Download Introduction to Java

Finally! …a taste of Java!
• History
– 1991 - James Gosling, Sun Microsystems, Inc.
• originally a language for programming home
appliances
– later (1994) used for World Wide Web
applications (since byte code can be downloaded
and run without compiling it)
• Eventually used as a general-purpose programming
language (for the same reason as above plus it is
object-oriented)
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Three Elements of Java
1. The Java Runtime Environment (JRE)
• Needed to run Java programs
• Most internet browsers now include the JRE.
• If you are going to use Java programs (as opposed
to creating them), this is all you need.
2. The Java programming language
• The language used to create programs.
3. The Java Application Programmer’s Interface (API)
• A (large) collection of pre-written programs in
the software development kit (SDK) that can be
used by your Java programs.
• The “interface” describes how to connect your
programs with the ones in the SDK.
2
Translation of Java to machine code
• A two step process
• Step 1: Compile source code to Java “byte code”.
– “Byte code” is a standard intermediate-level
language.
– Input: Java code in one or more files ending in
.java
– Output: Byte code in one or more files ending in
.class
• Step 2: Interpret Java byte code on a particular
machine
– The interpreter converts each byte code
statement to the specific type of machine code
and then runs that machine code.
3
Why 2 steps?
• Byte code is standard, so you can compile Java programs on any
machine, and run them on any other machine that has a Java
interpreter.
– This means that .class files can be downloaded from the
Internet and run locally.
– Only the original program developer needs a compiler and the
software development kit.
– Avoids having to re-compile the program for every
combination of hardware and operating system.
• The Java interpreter is called a “Virtual Machine” and is part of
the Java Runtime Environment.
– Java virtual machines are specific to one type of computer
and operating system.
– Most internet browsers now include a Java virtual machine.
4
Compile
Java
Source Code
(.java)
Java
Compiler
Run
Java
Byte Code
(.class)
Download
(optional)
.class
.class
Java Virtual
Machine A
Java Virtual
Machine B
(Machine
Code A)
(Machine
Code B)
5
Our First Java Program
• Let’s write a Java program to print
Hello, world!
on the screen
6
Our First Java Program
• Code to begin and end your program:
– (to be explained later)
public class Hello
{
public static void main( String[ ] args )
{
System.out.println( "Hello, world!" );
}
}
7
Java Program Structure
• All Java code must be part of a “class”, and a Java program is
structured as a set of classes.
• Every class has a name:
public class Hello
{
}
• Braces { and } are used to mark the start and end of the class.
• Other class information may also appear with the name:
– A “public” class is one that can be referred to by other
classes.
8
Comments
• Programmers use comments in their code to provide
information to the reader on what the code is doing.
• When a program is compiled, the compiler skips all
comments.
• It is common (and good) practice to use a comment at
the top of the code containing general information
about the file (known as a header).
9
Code Comments
• Single line comment
– Everything from // to the end of the line is the comment
some code // This is a comment
more code
• General comment
– Everything from /* to the next occurrence of */ is a
comment
– Can be part of a line code /* comment */ more code
– Can be several lines
code /* start of comment
more comment
end of comment */ more code
10
Sample Program Header
/**
* Hello.java
* Author
Brigitte Helen Boudreau
* Student ID
123456
*
This program is a simple program that displays
*
Hello, world!
*
* Modifications
*
08/09/2003
BHB
first created
*
25/08/2004
AWW
modified slightly
*/
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Components of Classes
• Inside a class, there can be
– “attributes”: they store some information
– “methods”: they do some work.
• In our first program, we have zero attributes and one method.
• Each method has 2 parts: a header, and a body.
– The header contains the name and other information about
the method.
• A user of the method needs to know this information.
– The body describes what the method should do.
• Braces { and } are used to mark the start and end of the
method body.
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Methods
• In our first program, we have one method.
public static void main(String[ ] args) // header
{
System.out.println( "Hello, world!" );
}
– The name of the method is main
• main is a special name: it means “when you run the
program, start here.”
– Other information in the method header will be explained
later; for now, every main method should have this exact
header.
– A method body consists of a set of statements.
• Each statement is terminated by a semi-colon ;
• Our main method has only one statement.
13
Statements
• Our first Java program has one statement:
System.out.println( "Hello, world!" );
• Whatever is between the double quotes will appear on the
screen, and then the output will move to the next line of the
screen.
• System is the name of a special class that comes with Java
– The . operator asks to go inside a class to an attribute or
method.
– The above asks to start the println method contained in
the out attribute of the System class.
– The information between quotes is passed to the println
method.
– On any computer with a Java virtual machine, the System
class keeps track of machine-specific information (such as
how to get information onto the screen).
14
Data in Java
• Java is a “typed” language:
– Every data value must have a type
– The type gives rules for what values are allowed,
and what operations can be done with the values.
• For the computer, values of a certain type usually
take a known amount of storage space in memory.
15
Java Primitive data types
• A data item has a PRIMITIVE type if it represents a
single value, which cannot be decomposed.
• Java has a number of pre-defined primitive data
types. In this course, we will use the following types:
int
represents integers (e.g. 3)
long
represents large integers
double
represents “real” numbers (e.g. 3.0)
char
represents single characters
boolean
represents Boolean (logical) values
16
Variables
• To store a value, we use a variable – a name referring
to the location in which the value is stored.
• Variables must be declared before they can be used.
• A variable declaration has three parts:
– The type of the variable
– The name of the variable
– (Optional) Assign an initial value to the variable.
• Example: int x = 3;
17
Assigning Values
• The equals sign = is used to represent assigning a
value to a variable.
• General form: <variable name> = <expression> ;
• In an assignment statement:
– The expression to the right of = is evaluated.
– The value is put in the memory location
represented by the variable name.
– Whatever value was previously associated with the
variable is replaced (lost).
• Example (assume x and y are declared previously):
x = 3 + y;
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The Boolean Type
• A Boolean variable is one which can have only 2
possible values: true or false. (These are not
numbers.)
• Boolean variables are used when you need to do logical
tests
– Example “Is X greater than 3?” x > 3
• Boolean operators:
– and (represented by && in Java)
– or (represented by || in Java)
– not (represented by ! in Java)
19
Truth Tables
• A TRUTH TABLE for a compound Boolean expression
shows the results for all possible combinations of the
simple expressions:
X
Y
true
true
true
false
false
true
false
false
X AND Y
x && y
X OR Y
x || y
NOT X
! x
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Type int
• A variable of type int may take values from
–2147483648 to 2147483647.
– Exceeding the range of legal values results in OVERFLOW, a
run-time error.
• The following operators are available for type int :
+ (addition)
– (subtraction, negation)
* (multiplication)
/ (integer division, where fraction is lost; result is an int)
Example: 7 / 3 = 2
% (remainder from division)
Example: 7 % 3 = 1
== != > < >= <= (comparisons: these take two
values of type int and return a
boolean value)
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Type long
• A variable of type long may take values from
–9223372036854775808L to
9223372036854775807L.
• To indicate the difference between an int constant
and a long constant, the character L is appended
– Example
– 1 is of type int
– 1L is of type long
• Values of type long take up twice as much memory as
values of type int.
22
Type double (Literals)
•
Type double represents “real” numbers approximately from
-1.7  10308 to 1.7  10308 with 15 accurate significant digits.
– While there are a lot of double values, the set of legal double
values is still finite, and so they are only an approximation to real
numbers.
– After a computation, the computer has to choose the closest
double value to a real result: this can introduce “round-off”
errors.
•
Format of large or small values:
12345600000000000.0 = 0.123456  1017 is 0.123456e17
0.00000123456 = 0.123456  10-5 is 0.123456e-5
•
If the value of e is more negative than about -308, the result is
UNDERFLOW, and the value will be replaced by zero. This happens
quietly, and is not a run-time error.
23
Type double (Operators)
• The following operators are available for type double :
+ (addition)
– (subtraction, negation)
* (multiplication)
/ (division in “real” numbers, result is a double)
> < (comparisons: these take two values of
type double and return a boolean value)
• WARNING: Using == (or != >= <=) to compare two values of
type double is legal, but NOT recommended, because of the
potential for round-off errors.
– Later on, we will see a safe way to do this.
24
Type char
• Characters are individual symbols, enclosed in single
quotes
• Examples
– letters of the alphabet (upper and lower case are
distinct) 'A', 'a'
– punctuation symbol (e.g. comma, period, question
mark)
– single blank space
– parentheses '(',')'; brackets '[',']';
braces '{','}'
– single digit ('0', '1', … '9')
– special characters such as '@', '$', '*', and so
on.
25
Special characters
• Some characters are treated specially because they cannot be
typed easily, or have other interpretations in Java.
– new-line character '\n'
– tab character '\t'
– single quote character '\''
– double quote character '\"'
– backslash character '\\'
• All of the above are single characters, even though they appear
as two characters between the quotes.
• The backslash is used as an escape character: it signifies that
the next character is not to have its “usual” meaning.
26
Strings (1)
• A STRING is a collection of characters.
– There is NO primitive data type in Java for a string.
• We will see later how to deal with strings in general.
• String literals (constants) can be used to help make your
program output more readable.
– String literals are enclosed in double quotes:
"This is a string”
• Watch out for:
– "a" (a string) versus 'a' (a character)
– " " (a string literal with a blank that has length 1) versus
"" (an empty string: a string literal of length 0)
– "257" (a string) versus 257 (an integer)
27
String Concatenation
• Strings can be CONCATENATED (joined) using the +
operator:
– "My name is" + "Alan" gives
"My name isAlan"
• String values can also be concatenated to values of
other types with the + operator.
– "The speed is " + 15.5 gives
"The speed is 15.5"
– Because one of the values for the + operator is a
string , the double is temporarily converted to a
string value "15.5" before doing the
concatenation.
28
Precedence of Operators
• Operators are evaluated left-to-right, with the following
precedence (all operators on the same line are treated equally):
() (expression) [] (array subscript) . (object member)
+ – (to indicate positive or negative values) ! (not)
* / %
+ - (for addition or subtraction of two values, concatenation)
< > >= <=
== !=
&&
||
= (assignment to variable)
29
Operator Precedence
• What is the order of evaluation in the following
expressions?
a + b + c + d + e
a + b * c - d / e
a / (b + c) - d % e
a / (b * (c + (d - e)))
30
The Math class
• The Java class Math provides constants and methods for
common mathematical functions (see text page 43):
Math.PI
the value of 
Math.E
the value of e
Math.abs(x)
absolute value
Math.sin(x)
sine x in radians (also cos, tan)
Math.asin(x)
arcsine (also acos, atan)
Math.log(x)
natural logarithm, x > 0
Math.exp(x)
ex
Math.pow(x,y)
xy
Math.sqrt(x)
square root
Math.floor(x)
next lower integer
Math.ceil(x)
next higher integer
Math.round(x)
closest integer
31
Random Numbers
• Random numbers are often needed for:
– simulations
– games
• The method Math.random() returns a random number x of
type double such that 0 ≤ x < 1.
• Repeated calls will produce a uniform distribution
– that is, the probability that any particular number will be
returned is exactly the same as any other number.
• To adjust the range of random numbers:
y = Math.floor(Math.random() * 5) + 1;
y will be an integer such that 1 ≤ y ≤ 5
32
Console Input and Output
• We have already seen how to print a value on the
screen:
System.out.println( aValue );
• Reading information from the keyboard is not
straightforward in Java
– What you type is treated as a collection of
characters, even if you want to enter a number.
– We have to read an entire line at once, and then
convert the characters to whatever data type is
needed.
33
Java Output methods
• There are two useful Java methods from the class
System:
System.out.println( aValue )
System.out.print( aValue )
– Method println() will append a new-line
character to the output, while print() does not.
• These methods are unusual in that the type of
aValue does not matter.
• Method println() with no parameters can be used
to print a blank line.
34
Java Input
• If one were to use Java’s input methods directly (see
text, section 2.8), the following is needed:
– At the start of the program, create a “buffered
reader”
– Whenever you want to read some input:
• Read a line of characters from the keyboard
• PARSE the input
– For example, if you are expecting the user to enter a
value for type double, parsing checks that the
characters that have been entered form a valid
double.
35
Simplified Keyboard Input
• To simplify your code, a class called Keyboard is
provided. You can download it from the course
website.
• In your assignments and labs, include this file, called
Keyboard.java, in the same directory as your
program. Then you may call the methods in this class
to read a value (or values) from the keyboard.
36
Methods in class Keyboard
• Here are some of the methods available in the class Keyboard.
• Keyboard.readInt()
– Reads a single integer value of type int
• Keyboard.readLong()
• Keyboard.readDouble()
• Keyboard.readChar()
– Read a single character from the keyboard.
• Keyboard.readString()
– Read all characters typed before the user hits ‘enter’
• Keyboard.readBoolean()
– Type the word true or false (no quotes) on the keyboard
and the appropriate boolean value will be created.
37
Examples of using Keyboard
int x = Keyboard.readInt( );
• If you type 123 and press ENTER, x will have the
value 123 .
• Method readDouble works the same way.
boolean b = Keyboard.readBoolean( );
• If you type false and press ENTER, b will have the
value false.
long x = Keyboard.readInt( );
• If you type 123 and press ENTER, x will have the
value of a long integer 123L.
38
What happens if the user
types the wrong value?
• If you use Keyboard.readInt(), you are expecting
the user to type a set of characters that form a legal
integer in the proper range for an int variable.
• If the user types something else:
– A “default value” will be returned that will match
the type of the variable
• false for a boolean
• the lowest possible integer
• a special “not a number” floating point value
• Later on, we will see how to check user input in our
programs.
39
Input and Output Prompts
• When the user is being asked to enter something
from the keyboard, your program should always print
a message first to tell the user what to enter.
• Example:
int x;
System.out.println(“Enter an integer from 0 to 3:”);
x = Keyboard.readInt();
• Likewise, when your program prints a value, there
should be a prompt to explain what the value is:
• Example:
System.out.println(“You typed:” + x );
40
Problem Solving
• We now have enough tools to start solving some
problems.
• For any problem, BEFORE you start writing a
program, determine:
– What are the input values that are needed from
the user?
– What results (outputs) do we need to determine?
– What other values are needed?
• math constants: , e, etc.
• physical constants
– What temporary values might need to be
calculated?
– How do we calculate the results?
41
Example 1
•
Calculate the area of a circle from its radius
• what input values do we need?
• what other values do we need?
• what is our output?
• how do we calculate the result?
42
Example 2
• Calculate the distance travelled by an object under
constant acceleration, given a specified time.
– What input values do we need?
– What other values do we need?
– What is the result?
– How do we calculate the result?
43
Example 3:
• Carbon-14 dating
– General radioactive decay equation:
• Q0: initial quantity of radioactive substance
• Q(t): quantity of radioactive substance at time t (in years)
• : radioactive decay constant for a specific element
• Q(t) = Q0e–t is the rate of decay equation
– Carbon-14 is continuously taken in by a living organism, and
the amount of material at death is assumed to be known.
– Once the intake stops, the carbon-14 decays at the above
rate, where  = 0.00012097 / year
– Measure the percentage of carbon-14 remaining
44
Example 3 continued:
• Solve for t to determine the age:
t decay
1
Q
  log

Q0
• Another useful thing to know: if half of the Carbon14 remains, the sample is about 5730 years old.
• Create a Java program to determine the age:
– what are the inputs?
– what are the outputs?
– what other values do we need?
– how do we calculate the formula?
45