COMP 110 Some more about objects and references

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COMP 110Some more about objects and references

• Luv Kohli
• October 10, 2008
• MWF 2-250 pm
• Sitterson 014

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Announcements

• Midterm will not cover any new material discussed
  after todays class, except for Lab 5 and todays
  in-class exercise
• Ivan Sutherland talk on Monday, 4pm, Sitterson
  Hall 011

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Questions?
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Today in COMP 110

• DecimalFormat
• Some more about objects and references

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DecimalFormat

• import java.text.DecimalFormat
• DecimalFormat df new DecimalFormat(0.00)
• double number 12.3456
• System.out.println(df.format(number))
• Output 12.35
• 0.00 is the pattern that the format method will
  use to format its output
• Two digits after the decimal point, one digit
  before (but it will display all digits if more
  than one before)
• Fractional portion will be rounded

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DecimalFormat

• DecimalFormats format method returns a
  StringBuffer, not a String, but you can still
  print out a StringBuffer
• See Appendix 6 (4th edition) or Appendix 4 (5th
  edition) for many more details
• Experiment with it on your own

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Variables of a class type

• Contain the memory address of the object named by
  the variable
• NOT the object itself
• Object is stored in some other location in memory
• The address to this other location is called a
  reference to the object
• Class types are also called reference types

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Writing the .equals() method

• public class Book
• private String name
• private int page
• public boolean equals(Book book)
• return (this.name.equals(book.name)
• this.page book.page)

Not quite the right way to write the equals
method but okay for now
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.equals()

• Every class has a default .equals() method if it
  is not explicitly written
• Does not necessarily do what you want
• You decide what it means for two objects of a
  specific class type to be considered equal
• Perhaps books are equal if the names and page
  numbers are equal
• Perhaps only if the names are equal
• Put this logic inside .equals() method

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Parameters of a primitive type

• public void increaseNum(int num)
• num
• public void doStuff()
• int x 5
• increaseNum(x)
• System.out.println(x)
• Prints 5. Why?
• num is local to increaseNum method does not
  change x

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Parameters of a class type

• public void changeBook(Book book)
• book new Book(Biology)
• public void doStuff()
• Book jacksBook new Book(Java)
• changeBook(jacksBook)
• System.out.println(jacksBook.getName())
• Prints Java. Why?
• book is local to changeBook, does not change
  jacksBook

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Parameters of a class type

• public void changeBook(Book book)
• book.setName(Biology)
• public void doStuff()
• Book jacksBook new Book(Java)
• changeBook(jacksBook)
• System.out.println(jacksBook.getName())
• Prints Biology. Why?
• book contains the same address as jacksBook!

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Monday