Primitive%20Data%20Types,%20etc.

1
Primitive Data Types, etc.

• Primitive Data Types
• Arithmetic expressions
• Relational operators
• Logical operators
• Conditional Statements
• if, if-else, switch
• Static Variables
• Constants
• Static Methods

2
Primitive Data Types

• A built-in data type
• Closely models the computers memory
• Predefined by the Java language
• Manipulated by built-in operators
• Do not have methods or instance variables (very
  different than objects)
• Java has 8 primitive data types
• byte, short, int, long, double, float, char,
  boolean

3
Integer Data Types

• Unlike mathematical numbers, which are infinite,
  numbers stored in a computer are restricted to a
  specific size
• Type Storage Range
• byte 1 byte (8 bits) -128 to 127
• short 2 bytes (16 bits) -32,768 to 32,767
• int 4 bytes (32 bits) -2,147,483,648 to
  2,147,483,647
• long 8 bytes (64 bits) -9,223,372,036,854,775,
  808 to 9,223,372,036,854,775,807

4
Integer Assignment

• Integer variables can be used in arithmetic
  expressions, sent as parameters, and assigned
  values, just like other variables
• references to objects are assigned a default
  value of null
• integers are assigned a default value of 0
• Color color //color null
• int number //number 0

5
Increment

• Preincrement operator
• Add 1, then assign/use value
• int a 3
• int b a //b4, a4
• Postincrement operator
• Assign/use old value, then add 1
• a 3
• b a //b3, a4
• a 3
• int c 10 a //c13, a4

6
Decrement

• Predecrement operator
• Subtract 1, then assign/use value
• a 3
• b --a //b2, a2
• Postdecrement operator
• Assign/use old value, then subtract 1
• a 3
• b a-- //b3, a2
• a 3
• c 10 a-- //c13, a2

7
Assignment Operators

• x x 5
• Can also be written as x 5
• a b 5
• Evaluates to a a (b 5)
• And not a a b 5

8
Example Code

• See Example1.java
• Use the (plus) operator to concatenate (add)
  Strings (characters enclosed in parentheses)
  together, as well as to convert a primitive data
  type to a String
• For example, this will display the String a3int
  a 3System.out.println("a" a)
• Output is displayed in the console window
• Method System.out.println() displays whatever is
  within the parentheses on a line

9
Follow Rules from Algebra

• Three rules for evaluation
• evaluation takes place from left to right, except
  that
• expressions in parentheses are evaluated first,
  starting at the innermost level, and
• operators are evaluated in order of precedence
• , /, and have precedence over and

10
Integer Division

• Integer division is different than the division
  that you are probably used to
• Cuts-off digits after the decimal point (does
  not round up)
• int i 1/3 // i0
• i 2/3 // i0
• i 3/3 // i1
• i 4/3 // i1
• i 5/3 // i1
• i 6/3 // i2

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Modulus Operator

• is the modulus operator
• Modulus is the remainder of a division
• 4 5 is 4
• 9 5 is 4
• 19 6 is 1
• 0 6 is 0
• 1 7 is 1
• 7 7 is 0
• 50 7 is 1

12
Class Exercise

• Evaluate the following expressions
• 2 4 3 - 7
• 10 3
• 2 9
• (2 3) (11 / 12)
• (4 4) 3 (2 - (11 / 3))

13
Fractional Numbers

• What if we want to represent fractional numbers?
• In Java we can use floats (floating point) or
  doubles (double precision floating point)
• floating point refers to where the decimal point
  is assumed to be internal implementation detail
• use doubles for scientific computation because
  they have larger range

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Floating Point Numbers

• Type Storage Range
• float 4 bytes (32 bits) -3.4 x 1038 to 3.4 x
  1038
• (7 significant digits)
• double 8 bytes (64 bits) -1.7 x 10308 to 1.7 x
  10308
• (15 significant digits)
• Example declarations
• float f 123.456f
• //f for float
• double d 123.456
• //double by default

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Two Kinds of Division

• Integer division
• Cuts-off digits after the decimal point (does
  not round up)
• int i 1/2 // i0
• Floating point division
• Returns a floating point (decimal point) value
• double d 1.0/2.0 // d0.5
• This returns an integer and then converts to a
  float
• d 1/2 // d0.0

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Mixing Integers Floats

• Can use different types of numbers together, but
  be careful!
• Can go from less to more precise, not the other
  way around!
• Java will not let you lose precision
  implicitly int myInt 4
• double myDouble 2.64
• myInt myDouble
• // cant assign a double to an int
• Change above assignment to
• myDouble myInt

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Mixing Integers Floats

• We can force Java to convert double to int
• called casting or coercion
• loss of precision
• myInt (int) myDouble // myInt is now 2
• Example code for last few slides
• See Example2.java

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Character Data Type

• Stores a 2 byte (16 bit) Unicode character
• See link from web page on Unicode
• Have to use single quotes around the character
• char character 'a'
• Some special characters, which are called escape
  sequences, begin with a backslash character (\)
• char newline '\n'
• char tab '\t'
• char double_quote '\"'
• char single_quote '\''
• char backslash '\\'

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Booleans

• Booleans, named in honor of British logician
  George Boole (1815-1864), are another base type
  (primitive data type) defined by Java.
• They can have one of two values true or false
• booleans are declared and assigned just like
  numbers
• default value is false
• boolean foo true
• boolean bar false
• foo bar // foo is now false...

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Boolean Expression

• A combination of operators operands that
  returns true or false
• Used in selection repetition statements
• Evaluated from left to right
• Operands are what the operators operate upon
• 3 Kinds of boolean operators
• Equality operators , !
• Relational operators gt, gt, lt, lt
• Logical operators , , ! (and, or, not)

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Logical Operators

• Logical and, or, not , , !
• Evaluated left to right
• Returns true or false
• Operands have to be boolean

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Comparing References

• We can check to see if two references are
  referring to the same instance of a class using
  or if they are referring to different
  instances using !
• Object object1 new Object()
• Object object2 new Object()
• boolean isEqual (object1 object2)
  // false
• object2 object1
• isEqual (object1 object2) // true

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Comparing References

• We can also check to see if a reference is not
  referring to anything
• Object object3 null
• boolean isNull (object3 null) //
  true
• object3 new Object()
• isNull (object3 null) // false

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Logical Operators And, Or, Not

• And () takes two boolean expressions and
  returns true only if both expressions are true
• Or () takes two boolean expressions and returns
  true if at least one expression is true
• Not (!) takes one boolean expression and returns
  its negation

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Logical Operators And, Or, Not

• Examples
• boolean bool1 (3 2) (2 lt 3) // false
• boolean bool2 (!bool1) (5.6 gt 8.0) // true
• boolean bool3 !(bool1 bool2) // true
• Example code for booleans and characters
• See Example3.java

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Class Exercise

• Was the output of the following code?
• See Tracing.java
• Make pairs discuss the problem with your
  partner
• Similar code will be on the next exam

27
Selection Statements

• Chooses which statement to be executed next
• Also called conditional statement
• Uses a boolean expression to choose which
  statement or group of statements to execute

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Selection 3 Ways

• 3 kinds of selection statements
• if structure
• single selection
• if-else structure
• double selection multiple selection
• switch structure
• multiple selection

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if Statements

• booleans help us make decisions with if
  statements
• Syntax
• if (ltboolean expressiongt)
• // code to be executed if expression is true
• Boolean expression is evaluated
• if true, code in body if statement is executed
  and then rest of method is executed
• if false, skips over body of if statement and
  continues with method

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if Statements

• If no parenthesis, then only the 1st statement
  following the if statement is associated with it
• if(a gt b)
• c 3
• d 4
• So d4 will always be evaluated
• Example code
• See Example4.java

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Checking booleans

• Note that you should never check if a boolean
  variable is true (or false) in the following way
• if (myBoolean true)
• System.out.println("true!")
• What if you mistyped and instead wrote
• if (myBoolean true)
• System.out.println("true!")

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Checking booleans

• The value true would be assigned to myBoolean and
  the conditional statement would always evaluate
  to true
• Assignments always evaluate to the result of the
  right side of the assignment
• This is a HUGE source of bugs!! Remember to
  always use when checking for equality!
• Instead, you should always say
• if (myBoolean)
• System.out.println("true!")

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if The Flow Chart
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Example Code

• See DaisyApplication.java
• Displays three daisies (Daisy.java)
• Clicking on a Petal calls method petalPlucked()
  of class Daisy, which increments numPetalsPlucked
• Example if statement
• See method petalPlucked() of Class Daisy
• Displays one of two messages
• Click on the leftmost Daisys Petals

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if-else

• What if we want to do something different when
  expression is false?
• We can add else clause to if!
• public void petalPlucked()
• numPetalsPlucked
• if (numPetalsPlucked ! 9)
• bubble.setText("Loves me not.")
• stem.setColor(Color.orange)
• else
• bubble.setText("Loves me.")
• center.setColor(Color.red)

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if-else

• We dont have to use braces if body of if or else
  is one statement
• public void petalPlucked()
• numPetalsPlucked
• if (numPetalsPlucked ! 9)
• bubble.setText("Loves me not.")
• else
• bubble.setText("Loves me.")

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if-else The Flow Chart
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Complex if statements

• We may want more than one condition in our
  if-statements
• We can add another if statement in elses body.
• if (ltboolean expressiongt)
• // body
• else if (ltboolean expressiongt)
• // body
• We can also nest our if statements by putting one
  inside another.

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Example Code

• See DaisyApplication.java
• Displays three daisies (Daisy2.java)
• Clicking on a Petal calls method petalPlucked()
  of class Daisy, which increments numPetalsPlucked
• Example if-else statements
• See method petalPlucked() of Class Daisy
• Displays several messages changes colors of
  Daisy
• Click on the middle Daisys Petals

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Short-Circuiting

• What is the value of n after the following code
  has executed?
• int n 1
• if (false (n 2))
• System.out.println("Went into the if clause")
• System.out.println(n)
• Answer n is 1
• since the left side of the condition evaluated to
  false the right side is not evaluated

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Short-Circuiting

• The same holds for and true
• if the left side of the conditional evaluates to
  true, the right side is not evaluated
• This is called short-circuiting
• part of the conditional is skipped
• Java takes a shorter route to evaluate conditional

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Short-Circuiting

• Can avoid short-circuiting by using and
  operators (both sides always evaluated)
• What if, in our above example, we always wanted
  to do the right side, which increments n?int n
  1if (false (n 2)) System.out.println(
  "Went into the if
  clause")System.out.println(n)
• If we used this instead, n would be 2 but we
  would still not go in to the if clause.

43
Side-effecting

• Note that the previous example updated a variable
  inside a conditional
• if(false (n 2))
• We only provide this as an example of C-style
  hacking which you might see when you look through
  someone elses code.
• In ICS 111, you should never write something like
  this
• Increment your variables outside of conditionals
• As youve seen, updating them inside conditionals
  makes the code confusing and hard to read.

44
If... If... If...

• What if we want lots of conditional statements?
• Could keep writing if-statements
• if (condition1) / body1 /
• else if (condition2) / body2 /
• else if (condition3) / body3 /
• else if (condition4) / body4 /
• ...
• else / bodyn /

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If... If... If...

• In English, this means
• if condition1, execute body1
• otherwise, if condition2, execute body2
• otherwise, if condition3, execute body3
• otherwise, if condition4, execute body4
• . . .
• otherwise, execute bodyN
• If all we want to do is something different for
  each value of a simple variable (e.g, age) cant
  we do something simpler?
• Yes! We can use a switch statement!

46
switch Statement

• switch is used to choose among more than two
  mutually exclusive choices, based on values of a
  variable.
• Very special form of general if-else-if
• switch is like vending machine.
• When coins are deposited into machine they fall
  into ramp with holes for each different type of
  coin.
• Smallest slot is for dimes largest, for quarters

47
switch Statement Selector

• switch can pick out one action from among many
  using a simple key called a selector
• Selectors cannot be floats, doubles, or
  references (objects)
• Of types we have encountered so far, can be ints,
  shorts, bytes, longs, or chars

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switch Statement Selector

• Value of selector determines action to be taken.
• Each possible value for selector should result in
  one action or set of actions (one block of code
  executed)
• Sometimes selector does not cover all cases.
• Use default value to specify what action(s)
  should be performed when selector does not fit
  any other values listed

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switch The Flow Chart


50
Dont Forget break Statement

• But theres a problem with code written according
  to the last diagram . . .
• If nothing alters flow of control, switch
  statements fall through to next case
• i.e., all case statements after one which matches
  selector are executed!
• Must have way to stop case from falling through
• break out of switch statement

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Amendment to Previous Diagram


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switch Statement

• So switch should usually include break statements
• If not, each case statement will be executed
  until encounter another break statement or reach
  end
• However, if we want several cases to execute the
  same statements
• Then list case statements one after another
  without using break statements

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Example Code

• See DaisyApplication.java
• Displays three daisies (Daisy3.java)
• Clicking on a Petal calls method petalPlucked()
  of class Daisy, which increments numPetalsPlucked
• Example switch statements
• See method petalPlucked() of Class Daisy
• Displays several messages changes colors of
  center stem
• Click on the right Daisys Petals

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static Class Variables

• Each instance of a class has different copies of
  instance variables. What if we want entire class
  to have same value for variable?
• Can use reserved word static to make class
  variables, of which there is only one copy for
  entire class.
• Each time any method of any instance changes the
  value of a static variable, all instances have
  access to that new value.
• Example code See SquaresFame.java (see Squares
  numberOfInstances getNumberOfInstances())

55
Constants

• Constants are used to represent values which
  never change in a program
• weve seen constants before the red, blue,
  green, etc., values of the java.awt.Color class
• They should be
• static, so there is exactly one value for entire
  class
• final, so their values cant be changed
• and public, so they are widely available

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Constants

• If we want a new value for SIDE, we can change
  our entire program very easily, by changing the
  value of our constant and recompiling
• See SquaresFrame.java give it a try!
• See code in Square class (at top of class)
• Constants never change after declaration
• ltconstantgt ltexprgt in a method wont work
• Cannot do this SIDE 2
• See code in Square constructor (towards the
  bottom)

57
Constants

• Constants have descriptive names literal numbers
  (such as 42) dont
• SIDE is called a symbolic constant
• 5 is called a literal constant
• Always use constants when possible
• makes your program easier to change
• makes your code easier to read
• Literal numbers should never (well, hardly ever)
  appear in programs.
• 0, 1 are common exceptions

58
Using Interfaces with Constants

• So far we have only used interfaces to declare
  methods we can also use them to declare
  constants
• What if many classes use same constants?
• Can put constants in an interface and have
  classes implement it!
• then public, static, and final are implicit
• interface SquareConstants
• int THICKNESS 5

59
Using Interfaces with Constants

• All classes that implement the SquareConstants
  interface have access to THICKNESS
• By changing the constants in the interface, we
  can change all the objects of all the classes
  that implement the interface
• For example, change THICKNESS and recompile run
  SquaresFrame.java
• What changes do you see?

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Java Math Class

• We can use Math class which is predefined by
  Java!
• Provides all kinds of cool methods
• Math.sin(x)
• Math.cos(x)
• Math.abs(x)
• Math.pow(x,y) raises x to the power of y
• Math.log(x)
• Math.sqrt(x) square root of x
• ... and more!

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Java Math Class

• Math also defines two symbolic constants
• Math.E 2.71...
• Math.PI 3.14...
• Why can we call methods on Math without
  instantiating it?
• Because its methods are static!

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static Class Methods

• static methods, or class methods, are invoked on
  behalf of entire class, not on specific instance
  of class no instance variables needed (nor are
  you allowed to access any)!
• In Math class, methods are declared
• public static double sin(double a)
• ...
• When calling static method, dont have to
  instantiate class, just send message to class
• double d Math.abs(-5.0) //d5.0

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Example Code

• Uses Math.random() method
• See Java 5.0 API for details on how this works
• Returns a double value with a positive sign,
  greater than or equal to 0.0 and less than 1.0.
• Used in Utilities.randomNumber()
• See Utilities.java
• Displays seven circles at random
• See RandomCirclesFrame.java

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Class Exercise

• Was the output of the following code?
• See Tracing2.java
• Similar code will be on the next exam
• In order to help you trace through the code, each
  box is has a different color
• On the actual exam, all the boxes will be black