Overview

• Collection Classes
• Java Arrays
• Cloning Arrays
• Abstract Data Type for a Bag of Integers:
  • Bags, Sets, Sequences
  • Specification: selected methods
  • Implementation: clone
• quiz, Chapters 1 and 2

Collection Classes

• in a collection class, each object contains a collection of elements
• examples: test scores for a course can be stored in a collection class
• examples: colors for a sampler can be stored in a collection class
• some languages distinguish two kinds of collection types:
  • uniform: all elements have the same type
  • non-uniform: elements may have the same type
  This course will have examples of both uniform and non-uniform collection classes.

Java Arrays Review

• arrays are themselves collections
• "int [] myArray;" is a uniform array of integers
• "myArray = new int [10];" creates an array with 10 elements numbered 0... 9.
• myArray.length == 10
• "myArray [7] = 19;" stores the value 19 into the 8th element of the array.
• "x = myArray [3];" gets the fourth element from the array and stores the value in x.
• NullPointerException is thrown for using an array that is null
• ArrayIndexOutOfBoundsException is thrown for using an index that is out of bounds

Cloning Arrays and Typecasts

• as for all classes, assignment makes a copy of the reference, not of the array
• clone method will copy the array
• copy = (int []) myArray.clone
• the type declaration in this assignment is called a type cast: it converts a value of type object to a value of type int array
• the typecast is needed because clone always returns an object
• the typecast will fail if the value is of the wrong underlying type
• example: casting a clone of MyClass to (int []) will throw ClassCastException

Array properties

• cannot grow or shrink -- fixed number of elements
• efficient random access to elements, by index -- O(1), i.e. constant time
• efficient sequential access -- O(1) for one access
• duplicate elements allowed
• slow search -- O(n)

Bags, Sets, Sequences

• bags:
  • can grow or shrink
  • duplicate elements allowed
  • no indices, no random access
  • usually fast addition, slow search, deletion
• sets are like bags, but:
  • no duplicate elements
  • addition, search deletion can be O(1)
• sequences:
  • can grow or shrink
  • efficient access (addition, deletion) to the front and sometimes the back
  • duplicate elements allowed

A Bag of Integers

• used to store a collection of integers, with possible duplicates
• constructor, add, remove, size, countHowMany
• addAll (join, merge two bags) and union (same as addAll, but result is a new bag)
• pre-allocate and control the capacity: get, increase, reduce the guaranteed capacity of the bag
• OutOfMemoryError exception may be thrown when changing the capacity

Specification

• public class IntArrayBag
• a collection of int numbers
• limitations:
  • maximum capacity limited by available memory -- may throw OutOfMemoryError
  • maximum capacity limited by Integer.MAX_VALUE = 2^{31} - 1 = 2,147,843,647
  • some operations require time O(n) for a bag of size n
• constructor public IntArrayBag() -- initial capacity is 10
• constructor public IntArrayBag(int initialCapacity)

Specification for add

• public void add (int element)
• adds the new element to the bag, increasing the capacity if necessary
• parameter: element is the element to add to the bag
• postcondition: a copy of the element has been added to the bag
• throws: OutOfMemoryError
• note: incrementing capacity beyond Integer.MAX_VALUE causes arithmetic overflow

Specification for remove

• public boolean remove (int target)
• if possible, removes one copy of the element from the bag
• parameter: target is the element to remove from the bag
• postcondition: if at least one instance of target was present in the bag, one copy is removed and the method returns true. Otherwise, the bag is unchanged and the method returns false.

Specification for clone

• public Object clone ()
• generates a copy of this bag
• returns: a new object whose contents are identical to those of this bag
• throws: OutOfMemoryError

Implementation of clone

public class IntArrayBag
   implements Cloneable
{ 
  private int [] data;
  ...
public Object clone()
{
  IntArrayBag answer;
  try {
    answer = (IntArrayBag) super.clone();
  } catch (CloneNotSupportedException e)
  {
    ...
  }
  answer.data = (int []) data.clone ();
  return answer;
}