Welcome to

1
Welcome to

• CS110 Introduction to Computing with Java class
  4
• June 3, 2008
• Dan PLETEA
• Department of Computer Science
• University of Massachusetts Boston

2
Expressions

• An Expression is a combination of one or more
  operators that usually perform a calculation.
• The operands might be literals, constants
  variables or other sources of data.
• E.g. int result // declaration
• result 20/4 //Expression
• result result 4 //Expression

3
Arithmetic Operators

• , - , do what you expect them to
• (Modulus or remainder operator) gives remainder
• 20 6 2
• 2000 4 (might be helpful to find if year is a
  leap year)
• 11 2 (if 1 then number is odd otherwise even)

4
Arithmetic Operators

• Division Operator (/)
• Integer Arithmetic / truncates
• 20/6 is 3
• Decimal Arithmetic works properly
• 20.0/6, 20/6.0, 20.0/6.0 are all 3.33333
• Unary Operators (only one operand)
• -3, 4 (rarely used)
• No built in operator for exponentiation.
• But Math class in java class library has methods
  that perform mathematical functions.
• Math.pow(2,3) 8.0, Math.sqrt(25) 5.0

5
Operator Precedence

• Operators can be combined to create more complex
  expressions.
• E.g. result 14 8/2
• The right hand expression is evaluated and the
  result is stored in result variable.
• Order of operator evaluation makes a big
  difference.

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Operator Precedence

• All expressions are evaluated according to an
  operator precedence hierarchy.
• E.g. result 14 8/2
• However precedence can be forced by using
  parentheses.
• E.g. result (14 8)/2
• Parentheses can be nested and the innermost
  nested expressions are evaluated first.
• E.g. result 3 ((18-4) /2 )

18
result
11
result
21
result
7
Operator Precedence

• , -, , /, Left to right associative
  meaning that arithmetic operators at the same
  level of precedence are performed left to right.
• and - have same precedence
• , /, have same precedence (higher than and
  -)
• E.g. result 3 (18-4)/2
• E.g. result 3 16 /4 - 2

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result
5
result
8
Operator Precedence

• A syntactically correct expression has matching
  left and right parentheses.
• When a variable is referenced in an expression,
  its current value is used to perform the
  calculation.
• E.g. int count 4, total 5, sum
• sum count total
• sum sum 1

9
sum
10
sum
9
Increment and Decrement operators

• Increment operator () adds 1 to the value and
  decrement operator (--) subtracts from the value
• sum // this will increment value of sum by 1
  and result is
• // stored back into the
  variable sum
• sum (postfix increment operator)
• sum (prefix increment operator)

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Increment and Decrement operators

• sum and sum are functionally equivalent.
• When these operators are used in a larger
  expression they can yield different results. If
  sum is 15 then
• total sum //total is 15 and sum is 16
• total sum //total is 16 and sum is 16

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Assignment Operators

• can be used as follows
• sum 5 // sum sum 5
• // following statement is equivalent to
• // sum sumtotal12/count
• sum total 12/count
• Likewise we can use -, , /, .

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Data Conversion

• It is sometimes helpful and necessary to convert
  data value of one type to another type.
• Careful about losing information.
• Suppose a variable of type long is converted to
  int value (narrowing conversion)- Loss of
  information.
• Suppose a variable of type int is converted to
  long value (widening conversion)- Generally no
  loss of information as more storage space
  available.

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Conversion Techniques

• Assignment Conversion
• long money 24567890987L
• int dollar 342567
• money dollar //int value is converted
• // to long
  variable
• If we try to do
• dollar money //compile time error
• But we can do this by dollar (int) money
  //Casting

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Conversion Techniques

• Promotion- This conversion occurs automatically
  when certain operators need to modify their
  operands in order to perform the operation.
• float sum 25.0F
• int count 3
• float result sum /count//count is promoted
• // to float automatically
• String totalStudents Total 24

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Conversion Techniques

• Casting- The most general form of conversion in
  Java. A cast is a java operator that is specified
  by a type name in parenthesis.
• dollar (int) money //Casting
• int total 10
• int count 4
• float result total /count //result 2.0
• float result (float) total /count //2.5

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Interactive programs

• Good programs read data from the user
  interactively during execution.
• This way new results can be computed each time
  the program is run.
• The Scanner class which is part of the standard
  Java class library(1.5) provide convenient
  methods for reading input values of various types.

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Scanner class

• Before sending requests to (or calling methods
  of) any class we must create its instance
  (object).
• Mostly use new reserved word to create a new
  object.
• Note that String is an exceptional case
• String greeting Hello! World
• String greeting new String (Hello! World)

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Scanner class

• Scanner keyboard new Scanner(System.in)
• Message invoking a method
• int i keyboard.nextInt()

nextInt() get number user types return
that number Somewhere in Scanner.java
i that number
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Add two numbers

• public class SumTwoNumbers
• //Execution always starts from main
• public static void main(String args)
• System.out.println(Enter two numbers on
  line)
• Scanner keyboard new Scanner(System.in)
• int num1 keyboard.nextInt()
• int num2 keyboard.nextInt()
• System.out.println(Sum is)
• System.out.println(num1 num2)

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More programs.

• Write a class AppleBasket.
• It should have only one method main.
• It asks the user for no. of baskets and no. of
  apples in each basket. It then prints the total
  no. of apples.
• Output should be something like this

Enter the number of apples in each
basket 6 Enter the number of baskets 10 The
total number of apples is 60
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More programs.

• Write a class ChangeMaker.
• It should have only one method main.
• It asks the user for a no. between 1 and 99. It
  tells the user one combination of coins that
  equals that amount of change.
• Output should be something like this

Enter a number between 1 and 99 87 87 cents in
coins can be given as 3 quarters 1 dimes 0
nickels 2 pennies
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Pseudo code for ChangeMaker

• Declare originalAmount, amount, quarters, dimes,
  nickels, pennies of type int
• Read the amount from user into the variable
  amount
• Assign amount to originalAmount
• Set the variable quarters equal to the maximum
  number of quarters in amount.
• Reset amount to the change left after giving that
  many quarters.
• Repeat steps 4 and 5 for dimes and nickels.
• Assign amount left to pennies
• Output originalAmount and the numbers of each
  coin.

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Homework

• Homework 1 is online
• Due next Thursday in class, on paper.
• Check the webpage for PA1 tomrow. The PA will be
  due next Thursday.