Java Collections and the ArrayList class

1
Java Collections and the ArrayList class

• What are Collections?
• Classes provided (in the java.util package) for
  managing collections of Objects
• Main types of collections
• Sets the set of people in this class
• no duplicates
• no inherent order
• no ith element
• you need to iterate over the elements, and add
  and remove elements
• no fixed size to the collection
• Lists phone messages to return
• might be duplicates
• entries are in order (perhaps in the order
  received)
• you can add to the end, or to the beginning, or
  to the middle
• you can access the ith element
• no fixed size to the collection
• Maps mapping of userids to passwords
• no duplicate keys
• one value per key
• need to access the value for a key quickly don't
  typically traverse the collection

2
The Array "Interface"

• What can you do with arrays?
• construct one of a fixed type and length
• get the ith element
• set the ith element
• "fake" a variable-size array with a maximum size
  and "last" pointer
• which is cumbersome because the "last" pointer
  has to be an additional parameter
• and eventually you might exceed the maximum and
  then you're truly stuck
• Additional things that would be useful
• increase and decrease the collection's size on
  demand
• add new elements to the array (to the ith
  position)
• shifting all other elements to the "right"
• which means dynamically increasing the size of
  the array
• remove elements from the array (from the ith
  position)
• shifting all other elements to the "left"
• which means dynamically decreasing the size of
  the array
• store objects of different data types in the
  array
• possibly useful, but less common
• make the array "non-primitive" so we dont have
  to have special syntax

3
List Collections

• List is actually an interface, and there are
  several classes that implement it
• ArrayList is one of them, and the one most
  commonly used in practice
• The List interface (part of it)
• add(int index, Object element)
• add the object at this index, shifting existing
  elements to the right
• add(Object element)
• add the object to the end of the list the same
  as add(size(), element)
• Object remove(int index)
• remove the object at this index, shifting
  remaining elements to the left
• boolean remove(Object element)
• remove the first occurrence of this object,
  shifting remaining elements to the left
• Object get(int index)
• Object set(int index, Object element)
• boolean contains(Object element)
• int indexOf(Object element)
• returns the index of the first occurrence of this
  object or -1 if it does not appear in the list
• Iterator iterator()
• returns an Iterator that allows sequential access
  through the list
• (all methods that do comparisons use the equals()
  method)

4
Sample Application Course class

• A Course has a name (string) and a collection of
  Students
• A Student has a first name, a last name and a
  year (1, 2, 3, 4)
• Public interface
• new Course(String name)
• String getName()
• void addStudent(Student aStudent)
• void removeStudent(Student aStudent)
• int numStudents()
• Iterator getStudents()
• void printClassList()
• print the class list to the console, in
  alphabetical order
• Application class
• (add some students to the course)
• print list of names of the students in the course
• for year 1, 2, 3, 4 report the percentage of
  students in that year