1. Arrays and Collections
Intro to the Java Collections Framework
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick

2. Modern programming languages support the concept of collections
That is, mechanisms to sort and manipulate many instances of the same type of data
In Java, an array is an indexed collection of data values of the same type
Arrays of both primitives or objects are possible
Integer arrays
String arrays
WinPlotter arrays
Winter 2004
CS-1010 Dr. Mark L. Hornick

3. Using constants to declare the array sizes does not always lead to efficient use of space
rainfall = new double [12];
Declaration of arrays with constants is called fixed-size array declaration.
Fixed-size array declaration may pose two problems; either:
Not enough capacity for the task at hand.
Wasted space (too much capacity)
Winter 2004
CS-1010 Dr. Mark L. Hornick

4. Java is not limited to fixed-size array declaration
int size;
int[] number;
size= Integer.parseInt(
JOptionPane.showInputDialog(null,
“Enter the size of an array:”));
number = new int[size]; // size determined from user input
Winter 2004
CS-1010 Dr. Mark L. Hornick

5. Any valid integer arithmetic expression is allowed for specifying the size of an array
size = Integer.parseInt(
JOptionPane.showInputDialog(null,””));
number = new int[size*size + 2* size + 5];
Winter 2004
CS-1010 Dr. Mark L. Hornick

6. Object Deletion A Before is executed After is executed 1 2 3 person 1
CS-1020
12/29/2018
Object Deletion
int delIdx = 1;
person[delIdx] = null;
Delete Person B by setting the reference in position 1 to null. A 1 2 3
person A B C D 1 2 3
person A C D
Before is executed
After is executed
In this approach, we simply leave the position of a deleted object to a null. With this approach, an array index positions will be a mixture of null and real pointers.
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick
Dr. Mark L. Hornick

7. Java Collections Framework
CS-1020
12/29/2018
Java Collections Framework
The java.util standard package contains other types of classes for maintaining a collection of objects.
We can add to, remove from, and retrieve objects in a collection.
A collection does not have a set limit to the number of objects we can add to it.
JCF includes classes that maintain collections of objects as sets, lists, or maps.
CS2851 is an entire course based on JCF
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick
Dr. Mark L. Hornick

8. Advantages to using JCF collection classes
Internal structure is hidden from view
Code is optimized for performance
Code is tested for quality
More flexible
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick

9. CS-1020
12/29/2018
ArrayList is a collection class that implements the following methods (and others)
boolean add( Object o )
Adds an object o to the list
void clear( )
Clears this list, i.e., make the list empty
Object get( int idx )
Returns the element at position idx
boolean remove( int idx )
Removes the element at position idx
int size()
Returns the number of elements in the list
There are total of 25 methods defined in the List interface. The five listed here are the very frequently used subset of those 25 methods.
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick
Dr. Mark L. Hornick

10. Generic ArrayList<E>
ArrayList<Double> salaryList
= new ArrayList<Double>();
E specifies the specific type of data that the ArrayList can manage
You must substitute a class name for E; you cannot create an ArrayList of primitives
This creates an instance, salaryList, of the ArrayList collection class. The elements in salaryList must be (references to) Doubles.
Double is the wrapper class for the primitive type double.
Generics allow you to create a class template that can use different types of objects when instantiated
CS-2851
Dr. Mark L. Hornick

11. Type conversion is done automatically through boxing and unboxing
Example: What if you want to insert a double value into an ArrayList<Double> at index 30? salaryList.add(30, );
This is called boxing: The automatic conversion of a primitive (double) value to the appropriate (Double) wrapper object
To retrieve the value, no explicit cast is needed:
double pay = /*(double)*/ salaryList.get(30);
This is because although the get() method returns a Double, a Double can be converted to a double primitive type. This is called unboxing.
CS-2851
Dr. Mark L. Hornick

12. ArrayList<E> usage features
What if salaryList needs to be expanded?
Done automatically!
What if you need to know the number of elements currently in salaryList?
salaryList.size()
What if you want to insert the element at index 30?
salaryList.add (30, );
// note: old element 30 becomes element 31, 31->32, etc
To retrieve the value, no cast is needed:
double sum = 0;
sum = sum + salaryList.get(30);
…provided that you use the generic version of ArrayList
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick

13. Built-in automatic sorting
Suppose you randomly add elements to an ArrayList
You can use the static sort() method of the Collections class to sort them
Collections.sort(list); // list is an ArrayList
Portions adapted with permission from the textbook author.
SE-1020
Dr. Mark L. Hornick