Data Types, Operators and Input

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Data Types, Operators and Input

• Introduction to Software Development
• Week 2 Lecture 2

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Objectives

• In this lecture, we will
• Investigate Data Types in more detail
• Review operators
• Discuss how to use methods of the class String

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What is the output of this program?

• public class Example1
• public static void main(String args)
• int x, y
• x 7
• y x 5 / 3
• System.out.println("The answer is " y)

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What is a data type?

• A data type defines
• amount of storage space reserved for data
• number of bytes
• how that data is interpreted
• the character 'A' and the number 65 both have the
  binary representation 00000000 01000001
• what operations are allowed and their effect
• "Hello" " Cathy" concatenates two Strings
• 4 5 adds two integers

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Java data types

• what about String ?
• Java class, not primitive type
• will look at later in lecture

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Java operators (arranged by precedence)
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Arithmetic operators and expressions

• int x, y
• x 5 assignment statement
• y x 2 an expression involving addition
• y x - 2 subtraction
• y x 2 multiplication
• y x / 2 division
• CAREFUL integer division / discards remainder
• modulus (mod) operator gives remainder
• y x 2

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Integer division and modulus example

• Given a large number of days, how many weeks and
  days does this represent?
• integer division by seven will give the number of
  whole weeks
• the number of days left over is the modulus ()
  of division by seven
• for example, 25 days is 3 weeks plus 4 days
• int largeNumberOfDays 25
• int weeks largeNumberOfDays / 7
• int days largeNumberOfDays 7

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Operator precedence

• The operators / take precedence over -
• see table of operators
• each row has precedence over the rows beneath
• when in doubt use brackets ( )
• highest precedence
• int x 7, y
• y x 5 / 3 8
• y (x 5) / 3 4

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Integer and floating point division

• floating point (float or double) division works
  as expected
• double x 5
• double y x / 3
• System.out.println("The answer is " y)
• output The answer is 1.6666666666666667
• the statement double x 5
• is an example of automatic type conversion
• the integer 5 is converted to the double 5.0 when
  it is stored in x

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Integer and floating point division

• Be careful !
• What is the output of
• double y 5 / 3
• System.out.println("The answer is " y)
• the expression is evaluated (using integer
  division) first
• then the integer result is converted to a double
  and stored in y
• solution is to use at least one double in the
  expression
• double y 5 / 3.0
• System.out.println("The answer is " y)

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Casting

• Automatic type conversion only occurs if there is
  no danger of losing information
• otherwise the compiler will give an error message
• int x 5.0
• "Tutorial2.java" possible loss of precision
  found double, required int at line 27,
  column 13
• We can force conversion between types by using a
  cast
• put desired type in brackets before the value to
  be converted
• int x (int)5.0 5
• int y (int)4.8 4
• int answer (int)(5.0 / 3.0) 1
• double answer2 (double)5 / 3 1.6667

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Increment and decrement operators

• Shortcut for increasing or decreasing an integer
  variable by 1
• prefix increment
• y x
• equivalent to
• x x 1 increase x by 1
• y x assign the new value to y
• postfix increment
• y x
• equivalent to
• y x assign the current value of x to y
• x x 1 then increase x by 1
• to avoid confusion, don't use in an expression
• normally used for counting, for example in loops
  x

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Unary and Binary and -

• int answer, a, b
• answer a b binary
• answer a unary
• not often used
• answer -a unary -
• unary and - have higher precedence than /
• and hence higher than binary and -
• but lower than increments and decrements
• int x 3, y 2, answer
• answer y x answer is 6
• not very good style!

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Assignment operators

• x 3 5 6
• Assignment is an operator with low priority
• right to left associativity
• most operators left to right
• x y z 7
• can have multiple assignments
• - /
• all same precedence, right to left associativity
• each has form
• variable op expression
• which is equivalent to
• variable variable op (expression)
• x 3 equivalent to x x 3
• x y 2 equivalent to x x (y 2)

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Constants

• sometimes it is desirable to define a constant
  value
• pi, conversion factor between centimetres and
  inches, VAT rate
• declare as follows
• final type name value
• final double PI 3.1416
• constants must be assigned a value when declared
• value cannot be subsequently changed
• can use it confidently in the program knowing its
  value will always be the same
• if we want to modify its value, say to 3.1415926,
  we only have to change the code in one place
• by convention, constants are named in UPPER_CASE
  in Java so that they stand out.

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String data type

• String is not a primitive data type
• unlike the types we have discussed so far
• a variable of type String contains zero to many
  characters in sequence
• String is a class
• part of Java language
• String objects have data
• the sequence of characters
• and useful methods

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Some String methods

• public char charAt(int index)
• returns the character at the specified index
• public int indexOf(char ch)
• returns the index (position, starting from 0) of
  the first occurrence of ch
• returns -1 if ch not found
• public int length()
• returns the length of the String
• public String replace(char ch1, char ch2)
• replaces every occurrence of the first parameter
  in the String with the second parameter
• returns the new String (original String is
  unchanged)
• public String substring(int startIndex)
• returns the substring starting at position
  startIndex
• see the Java documentation of the class String
  for more methods
• http//java.sun.com/j2se/1.5.0/docs/api/index.html

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Using String methods

• String phrase "I love Java"
• int phraseLength phrase.length()
• char firstLetter phrase.charAt(0)
• char lastLetter phrase.charAt(phraseLength
  1)
• String newString phrase.replace('o', 'i')
• System.out.println("The phrase is "
  phraseLength " letters long")
• System.out.println("In lower case the phrase is "
  phrase.toLowerCase())

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Wrapper classes for primitive types

• there are also Java classes corresponding to the
  primitive types
• Integer, Double, Float
• we don't normally use them
• but they have useful methods
• Integer.parseInt("523")
• converts the String "523" to the integer 523
• different
• representation in memory
• allowed operations
• we will learn much more about classes and objects
  in the next module

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Summary

• In this lecture we have
• Investigated Data Types in more detail
• Reviewed operators
• Learned how to use String methods
• Further reading
• Liang "Java Programming"
• Chapter 2 Primitive Data Types and Operations
• Section 7.2 The String class
• Sun Java website http//java.sun.com/
• Tutorial on Operators
• http//java.sun.com/docs/books/tutorial/java/nutsa
  ndbolts/operators.html
• documentation for the classes String and Scanner
• http//java.sun.com/j2se/1.5.0/docs/api/index.html