Chapter 2 Collections

CHAPTER 2
COLLECTIONS
Lecturer: Hồ Tiến Lâm
Contents
 Collection Interfaces
 Concrete Collections
 Legacy Collections
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Created by Ho Tien Lam
Contents
 Collection Interfaces
 Concrete Collections
 Legacy Collections
3
Created by Ho Tien Lam
Interfaces and Implementation
 The Java collection library separates
interfaces and implementations.
 Let’s look at that separation with a familiar
data structure, the
queue
.
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data structure, the
queue
.
 You use a queue when you need to collect
objects and retrieve them in a FIFO order.
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Interfaces and Implementation
(cont.)

 A minimal form of a queue interface:
interface Queue<E> {
void add(E element);
E remove();
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 This interface tells you nothing about how the
queue is implemented. There are two
implementations of a queue.
int size();
}
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Interfaces and Implementation
(cont.)
 One implementation uses a circular array and
one uses a linked list.
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Interfaces and Implementation
(cont.)
 When you use a queue in your program, you
don’t need to know which implementation is
actually used.

Use the interface type to hold the collection
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
Use the interface type to hold the collection
reference.
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Queue<Customer> expressLane = new

CircularArrayQueue<Customer>(100);
expressLane.add(new Customer("Harry"));
Interfaces and Implementation
(cont.)
 With this approach if you change your mind,
you can easily use a different implementation.
 You only need to change in one place, the
constructor → save time of modification
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constructor → save time of modification
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Queue<Customer> expressLane = new
LinkedListQueue<Customer>();
expressLane.add(new Customer("Harry"));
Interfaces and Implementation
(cont.)
 Among Java collection classes, there are
some classes whose name begins with
Abstract (AbstractQueue).

These classes are intended for library
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
These classes are intended for library
implementors.
 It is more convenient for developer to extend
AbstractQueue than to implement Queue
interface.
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Collection and Iterator Interfaces

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public interface Collection<E> {
boolean add(E element);
Iterator<E> iterator();

}
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 add: adds an element to the collection.
Returns true if adding the element actually
changes the collection. Otherwise, returns
false.
}
Collection and Iterator Interfaces
(cont.)
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public interface Collection<E> {
boolean add(E element);
Iterator<E> iterator();

}
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 iterator: returns an object that implements
the Iterator interface. You can use the iterator
object to visit the elements in the collection
one by one.
}
Collection and Iterator Interfaces
(cont.)
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public interface Iterator<E> {

E next();
boolean hasNext();
void remove();
}
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 next: visits the elements one by one. If you
reach the end of collection, this method throws
a NoSuchElementException.
 hasNext: returns true if the iterator still has
more elements to visit.
}
Collection and Iterator Interfaces
(cont.)
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Collection<String> c = ;
Iterator<String> iter = c.iterator();
while (iter.hasNext()) {
String element = iter.next();
do something with element
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do something with element
}
Collection and Iterator Interfaces
(cont.)
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 “for each” loop: simple loop
Collection<String> c = ;
for (String element : c) {
do something with element
}

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 This kind of loop works with any object that
implements the Iterable interface.
}
public interface Iterable<E> {
Iterator<E> iterator();
}
Collection and Iterator Interfaces
(cont.)
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 The Collection interface extends the
Iterable interface. So you can use “for
each” loop with any collection in Java library.

The order in which the elements are visited
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
The order in which the elements are visited
depends on the collection type.
 But you can be assured that you will encounter
all elements of the collection.
Collection and Iterator Interfaces
(cont.)
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 Example: How do next and remove method of
Iterator works?
A
B
C
D

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 it.next(); → returns A
 it.next(); → returns B
 it.remove(); → removes B
 it.remove(); → IllegalStateException
A
B
C
D
Collection and Iterator Interfaces
(cont.)
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 Generic Utility Methods:
int size()
boolean isEmpty()
boolean contains(Object obj)
boolean
containsAll
(Collection<?> c)
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boolean
containsAll
(Collection<?> c)
boolean equals(Object other)
boolean addAll(Collection<? extends E> from)
boolean remove(Object obj)
boolean removeAll(Collection<?> c)
void clear()
boolean retainAll(Collection<?> c)
Object[] toArray()

<T> T[] toArray(T[] arrayToFill)
Collection and Iterator Interfaces
(cont.)
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 The library supplies an abstract class that
leaves the fundamental methods size and
iterator abstract.
public abstract class AbstractCollection<E>
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implements Collection<E> {
public abstract Iterator<E> iterator();
public boolean contains(Object obj) {
for (E element : c) // calls iterator()
if (element.equals(obj)) return = true;
return false;
}
. . .
}
Collection and Iterator Interfaces
(cont.)
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 A concrete collection class can now extend the
AbstractCollection class.
 The implementor just needs to supply an
iterator
method, but the
contains
method
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iterator

method, but the
contains
method
has been taken care of by the superclass.
 This is an excellent design for a class
framework.
Contents
 Collection Interfaces
 Concrete Collections
 Legacy Collections
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Created by Ho Tien Lam
Concrete Collections
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 Linked Lists
 Array Lists
 Hash Sets

Tree Sets
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
Tree Sets
 Priority Queues
 Maps
 Specialized Set and Map Classes
Linked Lists
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 ArrayList class has a major drawback:
Removing an element from the middle of an
array is expensive because of the cost of

moving all elements beyond the removed one.
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Linked Lists (cont.)
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 The linked list will solve this problem because
a linked list stores each object in a separate
link. Each link also stores a reference to the
next link in the sequence.
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Linked Lists (cont.)
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 Example 1:
List<String> staff = new LinkedList<String>();
// LinkedList implements List
staff.add("Amy");
staff.add
("Bob");
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staff.add
("Bob");
staff.add("Carl");
Iterator iter = staff.iterator();
String first = iter.next(); // visit first element
String second = iter.next(); // visit second element
iter.remove(); // remove last visited element
Linked Lists (cont.)
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 Example 1:
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