Collections

1
Collections

• A First Glimpse

2
Collections

• A collection is a structured group of objects
• An array is a kind of collection
• A Vector is a kind of collection
• A linked list is a kind of collection
• Java 1.2 introduced the Collections Framework and
  provided many great implementations
• Vectors have been redefined to implement
  Collection
• Trees, linked lists, stacks, hash tables, and
  other classes are implementations of Collection
• Arrays do not implement the Collection interfaces

3
Types of Collection

• Java supplies several types of Collection
• Set cannot contain duplicate elements, order is
  not important
• SortedSet like a Set, but order is important
• List may contain duplicate elements, order is
  important
• Java also supplies some collection-like things
• Map a dictionary that associates keys with
  values, order is not important
• SortedMap like a Map, but order is important

4
Collections are ADTs

• Im not going to cover Collections just yet, but
  I want to use them as an example
• Collections are one of the best-designed parts of
  Java, because
• They are elegant they combine maximum power with
  maximum simplicity
• They are uniform when you know how to use one,
  you almost know how to use them all
• You can easily convert from one to another

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Uniformity through interfaces

• Much of the elegance of the Collections Framework
  arises from the intelligent use of interfaces
• For example, the Collection interface specifies
  (among many other operations)
• boolean add(Object o)
• boolean isEmpty()
• boolean remove()
• int size()
• Object toArray()
• Iterator iterator()

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Vectors

• The class Vector has been retrofitted to
  implement the Collection interface
• Vector supplies add(Object) and iterator()
  methods (among others)
• Lets look at creating a Vector and iterating
  through the elements

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The Iterator interface

• interface iterator // java.lang.util
• boolean hasNext()
• // Returns true if the iteration has more
  // elements.
• Object next()
• // Returns the next element in the //
  interation.
• void remove()
• // Removes from the underlying collection
  // the last element returned by the //
  iterator (optional operation).

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Using a Vector

• Collection numerals new Vector() numerals
  .add("one") numerals .add("two") numerals
  .add("three")
• Iterator iter numerals.iterator() while
  (iter.hasNext()) System.out.println(iter.n
  ext())
• Results one two three

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Using a TreeSet

• Collection numerals new TreeSet() numerals
  .add("one") numerals .add("two") numerals
  .add("three")
• Iterator iter numerals.iterator() while
  (iter.hasNext()) System.out.println(iter.n
  ext())
• Results one three two

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Conversions

• Vector v new Vector(numerals)
• TreeSet ts new TreeSet(v)

11
Back to arrays

• Arrays are not part of the new Collections
  Framework, but they havent been ignored
• Java 1.2 introduced the new Arrays class, with
  some useful operations, for example
• static void sort(Object a)
• static int binarySearch(Object a, Object key)
• static boolean equals(Object a, Object a2)
• static void fill(Object a, Object val)
• static void fill(Object a, Object val,
  int from, int to)
• static List asList(Object a)

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What to remember

• We havent really begun to study the Collections
  framework yet, but
• You should learn to use the Iterator interface
• You should examine and learn more about the
  Arrays package

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The End