Transcript Collections: A First Glimpse

Collections
A First Glimpse
26-Jul-16
Collections
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A collection is a structured group of objects
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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
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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
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Types of Collection
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Java supplies several types of Collection:
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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:
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Map: a “dictionary” that associates keys with values, order is
not important
SortedMap: like a Map, but order is important
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The Collections hierarchy
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Collections are ADTs
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I’m 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
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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
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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):
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boolean add(Object o)
boolean isEmpty()
boolean remove()
int size()
Object[] toArray()
Iterator iterator()
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Vectors
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The class Vector has been retrofitted to implement the
Collection interface
Vector supplies add(Object) and iterator()
methods (among others)
Let’s 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.next());
}
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.next());
}
Results:
one
three
two
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Conversions
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Vector v = new Vector(numerals);
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TreeSet ts = new TreeSet(v);
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Back to arrays
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Arrays are not part of the new Collections Framework,
but they haven’t been ignored
Java 1.2 introduced the new Arrays class, with some
useful operations, for example:
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static void sort(Object[] a)
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static int binarySearch(Object[] a, Object key)
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static boolean equals(Object[] a, Object[] a2)
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static void fill(Object[] a, Object val)
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static void fill(Object[] a, Object val,
int from, int to)
static List asList(Object[] a)
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What to remember
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We haven’t really begun to study the Collections
framework yet, but—
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You should learn to use the Iterator interface
You should examine and learn more about the Arrays
package
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The End
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