Java Basics (continued)

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Java Basics(continued)
Mr. Smith AP Computer Science A
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Basic Java Syntax and Semantics

• Methods, Messages, and Signatures
• Classes implement methods, and objects are
  instances of classes.
• Objects that share common behavior are grouped
  into classes.
• An object responds to a message only if its class
  implements a corresponding method.
• Messages are sometimes accompanied by parameters
  and sometimes not
• pen.move() // No parameter expected
• pen.moveDown(8) // One parameter expected

Java Concepts 2.3 (Objects, Classes, and
Methods), 2.4 (Method Parameters and return
values)
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Basic Java Syntax and Semantics

• Some methods return a value and others do not.
• To use a method successfully we must know
• What type of value it returns
• Its name (identifier)
• The number and type of the parameters it expects
• This information is called the methods signature.

Java Concepts 2.3 (Objects, Classes, and
Methods), 2.4 (Method Parameters and return
values)
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Basic Java Syntax and Semantics

• Programming Protocols Use camelCase
• When forming a compound variable name,
  programmers usually capitalize the first letter
  of each word except the first.
• (For example taxableIncome)
• All the words in a classs name typically begin
  with a capital letter (EmployeePayroll).
• Constant names usually are uppercase and words
  are separated with underlines (INCOME_TAX_RATE).
  

Java Concepts 2.1 (Types and Variables)
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Comments

• Comments are explanatory sentences inserted in a
  program in such a matter that the compiler
  ignores them.
• There are two styles for indicating comments
• Single line comments
• These include all of the text following a double
  slash (//) on any given line in other words,
  this style is best for just one line of comments
• Multiline comments
• These include all of the text between an opening
  / and a closing /

Java Concepts 1.6 (Compiling a Simple Program)
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Additional Operators

• Extended Assignment Operators
• The assignment operator can be combined with
  the arithmetic and concatenation operators to
  provide extended assignment operators. For
  example
• int a 9
• String s "hi"
• a 3 // Equivalent to a a 3
• a - 3 // Equivalent to a a 3
• a 3 // Equivalent to a a 3
• a / 3 // Equivalent to a a / 3
• a 3 // Equivalent to a a 3
• s " there" // Equivalent to ss " there"

Java Concepts 4.3 (Assignment, Increment, and
Decrement)
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Additional Operators

• Increment and Decrement
• Java includes increment () and decrement (--)
  operators that increase or decrease a variables
  value by one
• int m 7
• double x 6.4
• m // Equivalent to m m 1
• x-- // Equivalent to x x 1.0
• The precedence of the increment and decrement
  operators is the same as unary plus, unary minus,
  and cast.

Java Concepts 4.3 (Assignment, Increment, and
Decrement)
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Standard Classes and Methods

• Eight methods in the Math Class

static double random() Returns a double in the range 0.0, 1.0)
Java Concepts 4.4 (Arithmetic Operations and
Mathematical Functions)
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Standard Classes and Methods

• Using the Math class
• double absNum, powerNum, randomNum, sqrtNum
• absNum Math.abs(-30)
• powerNum Math.pow(-3, 3)
• randomNum Math.random()
• sqrtNum Math.sqrt(25.0)
• Results
• absNum has a value of 30.0 //Absolute value of
  -30
• powerNum has a value of -27.0 //-3 to the 3rd
  power
• randomNum has a value of ??? //Random number
  between //0 (inclusive) and 1
  (exclusive)
• sqrtNum has a value of 5.0 //Square root of 25.0

Java Concepts 4.4 (Arithmetic Operations and
Mathematical Functions)
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Standard Classes and Methods

• Random Numbers and Simulation
• The Random class of the Java library implements a
  random number generator.
• To generate random numbers, you construct an
  object of the Random class and then apply one of
  the following methods
• nextInt(n) returns a random integer between 0
  (inclusive) and n (exclusive) 0, n)
• nextDouble() returns a random floating point
  number between 0 (inclusive) and 1 (exclusive)
  0, 1)
• For example, if a contestant on Deal or No Deal
  randomly selects their briefcase from the 26
  initial briefcases
• import java.util.Random //Include this at top
  of program
• Random generator new Random()
• int briefcaseNum generator.nextInt(26) 1

Java Concepts 6.5 (Random Numbers and Simulation)
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Control Statements
While and if-else are called control statements.

• while (some condition)
• do stuff
• Do stuff repeatedly as long as the condition
  holds true

• if (some condition)
• do stuff 1
• else
• do stuff 2
• If some condition is true, do stuff 1, and if it
  is false, do stuff 2.

Java Concepts 5.1 (The if Statement), 6.1 (while
Loops)
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The if and if-else Statements

• Principal Forms
• In Java, the if and if-else statements allow for
  the conditional execution of statements.

if (condition) statement1
statement2
if (condition) statement1
statement2 else statement3
statement4
Java Concepts 5.1 (The if Statement)
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The if and if-else Statements

• Relational Operators
• The complete list of relational operators
  available for use in Java

Java Concepts 5.2 (Comparing values)
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The if and if-else Statements

• Relational Operators
• Which of the following if statements are
  incorrect?
• int x
• if ( x gt 10 )
• if ( x 10 )
• if ( x gtlt 10 )
• if ( x 10 )
• if ( x gt 10 )
• if ( x ! 10 )
• if ( !x 10 )

Incorrect
Incorrect
Incorrect
Java Concepts 5.2 (Comparing values)
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The while Statement

• The while statement provides a looping mechanism
  that executes statements repeatedly for as long
  as some condition remains true.
• while (condition) // loop test
• statement // one statement inside the loop body
• while (condition) // loop test
• statement // many statements
• statement // inside the
• ... // loop body

Java Concepts 6.1 (while Loops)
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The while Statement

• Common Structure
• Loops typically adhere to the following
  structure
• 1) initialize variables
• 2) while (condition) // perform test
• 3) Loop body - perform calculations or do
  something
• 4) Loop body - change variables involved in
  the condition
• In order for the loop to terminate, each
  iteration through the loop must move variables
  involved in the condition closer to satisfying
  the condition.

Java Concepts 6.1 (while Loops)
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The while Statement

• Write a while loop that will sum the numbers from
  1 to 100 (i.e. 123 100).
• Print the answer to the console.

Java Concepts 6.1 (while Loops)
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The while Statement

• int sum 0
• int count 1
• while (count lt 100)
• sum count
• count
• System.out.println(sum)

Java Concepts 6.1 (while Loops)
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DiceRoller

• DiceRoller (object class) and DiceRollerViewer
  client class
• Write a DiceRoller class (similar to Yahtzee)
  to
• Allow the person to initially roll five dice.
  Each die has six faces representing numbers from
  1 to 6, so the program should randomly pick a
  number between 1 and 6.
• Print the results of each die to the console.
• After the first roll, the person should input the
  number of dice (between 0 and 5) that they want
  to roll again.
• If they enter 0, then dont roll the dice again.
  
• Else, roll that many dice and print the results
  of each die to the console again.
• Repeat the prompt one more time to see if the
  person wants to roll some of the dice again (for
  a maximum of 3 rolls).
• Write a DiceRollerGame client class to
• Instantiate a DiceRoller object
• Run a method in DiceRoller (such as playGame() )
  to play the game.
• EXTRA CREDIT (5 points) Have the player
  continue to take a turn (3 rolls) until they want
  to stop.
• HINT You may want to use
• the Scanner class to input data,
• the Random class to generate a random number to
  simulate rolling a die,

Java Concepts 6.5 (Random Numbers and Simulation)
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The for Statement

• The for statement combines counter
  initialization, condition test, and update into a
  single expression.
• The form for the statement
• for (initialize counter test counter update
  counter)
• statement // one statement inside the loop
  body
• for (initialize counter test counter update
  counter)
• statement // many statements
• statement // inside the
• . . . // loop body

Java Concepts 6.2 (for Loops)
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The for Statement

• Declaring the Loop Control Variable in a for
  Loop.
• The for loop allows the programmer to declare the
  loop control variable inside or outside of the
  loop header.
• The following are equivalent loops that show
  these two alternatives
• int i //Declare control variable above loop
• for (i 1 i lt 10 i)
• System.out.println(i)
• for (int i 1 i lt 10 i) //Declare var. in
  loop
• System.out.println(i)

Use this technique if you need to reference the
value of i later
Use this technique if you only use variable i
within the loop
Java Concepts 6.2 (for Loops)
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the break Statement

• The break statement can be used for breaking out
  of a loop early (before the loop condition is
  false).
• break statements can be used similarly with both
  for loops and while loops (break terminates the
  loop immediately).
• int sum 0 for (int i 1 ilt50 i)
  sum (i 2) if (sum gt 100)
  break

Java Concepts 6.4 Advanced Topic (break and
continue Statements)
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Nested Control Statements and the break Statement

• Control statements can be nested inside each
  other in any combination that proves useful.
• If you are nesting loops, the break statement
  only breaks out of the loop the program is
  currently in (the inside loop).
• int sum 0 for (int i 1 ilt100 i)
  for (int j 1 jlt100 j) sum
  (i j) if (sum gt 100)
  break System.out.println(The sum is
  sum) sum 0

Breaks out of this loop (not the outside loop)
Java Concepts 6.4 Advanced Topic (break and
continue Statements)
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Sentinel

• A sentinel is a data value that is used to denote
  the end of a data list or data input.
• The value cannot be a valid data value
• Usually 99999 or an unreachable high value or it
  could be a character such as Q for Quit. Note
  if you want the sentinel value to be Q then the
  value being entered needs to be a string.
• Example Sum up the ages of your family members
  int ageSum 0while (true)
  System.out.println(Enter age of family member
  (or 999 to quit)) int age in.nextInt()
  if (age 999) break ageSum
  ageSystem.out.println(Total age of all
  family members is ageSum)

Java Concepts 6.4 (Processing Sentinel Values)
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Sentinel

• Write a segment of code that will have the user
  input their grades and print the average
  afterwards. The user should enter a sentinel
  value when there are no more grades to enter.

Java Concepts 6.4 (Processing Sentinel Values)
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Sentinel

• Write a segment of code that will have the user
  input their grades and print the average
  afterwards. The user should enter a sentinel
  value when there are no more grades to enter.
• POSSIBLE OPTION
• int sum 0, numGrades 0, grade
• while (true)
• System.out.print(Enter a grade or 999 to end
  )
• grade in.nextInt()
• if (grade 999)
• break
• sum grade
• numGrades 1
• System.out.println(Average is (sum /
  numGrades )

What bugs do you see in this line of code?

• Missing parentheses
• Need to convert to a double
• Possible Divide by Zero error

Java Concepts 6.4 (Processing Sentinel Values)
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CircleCalc

• CircleCalc and CircleCalcViewer classes
• Write a class to
• Have the user input the diameter of a circle
  (floating point number) and the program outputs
  the
• Radius (how do you get the radius if you know the
  diameter?)
• Circumference
• Area
• Note you should use Math.PI in these
  calculations
• Use a sentinel controlled loop (sentinel value
  should be 99999) to keep asking the user the
  diameter of different circles.
• Test input for validity.
• EXTRA CREDIT (5 points) See next page
• What is the advantage of using a sentinel
  controlled loop?
• It allows the user to stop the looping whenever
  they want

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CircleCalc Extra Credit(5 extra points)

• CircleComponent and CircleGraphicalViewer
  classes
• Write classes to
• In the client class, have the user input the
  diameter of a circle (floating point number) and
  instantiate a component object to
• Pass the diameter to a Component object
• Calculate radius, circumference, and area (Note
  you should use Math.PI in these calculations)
• Draw the circle in a frame and print the
  calculated values beside it. The diameter of the
  circle (in pixels) should match what the user
  entered.
• Use a sentinel controlled loop (sentinel value
  should be 99999) to keep asking the user the
  diameter of different circles.
• Test input for validity.