Generics Types and Collections
This is a deeper review on data structures specific to Java. Many of these Data Structures are referred to as Collections. Using Collections requires deeper understand of Objects and the Generic Type.
- Arrays, ArrayList, 2D Arrays
- Collection Framework in Java
- Generics. The purpose of this "class" is to show how we can combine any Data Type into a super class. In fact, this is what many Computer Languages do, is they make general methods and properties for all Data within the language.
- Alphabet. This class is used to store the alphabet.
- Animal. This class is used to store properties on Animals.
- Cupcake. This class is used to store properties of Cupcakes.
- Hack Helpers
- Hacks
- Hacks: Code
- Hacks
Arrays, ArrayList, 2D Arrays
Most "Data Structures" conversations begin with Arrays, which are built into most Computer Programming Languages. College Board has CSA Units 6-8 which discuss Arrays, ArrayLists, and 2-Dimensional Arrays.
Arrays, 2D Arrays, and ArrayLists are important data structures in computer science, and they are the subject of two FRQs in each AP Computer Science A exam. Here are some types of FRQs that may focus on these topics:
- Array/ArrayList implementation: You may be asked to implement an Array or ArrayList, including methods to add, remove, and access elements.
- Array/ArrayList traversal: You may be given an Array or ArrayList and asked to traverse it, perform operations on each element, and/or modify the array or list.
- Array/ArrayList searching and sorting: You may be asked to implement or modify code to search for an element in an array or list, or to sort the elements of an array or list.
- 2D Arrays or Multi-dimensional arrays: You may be asked to implement or modify code that uses a multi-dimensional array, and to perform operations on elements of the array.
- ArrayList vs. Array: You may be asked to compare and contrast the characteristics of ArrayLists and Arrays, and to explain when it is appropriate to use one data structure over the other.
- Big-O complexity: You may be asked to analyze the time and space complexity of algorithms that use Arrays or ArrayLists, and to compare the efficiency of different algorithms.
Collection Framework in Java
A deeper dive into Data Structures continues with Linked Lists (LL) which are the foundation for Stacks and Queues, which we have used. Java has implemented a Collection framework that has established common methods to assist in using many of these Data Structures.
Queue<String> queue = new LinkedList<>(); // Queue interface uses LL implementation
queue.add("John");
queue.add("Jane");
queue.add("Bob");
Queue<String> queue = new LinkedList<>(); // Queue interface uses LL implementation
queue.add("John");
queue.add("Jane");
queue.add("Bob");
// Collections has a toArray convertion
Object[] arr = queue.toArray();
// Empty queue
System.out.println("Empty Queue");
while (queue.size() > 0) // Interate while size
System.out.println(queue.remove());
// Iterate of array
System.out.println("Iterate over Array");
for (Object a : arr) // Type is Object from convertion
System.out.println(a);
Generics. The purpose of this "class" is to show how we can combine any Data Type into a super class. In fact, this is what many Computer Languages do, is they make general methods and properties for all Data within the language.
- This class is abstract, meaning it is not used unless extended.
- The keyword interface is used to ensure people specify "interface" in their implementation. This can be used for things like sorting and searching information within the class.
- Every object in Java is inherited from Data Type "Object". This is shown in toString() method @Overrides below. The toString() method has a prototype implementation in "Object". Each extended class that @Overrides toString() and can be used to create a string representation of its "Object".
/* This is wrapper class...
Objective would be to push more functionality into this Class to enforce consistent definition
*/
public abstract class Generics {
public final String masterType = "Generic";
private String type; // extender should define their data type
// generic enumerated interface
public interface KeyTypes {
String name();
}
protected abstract KeyTypes getKey(); // this method helps force usage of KeyTypes
// getter
public String getMasterType() {
return masterType;
}
// getter
public String getType() {
return type;
}
// setter
public void setType(String type) {
this.type = type;
}
// this method is used to establish key order
public abstract String toString();
// static print method used by extended classes
public static void print(Generics[] objs) {
// print 'Object' properties
System.out.println(objs.getClass() + " " + objs.length);
// print 'Generics' properties
if (objs.length > 0) {
Generics obj = objs[0]; // Look at properties of 1st element
System.out.println(
obj.getMasterType() + ": " +
obj.getType() +
" listed by " +
obj.getKey());
}
// print "Generics: Objects'
for(Object o : objs) // observe that type is Opaque
System.out.println(o);
System.out.println();
}
}
public class Alphabet extends Generics {
// Class data
public static KeyTypes key = KeyType.title; // static initializer
public static void setOrder(KeyTypes key) {Alphabet.key = key;}
public enum KeyType implements KeyTypes {title, letter}
private static final int size = 26; // constant used in data initialization
// Instance data
private final char letter;
/*
* single letter object
*/
public Alphabet(char letter)
{
this.setType("Alphabet");
this.letter = letter;
}
/* 'Generics' requires getKey to help enforce KeyTypes usage */
@Override
protected KeyTypes getKey() { return Alphabet.key; }
/* 'Generics' requires toString override
* toString provides data based off of Static Key setting
*/
@Override
public String toString()
{
String output="";
if (KeyType.letter.equals(this.getKey())) {
output += this.letter;
} else {
output += super.getType() + ": " + this.letter;
}
return output;
}
// Test data initializer for upper case Alphabet
public static Alphabet[] alphabetData()
{
Alphabet[] alphabet = new Alphabet[Alphabet.size];
for (int i = 0; i < Alphabet.size; i++)
{
alphabet[i] = new Alphabet( (char)('A' + i) );
}
return alphabet;
}
/*
* main to test Animal class
*/
public static void main(String[] args)
{
// Inheritance Hierarchy
Alphabet[] objs = alphabetData();
// print with title
Alphabet.setOrder(KeyType.title);
Alphabet.print(objs);
// print letter only
Alphabet.setOrder(KeyType.letter);
Alphabet.print(objs);
}
}
Alphabet.main(null);
/*
* Animal class extends Generics and defines abstract methods
*/
public class Animal extends Generics {
// Class data
public static KeyTypes key = KeyType.title; // static initializer
public static void setOrder(KeyTypes key) { Animal.key = key; }
public enum KeyType implements KeyTypes {title, name, age, color}
// Instance data
private final String name;
private final int age;
private final String color;
/* constructor
*
*/
public Animal(String name, int age, String color)
{
super.setType("Animal");
this.name = name;
this.age = age;
this.color = color;
}
/* 'Generics' requires getKey to help enforce KeyTypes usage */
@Override
protected KeyTypes getKey() { return Animal.key; }
/* 'Generics' requires toString override
* toString provides data based off of Static Key setting
*/
@Override
public String toString()
{
String output="";
if (KeyType.name.equals(this.getKey())) {
output += this.name;
} else if (KeyType.age.equals(this.getKey())) {
output += "00" + this.age;
output = output.substring(output.length() - 2);
} else if (KeyType.color.equals(this.getKey())) {
output += this.color;
} else {
output += super.getType() + ": " + this.name + ", " + this.color + ", " + this.age;
}
return output;
}
// Test data initializer
public static Animal[] animals() {
return new Animal[]{
new Animal("Lion", 8, "Gold"),
new Animal("Pig", 3, "Pink"),
new Animal("Robin", 7, "Red"),
new Animal("Cat", 10, "Black"),
new Animal("Kitty", 1, "Calico"),
new Animal("Dog", 14, "Brown")
};
}
/* main to test Animal class
*
*/
public static void main(String[] args)
{
// Inheritance Hierarchy
Animal[] objs = animals();
// print with title
Animal.setOrder(KeyType.title);
Animal.print(objs);
// print name only
Animal.setOrder(KeyType.name);
Animal.print(objs);
}
}
Animal.main(null);
public class Cupcake extends Generics {
// Class data
public static KeyTypes key = KeyType.title; // static initializer
public static void setOrder(KeyTypes key) {Cupcake.key = key;}
public enum KeyType implements KeyTypes {title, flavor, frosting, sprinkles}
// Instance data
private final String frosting;
private final int sprinkles;
private final String flavor;
// Constructor
Cupcake(String frosting, int sprinkles, String flavor)
{
this.setType("Cupcake");
this.frosting = frosting;
this.sprinkles = sprinkles;
this.flavor = flavor;
}
/* 'Generics' requires getKey to help enforce KeyTypes usage */
@Override
protected KeyTypes getKey() { return Cupcake.key; }
/* 'Generics' requires toString override
* toString provides data based off of Static Key setting
*/
@Override
public String toString() {
String output="";
if (KeyType.flavor.equals(this.getKey())) {
output += this.flavor;
} else if (KeyType.frosting.equals(this.getKey())) {
output += this.frosting;
} else if (KeyType.sprinkles.equals(this.getKey())) {
output += "00" + this.sprinkles;
output = output.substring(output.length() - 2);
} else {
output = super.getType() + ": " + this.flavor + ", " + this.frosting + ", " + this.sprinkles;
}
return output;
}
// Test data initializer
public static Cupcake[] cupcakes() {
return new Cupcake[]{
new Cupcake("Red", 4, "Red Velvet"),
new Cupcake("Orange", 5, "Orange"),
new Cupcake("Yellow", 6, "Lemon"),
new Cupcake("Green", 7, "Apple"),
new Cupcake("Blue", 8, "Blueberry"),
new Cupcake("Purple", 9, "Blackberry"),
new Cupcake("Pink", 10, "Strawberry"),
new Cupcake("Tan", 11, "Vanilla"),
new Cupcake("Brown", 12, "Chocolate"),
};
}
public static void main(String[] args)
{
// Inheritance Hierarchy
Cupcake[] objs = cupcakes();
// print with title
Cupcake.setOrder(KeyType.title);
Cupcake.print(objs);
// print flavor only
Cupcake.setOrder(KeyType.flavor);
Cupcake.print(objs);
}
}
Cupcake.main(null);
Hack Helpers
Below is a starter Queue and a Linked List implementation. This implements Generic type and implements Iterable to support Java ForEach (enhanced For) loops.
In my experience, building your own Data Structures can help you to understand fundamentals of a Computer Language. To use a Data Structure you will need data. The developer working with LL, Stacks, and Queues needs to can learn how to manage different Data Types, this helps you learn about the Java Data Type Object as a generic form of an instance of a class and the Generic type <T> as generic for of a Data Type within a class definition.
/**
* Implementation of a Double Linked List; forward and backward links point to adjacent Nodes.
*
*/
public class LinkedList<T>
{
private T data;
private LinkedList<T> prevNode, nextNode;
/**
* Constructs a new element
*
* @param data, data of object
* @param node, previous node
*/
public LinkedList(T data, LinkedList<T> node)
{
this.setData(data);
this.setPrevNode(node);
this.setNextNode(null);
}
/**
* Clone an object,
*
* @param node object to clone
*/
public LinkedList(LinkedList<T> node)
{
this.setData(node.data);
this.setPrevNode(node.prevNode);
this.setNextNode(node.nextNode);
}
/**
* Setter for T data in DoubleLinkedNode object
*
* @param data, update data of object
*/
public void setData(T data)
{
this.data = data;
}
/**
* Returns T data for this element
*
* @return data associated with object
*/
public T getData()
{
return this.data;
}
/**
* Setter for prevNode in DoubleLinkedNode object
*
* @param node, prevNode to current Object
*/
public void setPrevNode(LinkedList<T> node)
{
this.prevNode = node;
}
/**
* Setter for nextNode in DoubleLinkedNode object
*
* @param node, nextNode to current Object
*/
public void setNextNode(LinkedList<T> node)
{
this.nextNode = node;
}
/**
* Returns reference to previous object in list
*
* @return the previous object in the list
*/
public LinkedList<T> getPrevious()
{
return this.prevNode;
}
/**
* Returns reference to next object in list
*
* @return the next object in the list
*/
public LinkedList<T> getNext()
{
return this.nextNode;
}
}
import java.util.Iterator;
/**
* Queue Iterator
*
* 1. "has a" current reference in Queue
* 2. supports iterable required methods for next that returns a generic T Object
*/
class QueueIterator<T> implements Iterator<T> {
LinkedList<T> current; // current element in iteration
// QueueIterator is pointed to the head of the list for iteration
public QueueIterator(LinkedList<T> head) {
current = head;
}
// hasNext informs if next element exists
public boolean hasNext() {
return current != null;
}
// next returns data object and advances to next position in queue
public T next() {
T data = current.getData();
current = current.getNext();
return data;
}
}
/**
* Queue: custom implementation
* @author John Mortensen
*
* 1. Uses custom LinkedList of Generic type T
* 2. Implements Iterable
* 3. "has a" LinkedList for head and tail
*/
public class Queue<T> implements Iterable<T> {
LinkedList<T> head = null, tail = null;
/**
* Add a new object at the end of the Queue,
*
* @param data, is the data to be inserted in the Queue.
*/
public void add(T data) {
// add new object to end of Queue
LinkedList<T> tail = new LinkedList<>(data, null);
if (this.head == null) // initial condition
this.head = this.tail = tail;
else { // nodes in queue
this.tail.setNextNode(tail); // current tail points to new tail
this.tail = tail; // update tail
}
}
/**
* Returns the data of head.
*
* @return data, the dequeued data
*/
public T delete() {
T data = this.peek();
if (this.tail != null) { // initial condition
this.head = this.head.getNext(); // current tail points to new tail
if (this.head != null) {
this.head.setPrevNode(tail);
}
}
return data;
}
/**
* Returns the data of head.
*
* @return this.head.getData(), the head data in Queue.
*/
public T peek() {
return this.head.getData();
}
/**
* Returns the head object.
*
* @return this.head, the head object in Queue.
*/
public LinkedList<T> getHead() {
return this.head;
}
/**
* Returns the tail object.
*
* @return this.tail, the last object in Queue
*/
public LinkedList<T> getTail() {
return this.tail;
}
/**
* Returns the iterator object.
*
* @return this, instance of object
*/
public Iterator<T> iterator() {
return new QueueIterator<>(this.head);
}
}
/**
* Queue Manager
* 1. "has a" Queue
* 2. support management of Queue tasks (aka: titling, adding a list, printing)
*/
class QueueManager<T> {
// queue data
private final String name; // name of queue
private int count = 0; // number of objects in queue
public final Queue<T> queue = new Queue<>(); // queue object
/**
* Queue constructor
* Title with empty queue
*/
public QueueManager(String name) {
this.name = name;
}
/**
* Queue constructor
* Title with series of Arrays of Objects
*/
public QueueManager(String name, T[]... seriesOfObjects) {
this.name = name;
this.addList(seriesOfObjects);
}
/**
* Add a list of objects to queue
*/
public void addList(T[]... seriesOfObjects) { //accepts multiple generic T lists
for (T[] objects: seriesOfObjects)
for (T data : objects) {
this.queue.add(data);
this.count++;
}
}
/**
* Print any array objects from queue
*/
public void printQueue() {
System.out.println(this.name + " count: " + count);
System.out.print(this.name + " data: ");
for (T data : queue)
System.out.print(data + " ");
System.out.println();
}
}
/**
* Driver Class
* Tests queue with string, integers, and mixes of Classes and types
*/
class QueueTester {
public static void main(String[] args)
{
// Create iterable Queue of Words
Object[] words = new String[] { "seven", "slimy", "snakes", "sallying", "slowly", "slithered", "southward"};
QueueManager qWords = new QueueManager("Words", words );
qWords.printQueue();
// Create iterable Queue of Integers
Object[] numbers = new Integer[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
QueueManager qNums = new QueueManager("Integers", numbers );
qNums.printQueue();
// Create iterable Queue of NCS Generics
Animal.setOrder(Animal.KeyType.name);
Alphabet.setOrder(Alphabet.KeyType.letter);
Cupcake.setOrder(Cupcake.KeyType.flavor);
// Illustrates use of a series of repeating arguments
QueueManager qGenerics = new QueueManager("My Generics",
Alphabet.alphabetData(),
Animal.animals(),
Cupcake.cupcakes()
);
qGenerics.printQueue();
// Create iterable Queue of Mixed types of data
QueueManager qMix = new QueueManager("Mixed");
qMix.queue.add("Start");
qMix.addList(
words,
numbers,
Alphabet.alphabetData(),
Animal.animals(),
Cupcake.cupcakes()
);
qMix.queue.add("End");
qMix.printQueue();
}
}
QueueTester.main(null);
Hacks
Blog or illustrate understanding of the following.
- Watch the college board video Classes and Objects* Blog and Define the details of a Class: Access modifier, constructor, modifiers/setters, getters, etc.
- Build example code in jupyter notebook with Linked List, Queues, and Stacks.
- Show familiarity with managing data (aka nodes in LL) in these structures.
- Show familiarity with Generic data and ForEach loop support, similar to ArrayLists T. Here is sample Java Generic T and the Java Iterable interface by Geeks4Geeks.
Hacks: Code
-
Challenge #1, Add and Delete elements from Queue. Working with the code that is given, you will need to adjust Add and write Delete, to output from the Queue as follows.
Enqueued data: seven Words count: 1, data: seven Enqueued data: slimy Words count: 2, data: seven slimy Enqueued data: snakes Words count: 3, data: seven slimy snakes Enqueued data: sallying Words count: 4, data: seven slimy snakes sallying Enqueued data: slowly Words count: 5, data: seven slimy snakes sallying slowly Enqueued data: slithered Words count: 6, data: seven slimy snakes sallying slowly slithered Enqueued data: southward Words count: 7, data: seven slimy snakes sallying slowly slithered southward Dequeued data: seven Words count: 6, data: slimy snakes sallying slowly slithered southward Dequeued data: slimy Words count: 5, data: snakes sallying slowly slithered southward Dequeued data: snakes Words count: 4, data: sallying slowly slithered southward Dequeued data: sallying Words count: 3, data: slowly slithered southward Dequeued data: slowly Words count: 2, data: slithered southward Dequeued data: slithered Words count: 1, data: southward Dequeued data: southward Words count: 0, data: null
-
Challenge #2, perform a merge or combination of 2 Queue's that are ordered. This is a foundation step for the algorithm used in Merge sorting. IMO, this algorithm is easier if you "peek" at data at the head of the queue, prior to performing dequeue action.
// Start with two ordered Queue's
(1st Queue) 1 -> 4 -> 5 -> 8 -> nil
(2nd Queue) 2 -> 3 -> 6 -> 7 -> nil
// Finish with a 3rd Queue
1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 nil
- Challenge #3, Shuffle the Queue. Iterate through the Queue and change data with another random position in the queue.
// Start with ordered Queue
1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 nil
// Finish with shuffled Queue
2 -> 8 -> 6 -> 1 -> 3 -> 4 -> 7 -> 4 nil
- Challenge #4, Build a Stack and use it to reverse the order of a Queue. The Queue is a LIFO Data Structure, the Stack is a FIFO data structure, so code is similar but most everything is reversed.
// Place elements into Queue
(Head) 1 -> 2 -> 3 -> nil
// Print out the following:
1 2 3
// Place elements from Queue to Stack
(Top) 3 -> 2 -> 1 -> nil
// Print out the following:
3 2 1
- Advanced Challenge #5, Implement a Stack from your LL into a new Jupyter Notebook ... Here is a former solution
/**
* Driver Class
* Tests queue with string, integers, and mixes of Classes and types
*/
class QueueTester {
public static void main(String[] args)
{
// Create iterable Queue of Words
Object[] words = new String[] { "seven", "slimy", "snakes", "sallying", "slowly", "slithered", "southward"};
QueueManager qWords = new QueueManager("Words", words );
qWords.addList();
qWords.printQueue();
// Create iterable Queue of Integers
Object[] numbers = new Integer[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
QueueManager qNums = new QueueManager("Integers", numbers );
// qNums.printQueue();
// Create iterable Queue of NCS Generics
Animal.setOrder(Animal.KeyType.name);
Alphabet.setOrder(Alphabet.KeyType.letter);
Cupcake.setOrder(Cupcake.KeyType.flavor);
// Illustrates use of a series of repeating arguments
QueueManager qGenerics = new QueueManager("My Generics",
Alphabet.alphabetData(),
Animal.animals(),
Cupcake.cupcakes()
);
// qGenerics.printQueue();
// Create iterable Queue of Mixed types of data
QueueManager qMix = new QueueManager("Mixed");
qMix.queue.add("Start");
qMix.addList(
words,
numbers,
Alphabet.alphabetData(),
Animal.animals(),
Cupcake.cupcakes()
);
qMix.queue.add("End");
// qMix.printQueue();
// for (Object o : words) {
// System.out.println("Enqueued Data: " + o);
// }
}
}
QueueTester.main(null);