Download CSCI1402 Introductory Java Programming

CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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CSCI1402 Introductory Java Programming
Using class ArrayList (cont)
Collections
 Example 10,000 students registered at the
university.
 Computer application to manage the student data will
use a Student object to store details of each
individual student .
 Therefore,10,000 students objects needed
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Display the details of each student:
System.out.println(student1.toString());
System.out.println(student2.toString());
System.out.println(student3.toString());
System.out.println(student4.toString());
System.out.println(student5.toString());
System.out.println(student6.toString());
.
.
System.out.println(student9999.toString());
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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 We want to be able to store the student objects in
some kind of data structure that will allow us to
process the collection of students as a whole
 We can do this with an array:
Student[] students = new Student(10000);
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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 Student objects need to be added one by one e.g.
Student s1 = new Student(“Safia”, ‘F’);
students[0] = s1;
OR
students[0] = new Student (“Safia”, ‘F’);
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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 Once added, the array can be processed as a whole.
e.g display the names of all of the students
for(int i = 0; i<students.length;i++)
{
System.out.println(students[i].getName());
}
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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A problem with using an array:
 Unused elements of the array are initialised to null
 We have to ensure that we don’t request a null to
execute a method
Safia Malik Sue Ben null null null null ...
18
21
19
50
0
1
2
3
4
5
6
7
...
a nullPointerException is
thrown
getName()
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Solution 1
Check each object before sending the getName()
message
for(int i = 0; i<students.length;i++)
{ //begin for loop
if(students[i] ! = null)
{//begin do if not a null
System.out.println(students[i].getName());
} // end do if not a null
} // end for loop
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Solution 2
 If you know that objects will always be stored in a
contiguous block starting at position 0 then stop the
loop when the first null is reached
for(int i = 0; i<students.length && students[i] ! = null;i++)
{ //begin for loop
System.out.println(students[i].getName());
} // end for loop
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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A better solution
Use a java ArrayList - it does the work for you
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Another (more serious) problem with using an array:
 An array is a static fixed –size structure
 It cannot grow
 We have to specify it’s size when we create it
 If it becomes full, then the only thing we can do is to
create another array that is bigger, copy all of the
objects from the old array into the new array etc.
 If the students array already stored 10,000 students
and another student wanted to enrol then this would
need to be done
 This involves a lot of extra coding
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
Solution
Don’t enrol any more students
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Another Solution
 Create a much bigger array to start with
This can be very wasteful of the computer’s memory
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
A better solution
Use a java ArrayList  it is a dynamic data structure
 it grows on demand
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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 The java ArrayList is provided in the java API
(Application Programmers Interface)
 The API contains ready – made classes that are
available for programmers to use –ArrayList is one of
them
 ArrayList is one of many classes provided in the API to
manage collections of objects
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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This is the specification for the ArrayList:
java.util
Class ArrayList
Constructor Summary
ArrayList()
Constructs an empty list.
ArrayList(int initialCapacity)
Constructs an empty list with the specified initial
capacity.
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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Method Summary
add(int index, Object element)
Inserts the specified element at the specified position
in this list.
add(Object o)
Appends the specified element to the end of this list.
addAll(Collection c)
Appends all of the elements in the specified
Collection to the end of this list, in the order that they are
returned by the specified Collection's Iterator.
clear()
Removes all of the elements from this list.
void
boolean
boolean
void
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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contains(Object elem)
Returns true if this list contains the specified
element.
Object
get(int index)
Returns the element at the specified position in this
list.
indexOf(Object elem)
Searches for the first occurence of the given
argument, testing for equality using the equals method.
isEmpty()
Tests if this list has no elements.
boolean
int
boolean
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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lastIndexOf(Object elem)
Returns the index of the last occurrence of the
specified object in this list.
Object remove(int index)
Removes the element at the specified position in this
list.
int
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CSCI1402 Introductory Java Programming
Week 20 Lecture 1
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set(int index, Object element)
Replaces the element at the specified position in this
list with the specified element.
int
size()
Returns the number of elements in this list.
String
toString()
Returns a String representation of all of the objects in the
ArrayList
Object
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