints are stored in 4 bytes (32 bits)
In 32 bits, we can store at most 232 different
numbers
What happens if we take the largest of these,
and add 1 to it?
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ERROR!
This is known as overflow: trying to store
something that does not fit into the bits reserved
for a data type.
http://en.wikipedia.org/wiki/Arithmetic_overflow
Overflow errors are NOT automatically detected!
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It’s the programmer’s responsibility to prevent these.
The actual result in this case is a negative number.
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int n = 2000000000;
System.out.println(n * n);
// output: -1651507200
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the result of n*n is 4,000,000,000,000,000,000 which needs 64-bits:
---------- high-order bytes ------00110111 10000010 11011010 11001110
---------- low order bytes -------10011101 10010000 00000000 00000000
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In the case of overflow, Java discards the high-order bytes, retaining
only the low-order ones
In this case, the low order bytes represent 1651507200, and since the
right most bit is a 1 the sign value is negative.
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What happens if we create a double value of 1, and then
keep dividing it by 10?
Answer: eventually, it becomes 0.
This is known as underflow: a condition where a
calculated value is smaller than what can be represented
using the number of bytes assigned to its type
http://en.wikipedia.org/wiki/Arithmetic_underflow
Again, Java does not detect this error; it’s up to the
programmer to handle it.
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Legal assignment
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Left is a single variable
Right is a legal expression
Prefix and postfix increment/decrement,
p79
Combined assignment, p73
Initialization & Declaration, p57
Constants (i.e., final), P60
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Shorthand
<variable>++;
<variable>--;

Examples:
int x = 2;
x++;
Equivalent longer version
<variable> = <variable> + 1;
<variable> = <variable> - 1;
// x = x + 1;
// x now stores 3
double gpa = 2.5;
gpa++;
// gpa = gpa + 1;
// gpa now stores 3.5
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
after executing
int m = 4;
int result = 3 * (++m)
result has a value of 15 and m has a value of 5
after executing
int m = 4;
int result = 3 * (m++)
result has a value of 12 and m has a value of 5
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
Java has several combined operators that allow you to quickly
modify a variable's value.
Shorthand
<variable>
<variable>
<variable>
<variable>
<variable>

+=
-=
*=
/=
%=
Equivalent longer version
<exp>;
<variable> =
<exp>;
<variable> =
<exp>;
<variable> =
<exp>;
<variable> =
<exp>;
<variable> =
<variable>
<variable>
<variable>
<variable>
<variable>
+
*
/
%
(<exp>);
(<exp>);
(<exp>);
(<exp>);
(<exp>);
Examples:
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x += 3 - 4;
gpa -= 0.5;
number *= 2;
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// x = x + (3 - 4);
// gpa = gpa – (0.5);
// number = number * (2);
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
A variable can be declared and assigned
an initial value in the same statement.
Declaration/initialization statement syntax:
<type> <name> = <expression>;

Examples:
double myGPA = 3.95;
int x = (11 % 3) + 12;
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
It is legal to declare multiple variables on one line:
<type> <name>, <name>, ..., <name>;

Examples:
int a, b, c;
double x, y;

It is also legal to declare/initialize several at once:
<type> <name> = <expression> , ..., <name> =
<expression>;

Examples:
int a = 2, b = 3, c = -4;
double grade = 3.5, delta = 0.1;

NB: The variables must be of the same type.
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
To avoid confusion, always name constants
(and variables).
area = PI * radius * radius;
is clearer than
area = 3.14159 * radius * radius;
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Place constants near the beginning of the
program, CircleCalculation2.java, p108.
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http://www.cis.temple.edu/~jiang/CircleCalculation2.pdf
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Once the value of a constant is set (or
changed by an editor), it can be used (or
reflected) throughout the program.
public static final double INTEREST_RATE =
6.65;

If a literal (such as 6.65) is used instead,
every occurrence must be changed, with the
risk than another literal with the same value
might be changed unintentionally.
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Syntax
public static final
Variable_Type <name> = <Constant>;
Examples
public static final double
PI = 3.14159;
public static final String MOTTO =
"The customer is always right.";
By convention, uppercase letters are used for
constants.
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Swap.java
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http://www.cis.temple.edu/~jiang/Swap.pdf
Payroll.java
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http://www.cis.temple.edu/~jiang/Payroll.pdf
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71
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Math.PI, Math.pow, Math.sqrt, etc. (p412)
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String
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text processing: Two data types involved
char
String
Represents individual characters
Represents sequences of characters
Primitive type
Object type (i.e., not primitive)
Written with single quotes
Written with double quotes
e.g.:
‘T’
‘t’
‘3’
‘%’
‘\n’
e.g.:
“We the people”
“1. Twas brillig, and the slithy toves\n”
“”
“T”
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char: A primitive type representing single characters. P52.
Individual characters inside a String are stored as char
values.
Literal char values are surrounded with apostrophe
(single-quote) marks, such as 'a' or '4' or '\n' or
'\''
Example, p67.
char letter = 'S';
System.out.println(letter);
System.out.println((int)letter);
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// prints S
// prints 83,
// explained on p932
74
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Most programming languages use the ASCII
character set.
Java uses the Unicode character set which
includes the ASCII character set.
The Unicode character set includes characters
from many different alphabets (but you
probably won't use them).
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String: an object type for representing sequences
of characters
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Sequence can be of length 0, 1, or longer
Each element of the sequence is a char
We write strings surrounded in double-quotes
We can declare, initialize, assign, and use String variables
in expressions just like other data types
String s = “Hello, world\n”;
System.out.println(s);
s = s + “I am your master\n”;
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//
//
//
//
declare, init
use value
concatenate
and assign
76
Unlike primitive types, String can have methods, P86.
Here is a list of methods for strings:
Method name
Description
charAt(index)
returns the character at the given index
indexOf(str)
returns the index where the start of the given string appears in this
string (-1 if not found)
length()
returns the number of characters in this string
substring(index1,index2)
returns the characters in this string from index1 up to, but not
including, index2
toLowerCase()
returns a new string with all lowercase letters
toUpperCase()
returns a new string with all uppercase letters
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
Let s be a variable of type String
General syntax for calling a String method:
s.<method>(<args>)

Some examples:
String s = “Cola”;
int len = s.length();
// len == 4
char firstLetter = s.charAt(0); // ‘C’
int index = s.indexOf(“ol”); // index == 1
String sub = s.substring(1,3); // “ol”
String up = s.toUpperCase(); // “COLA”
String down = s.toLowerCase(); // “cola”
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Displaying message
Input P116-118
 converting a string to number, p123
Output P121-122
 converting a number to string
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http://www.cis.temple.edu/~jiang/Payroll2.pdf
79
Summary of programs in
discussion

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Welcome.java
Welcome.java
Hello.java
Exercises (slide 17-18, 49, 54)
FirstProgram.java
Variable.java
ChangeMaker.java
CircleCalculation2.java
Swap.java
Payroll.java
Payroll2.java (a similar program ChangeMakerWindow.java)
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Summary of Concepts
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Running environment and execution of program (see lab work)
Template of java program, i.e., file, class, and main (see in lab work)
Program debug (memory tracking)
println and print (P93), escape sequence (P89)
Input via keyboard and plain text output (P96-97)
I/O via JOptionPane (showInputDialog P116-8, showMessageDialog P121-2)
Variable (P50-1), name (P55, P103), assignment (P55), declaration (P51), type
(P52), initialization (P57)
Constants (P60), type compatibility (P63-4) and type cast (P65-66)
String, its conversion to number P123 and vice versa (concatenation P82)
Arithmetic operators (e.g., %, P68), precedence order (P72)
Imprecision (round-off error), and overflow (online materials)
Prefix and postfix increment/decrement (P78,79), combined assignment (P73)
Math class, P410-413
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Other materials (FYI, not
required for test)



Printf (p101)
Delimiters for input (p99)
Windows program (p125)
http://www.cis.temple.edu/~jiang/ChangeMakerWind
ow.pdf
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