Conditions

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Chapter 6

• Conditions

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This chapter discusses

• Conditions and conditional statements.
• Preconditions, postconditions, and class
  invariants.
• Boolean expressions.

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Conditions

• postcondition a condition the implementor
  (server) guarantees will hold when a method
  completes execution.
• invariant a condition that always holds true.
• class invariant an invariant regarding
  properties of class instances that is, a
  condition that will always be true for all
  instances of a class.

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Counter class

• class invariant component variable tally will
  always be greater than or equal to zero.
• This holds true with the methods currently
  defined.
• postcondition the method count must make sure
  that tally is greater or equal to zero when the
  method completes execution.

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Counter class (cont.)

• Adding a method decrementCount threatens the
  class invariant constraint that tally be greater
  than or equal to zero.
• public void decrementCount ()
• tally tally - 1
• We must guard the assignment statement with a
  condition statement.

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Specifications

• Postconditions and class invariants are part of
  class specification but not the implementation.
  Therefore they should be included in comments but
  not in the implementation.
• /
• Current countthe number of items
• counted.
• ensure
• count gt 0
• /
• public int count ()

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Specifications (cont.)

• private int tally //current count
• //invariant
• // tally gt 0

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if-then statement

• Syntax if (condition) statement
• A composite statement.

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Counter class

• /
• Decrement positive count by 1
• /
• public void decrementCount ()
• if (tally gt 0)
• tally tally - 1

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Explorer Class

• /
• Damage (hit points) required to defeat
• this Explorer.
• ensure
• stamina gt 0
• /
• public int stamina ()
• return staminaPoints
• private int staminaPoints//current stamina
• //invariant //staminaPoints gt 0

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Explorer Class

• If stamina reaches 0, an explorer is defeated.
• One possible solution
• public void takeHit (int hitStrength)
• if (hitStrength lt staminaPoints)
• staminaPoints staminaPoints -
  hitStrength
• But this rarely lets the staminaPoints reach
  zero.

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Another possible approach

• public void takeHit (int hitStrength)
• if (hitStrength lt staminPoint)
• staminaPoints staminaPoints -
  hitStrength
• if (hitstrength gt staminaPoints)
• staminaPoints 0
• What is wrong with this approach?

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Another possible approach (cont.)

• It may meet the first condition and then in its
  changed state, meet the second condition as well.

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If-then-else statement

• Syntax if (condition) statement1
  else statement2

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Explorer class

• public Explorer( , int stamina, )
• if (stamina gt0)
• staminaPoints stamina
• else
• staminaPoints 0
• public void takeHit (int hitStrength)
• if (hitStrength lt staminaPoints)
• staminaPoints staminaPoints -
  hitStrength
• else
• staminaPoints 0

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Explorer constructor

• What should we do if the constructor is called
  with a negative value for the parameter stamina?
• public Explorer (String name, rooms.Room
  location, int hitStrength,
• int stamina)
• if (stamina gt 0)
• staminaPoints stamina
• else
• staminaPoints 0

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Compound statements

• Syntax statement1 statement2
• In this statement,
• if (hitStrength lt staminaPoints)
• staminaPoints staminaPoints - hitStrength
• else
• staminaPoints 0
• strengthPoints 0
• The last statement is not part of the else
  condition.

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Compound statements (cont.)
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Compound statements (cont.)

• Braces are used to create a block or compound
  statement, which is a single composite statement.
• if (condition) if (condition)
• statement1 statement1
• statementn statementn
• else
• statement1
• statementn

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Compound statements (cont.)

• if (hitStrength lt staminaPoints)
• staminaPoints staminaPoints - hitStrength
• else
• staminaPoints 0
• strengthPoints 0

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Relational expressions

• Relational operators
• lt less than
• lt less than or equal
• gt greater than
• gt greater than or equal
• equal (A single denotes assignment.)
• ! not equal
• A relational expression consists of two
  expressions joined with a relational operator.

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Relational expressions (cont.)

• Let i110, i2-20, i330.
• i1 lt 12 -gt true
• i1 lt 10 -gt true
• i3 i1 -gt false
• i1 ! 2 -gt true
• i1 lt 10 -gt false
• i2 gt 0 -gt false
• 2i1 i240 -gt true
• i2(-1) ! i1i1 -gt false

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Relational expressions (cont.)

• An expression like altbltc is illegal.
• If one operand is an int and the other is a
  double, the int is first converted to a double.
  10 gt 2.5 ? 10.0 gt 2.5
• Since floating point values are approximations
  for real numbers, we should avoid using equality
  or inequality operators with them.
• (1.0/6.01.0/6.01.0/6.0
• 1.0/6.01.0/6.01.0/6.0) 1.0 ? false

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Boolean variables

• Booleans can be stored as variables.
• private boolean tooBig
• tooBig true
• tooBig i1 gt 10

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Boolean operators

• ! not
• and
• or
• ! booleanExpression
• booleanExpression booleanExpression
• booleanExpression booleanExpression
• A boolean expression evaluates to either true or
  false.

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The not operator

• Not reverses boolean values. i.e. !true ?
  false !false ? true
• i110
• !( i1 gt 9) ? !(true) ? false
• Not has high precedence.
• ! i1 gt 9 ? (!i1) gt 9 ? illegal
• Generally avoid the not operator.

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And and or

• (i1gt10)(i110)?falsetrue?true
• (i1gt10)(i1lt0)?falsefalse?false
• (i1gt0)(i1lt5)?truefalse?false
• (i1gt0)(i1lt20)?truetrue?true

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And and or (cont.)

• The and have lower precedence than the
  relational operators, but parentheses are still
  useful because they enhance readability.
• The and are lazy operators that evaluate
  only the left operand only when that suffices.

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And and or (cont.)

• Consider
• (5 4) ????
• (5 ! 4) ????
• Does it matter what the second operands are?
• This can protect against runtime errors such as
  an attempt to divide by zero.
• Example (x 0) (y/x lt 10)
• (x ! 0) (y/x lt 10)

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DeMorgans laws

• Useful for simplifying expressions.
• !(b1 b2) ? !b1 !b2
• !(b1 b2) ? !b1 !b2
• !(i1gt5 i1lt8) ? !(i1gt5) !(i1lt8)

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Operation precedence
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Handling multiple cases

• Consider a method that determines if a year is a
  leap year.
• The Gregorian calendar stipulates that a year is
  a leap year if it is divisible by 4, unless it is
  also divisible by 100, in which case it is a leap
  year if and only if it is divisible by 400. For
  example, 1900 is not a leap year, but 2000 is.
• (year 100 ! 0 year 4 0)
• (year 100 0 year 400 0)

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LeapYear method

• Another way of representing these rules uses
  cases and nested conditional statements.
• public boolean isLeapYear (int year)
• boolean aLeapYear
• if (year 4 0)
• if (year 100 0)
• aLeapYear (year 400 0)
• else
• aLeapYear true
• else
• aLeapYear false
• return aLeapYear

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LeapYear method (cont.)
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TrafficSignal class

• The traffic signal class cycles through 4 states
  
• LEFT (left turn arrow)
• GO (green light)
• CAUTION (yellow light)
• STOP (red light)
• The component variable currentState will hold one
  of these states.

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TrafficSignal class (cont.)

• advance moves currentState to the next state.
• public void advance ()
• private int currentState
• if (currentState LEFT)
• currentState GO
• else if (currentState GO)
• currentState CAUTION
• else if (currentState CAUTION)
• currentState STOP
• else //currentState STOP
• currentState LEFT

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TrafficSignal class (cont.)
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Dangling else

• Which if statement is this else statement
  associated with?
• if (condition1)
• if (condition2)
• statement1
• else
• statement2

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Dangling else (cont.)

• It is associated with the second if (B).
• To associate it with the first, add braces around
  everything contained between the first if and the
  else.
• if (condition1)
• if (condition2)
• statement1
• else
• statement2

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Example combination lock

• Responsibilities
• Know
• The combination
• whether unlocked or locked
• Do
• lock
• unlock

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CombinationLock class

• Class CombinationLock
• Queries
• is open
• Commands
• lock
• unlock

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Class CombinationLock specifications

• public class CombinationLock
• Contructor
• public CombinationLock (int combination)
• Queries
• public boolean isOpen()
• Commands
• public void close ()
• public void open(int combination)

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Class CombinationLock implementation

• Component variables
• private int combination
• private boolean isOpen

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Class CombinationLock implementation (cont.)

• The straightforward implementations
• public boolean isOpen ()
• return isOpen
• public void close ()
• isOpen false

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Class CombinationLock implementation (cont.)

• When the constructor is being executed, there are
  two distinct variables with the same name.
  (Component variable and local variable
  combination).
• The keyword this refers to the current object.
  Therefore this.combination refers to the
  component variable.
• If the variable does not include the object
  reference this in front of it, it is a reference
  to the local variable.

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Class CombinationLock implementation (cont.)
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Class CombinationLock implementation (cont.)

• public CombinationLock (int combination)
• this.combination combination
• isOpen false
• We could write the second assignment as
• this.isOpen false
• But there is no ambiguity, so it is not needed.

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Class CombinationLock implementation (cont.)

• A final method
• public void open (int combination)
• if (this.combination combination)
• isOpen true

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Digit by digit lock

• This lock has a 3 digit combination.
• To open the lock, the client provides the digits
  one at a time.
• If the client enters the three digits of the
  combination in order, the lock opens.
• It doesnt matter how many digits the client
  provides, as long as the combination is given.

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Digit by digit lock (cont.)

• Digit Entered
• 4
• 1
• 2
• 4
• 3
• 1
• 2
• 3
• Digit Entered
• 1
• 2
• 3
• 4
• 7

Lock Status closed closed closed closed closed clo
sed closed open Lock Status closed closed open op
en open
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Class CombinationLock

• public class CombinationLock
• Constructor
• public CombinationLock (int combination)
• require combination gt 0
  combination lt999
• Queries
• public boolean isOpen ()
• Commands
• public void close ()
• public void enter (int digit)
• require digit gt0 digit lt9

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Precondition

• A condition the client of a method must make sure
  holds when the method is invoked.
• The constructor and method enter have certain
  requirements that must be met for them to execute
  properly. These are the requires found it the
  definition on the previous slide.
• Somewhat like the unleaded gas only sign near
  the gas cap on an automobile.

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CombinationLock responsibilities

• Know
• the 3-digit combination.
• whether locked or unlocked.
• the last three digits entered.
• Do
• lock.
• unlock, when given the proper combination.
• accept a digit.

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Getting digits from integers.

• Using the and / operator, we can extract each
  digit.
• Suppose a combination 123.
• 123 10 -gt 3
• 123 / 10 -gt 12
• 12 10 -gt 2
• 12 / 10 -gt 1
• 10 gets the last digit and / 10 get the
  remaining digits.

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CombinationLock implementation

• // entered1, entered2, entered3 are the last
  three
• // digits entered, with entered3 the most recent.
• // a value of -1 indicates the digit has not been
• // entered.
• private int entered1
• private int entered2
• private int entered3
• // invariant
• // entered1 gt -1 entered1 lt 9
• // entered2 gt -1 entered2 lt 9
• // entered3 gt -1 entered3 lt 9

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Weve covered

• Boolean expressions.
• Conditional statements.
• if (condition)
• statement
• if (condition)
• statement1
• else
• statement2
• Compound statements statement1 statement2
  statementn.
• Preconditions, postconditions, invariants.

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Glossary
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Glossary (cont.)