PAL Worksheet ArrayList CSC20
Topics: Collections Interface, List Interface, ArrayList implementation
Ref: Polymorphism and Interfaces Worksheet.
The Java Collection framework is a set of interfaces and classes that help
manage objects. The List interface extends Collection and two common
concrete classes that implementations List are ArrayList and LinkedList. We
will explore ArrayList in this worksheet.
Standard arrays in Java have notable limitations, such as their fixed length.
Once an array is filled to capacity, it cannot be re-sized and a new array must
be created of larger sizer. Insertion and deletion for a standard array are also
cumbersome, requiring existing array elements to be shifted around.
The ArrayList class provides a dynamic array-like data structure which can
grow or shrink dynamically as elements are added or removed. The type of
objects that it stores needs to be specified but the size ie number of objects
does not need to be specified.
Here is how to create an ArrayList object and add some elements into it that
demonstrates this.
ArrayList<Integer> arraylist = new ArrayList<>();
arraylist.add(2);
arraylist.add(4);
arraylist.add(6);
Copy paste this code and see it run. Be sure to import java.util.ArrayList
Change the declaration to
List<Integer> arraylist = new ArrayList<>();
Does this work? Why or why not?
Note that collections like ArrayList store objects, not primitives such as int. So,
the above arraylist stores elements of type Integer. This is a ‘wrapper’ class
which encapsulates an int and provides functionalities to the primitive int
data types. Java will automatically convert a primitive type to the
corresponding object if needed.
PAL Worksheet ArrayList CSC20
Ex. 1) Write Java code to create a standard integer array called std_array of
size 3, and an Integer ArrayList called ‘array_list’. Provide each the following
elements: [0, 2, 4].
Ex. 2) Now, expand the Problem 1 code by inserting the elements [6, 8, 10] at
the end of both std_array and array_list. You will need to re-size std_array. You
could do this by writing a method that creates a new array that is twice the
size of an incoming array and copies the elements over before returning it.
Look up other ArrayList method to retrieve (get), delete (remove) and change
(set) elements. Notice that arraylists have indices much the same as a regular
array! Many of the ArrayList methods use indices to search and manipulate
arraylists. For example:
We can retrieve the element 4 at index 2 in the ArrayList from Problem 2
using the get method.
// array_list contains Integers [0,2,4,6,8,10]
array_list.get(2);
Ex. 3) To practice getting items, write a Java method which can reverse our
array_list. First, create a new empty ArrayList called temp. Then iterate
through array_list backwards, and within each loop cycle, retrieve an element
from array_list using get(), and use add() to insert it into temp. Use the method
size() to get the size of an ArrayList.
ArrayLists also have the remove() method to delete elements by index
position. The method dynamically re-arranges the remaining elements after
the deletion.
Ex. 4) Given the following Integer ArrayList scores_list of student exam scores:
[84, 70, 90, 67, 77, 94, 92, 71, 80, 95], write some lines of code that remove all
the odd-valued scores, i.e. 67, 77, 71 and 95
It’s often important to remove duplicate elements in an ArrayList. Look up the
remove method version that removes objects by value. Note that
remove(value) will remove the first occurrence of the provided value. What if
there are multiple occurrences of the value?
PAL Worksheet ArrayList CSC20
Ex. 5) Write a Java method to remove multiple occurrences of a certain value
in a given String ArrayList. Use the contains() method to help you, which
checks if a certain value is present in the list and returns the appropriate
boolean.
Bonus) Write a Java method called uniqueArrayList() which takes an Integer
ArrayList and returns an ArrayList of its unique elements.
Ex. 6) Write a few lines of code to create an ArrayList of Cones (use the class
in the Inheritance worksheet). Add five Cone objects to this arraylist. Now
insert a new Cone object at every other position in this arraylist. Finally delete
all the Cone objects of the original arraylist that you created.