Finally a taste of Java

1
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)

2
Three Elements of Java

• 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.
• The Java programming language
• The language used to create programs.
• The Java Application Programmers 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.

3
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.

4
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.

5
Compile
Java Source Code (.java)
Java Byte Code (.class)
Java Compiler
Download (optional)
.class
.class
Java Virtual Machine A
Run
Java Virtual Machine B
(Machine Code A)
(Machine Code B)
6
Our First Java Program

• Lets write a Java program to print
• Hello, world!
• on the screen

7
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!" )

8
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.

9
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).

10
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

11
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
• /

12
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.

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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).

15
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.

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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

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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

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Assigning Values

• The equals sign is used to represent assigning
  a value to a variable.
• General form ltvariable namegt ltexpressiongt
• 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 gt 3
• Boolean operators
• and (represented by in Java)
• or (represented by in Java)
• not (represented by ! in Java)

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Truth Tables

• A TRUTH TABLE for a compound Boolean expression
  shows the results for all possible combinations
  of the simple expressions

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Type int

• A variable of type int may take values
  from2147483648 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
• ! gt lt gt lt (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
  from9223372036854775808L 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.

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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.

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Type double (Operators)

• The following operators are available for type
  double
• (addition)
• (subtraction, negation)
• (multiplication)
• / (division in real numbers, result is a
  double)
• gt lt (comparisons these take two values of
  type double and return a boolean value)
• WARNING Using (or ! gt lt) 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.

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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 '_at_', '', '', and so
  on.

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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.

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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)

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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.

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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)
• lt gt gt lt
• !
• (assignment to variable)

30
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)))
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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 gt 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

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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 lt 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

33
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.

34
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.

35
Java Input

• If one were to use Javas 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.

36
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.

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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.

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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.

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What happens if the usertypes 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.

40
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 )

41
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?

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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?

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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?

44
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

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Example 3 continued

• Solve for t to determine the age
• Another useful thing to know if half of the
  Carbon-14 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?