Introduction

Java generic is a concept in programming that allows type and method to operate on any variable type or object. In 2004, a generic was added to Java to expand the language's type system.
Java generic allows you to declare variables and methods that can be used to manipulate any data type
The generic data structure is a container that can hold any type of data. This means that a class or method can be declared for different types of variables or objects without overloading the method.

Prerequisites

The prerequisites to follow along in this tutorial, the reader should have the following:
Basic knowledge of Java.
Java installed.
Any Java IDE of their choice.
An understanding of collections and their methods.
Basic understanding of generic

The motivation of generic methods and classes

Method overloading is a concept often used to perform operations that has similar syntax but different signature(different data type, different number of parameter). With the generic concept. The implementation of an algorithm that works with different collection types can be customized. This method is type-safe and simple to comprehend. This means that generic help in implementing a single method that can function with any data type.

Method overloading

public class OverLoadMethod {
public static void printElement(Integer[] inputElement){
for (Integer element: inputElement){
System.out.printf("%s ",element);
}
System.out.println();
}
public static void printElement(Double[] inputElement){
for (Double element: inputElement){
System.out.printf("%s ",element);
}
System.out.println();
}
public static void printElement(Character[] inputElement){
for (Character element: inputElement){
System.out.printf("%s ",element);
}
System.out.println();
}
public static void main(String[] args) {
Integer[] integerElement = {1,2,3,4,5,6};
Double[] doublesElement = {1.0,2.0,3.3,4.2,5.1,6.9};
Character[] charactersElement = {'W', 'E', 'L',' C',' O',' M', 'E'};
System.out.printf("Array integerElement contains: %n");
printElement(integerElement);
System.out.printf("Array doubleElement contains: %n");
printElement(doublesElement);
System.out.printf("Array characterElement contains: %n");
printElement(charactersElement);
}
}

From the code above, we declare a class called OverLoadMethod. The class contain three overloading methods of printElement. Each method used a parameter of type-wrapper of arrays of Integer, Double and Character and return void. In each of the methods, we used an enhanced loop to loop through the array of each method and print the content of the array with a print format. System.out.println() is used to print empty lines and give space to the output. public static void main(String[] args) declares the main method which serves as the entry point of the code. The method contains an array declaration of type-wrapper of Integer, Double and Character. In the main method, the printElement method was called to operate on each array declaration.
Note: we could as well use the primitive data type for the operation. But Because we are performing operations on generic methods and classes that only work on reference data types.
Generic Implementation of the Method

public class GenericMethod {
public static <T> void printElement(T[] inputElement){
for (T element: inputElement){
System.out.printf("%s ",element);
}
System.out.println();
}
public static void main(String[] args) {
Integer[] integerElement = {1,2,3,4,5,6};
Double[] doublesElement = {1.0,2.0,3.3,4.2,5.1,6.9};
Character[] charactersElement = {'W', 'E', 'L',' C',' O',' M', 'E'};
System.out.printf("Array integerElement contains: %n");
printElement(integerElement);
System.out.printf("Array doubleElement contains: %n");
printElement(doublesElement);
System.out.printf("Array characterElement contains: %n");
printElement(charactersElement);
}
}

From the above code, We created a class called GenericMethod. The class declares a generic method called printElement which takes the generic type of arrays as a parameter and returns void. for (T element: input element) This statement is used to loop through the array. System. out.print("%s ", element) This statement allows us to print each element with print format.
We also create a main method that serves as an entry point for the application. The main method has three arrays of different types of wrappers as Integer, Double and Character. Each of the arrays is used as an argument to process the printElement method.

Generic Data Structure

The generic data structure is a way of format organizing, retrieving, storing and processing data of any data type in a single data structure. This means that the data structure can take in any data type.
In the tutorial, we will be discussing Generic memory Allocation, List, LinkedList, Exception, Stack in different forms, Queue And Tree.

Generic Memory Allocation

The creation, usage and maintenance of generic classes need the allocation of memory. This allocation is done dynamically at the time of execution. This is the time to store a new node and release the node which is no longer needed by the program. The release of the node which is no longer used by the program is done by Java Automatic Garbage collection. Garbage collection clean object whose reference is no longer used in the program. The limit to the memory allocation can be determined by the size of the physical memory or virtual memory of the computer. But because different applications share the memory space available. The memory allocation of the Generic data structure is limited to a smaller space.

Node<Integer> newNodeToAdd = new Node<Integer>(10);

In the declaration, we allocate Node and assign the reference to newNodeToAdd.
With the Allocation, if the memory available is low the program throws an exception of OutOfMemoryError.

Implementing Generic List and LinkedList

class ListNode<T> {
T data;
ListNode<T> nextNode;
public ListNode(T object) {
this(object, null);
}
public ListNode(T object, ListNode<T> node) {
this.data = object;
this.nextNode = node;
}
public T getData() {
return data;
}
public ListNode<T> getNextNode() {
return nextNode;
}
}

In the above code, we declare a class called ListNode. The class has two variables, which are data which is typically generic and nextNode of a generic list. We declare a first constructor which takes the object as a parameter of type generic. We also declare another constructor of a two-parameter. These are object and nodelist, both are of type generic. The code has two getter methods which are getData and getNextNode which both respectively return data and nextNode(List).

Implementing list

import exceptions.EmptyListException;
public class List<T> {
private ListNode<T> firstNode;
private ListNode<T> lastNode;
private final String name;
public List()
{
this("list");
}
// constructor creates an empty list with a name
public List(String listName)
{
name = listName;
firstNode = lastNode = null;
}
public void addToTheFront(T insertItem){
if (isEmpty()){
firstNode = lastNode = new ListNode<T>(insertItem);
} else firstNode = new ListNode<>(insertItem, firstNode);
}
public void addToTheBack(T insertItem) {
if (isEmpty()){
firstNode = lastNode = new ListNode<T>(insertItem);}
else lastNode = lastNode.nextNode = new ListNode<>(insertItem);
}
public T removeFromFront() throws EmptyListException
{
if (isEmpty()){
throw new EmptyListException(name);
}
T removedItem = firstNode.data;
if (firstNode == lastNode)
firstNode = lastNode = null;
else
firstNode = firstNode.nextNode;
return removedItem;
}
public T removeFromBack() throws EmptyListException {
if (isEmpty()){
throw new EmptyListException(name);
}
T removeItem = firstNode.data;
if (firstNode==lastNode){
firstNode=lastNode=null;
}else{
firstNode = firstNode.nextNode;
}
return removeItem;
}
public void print() {
if (isEmpty()){
System.out.printf("Empty %s%n", name);
}
System.out.printf("This %s is: ", name);
ListNode<T> current = firstNode;
while(current !=null){
System.out.printf("%s ", current.data);
current = current.nextNode;
}
System.out.println();
}
boolean isEmpty() {
return firstNode==null;
}
}

From the code above the class imports exceptions from the customised exception class. Next, we declare a class List with type holder . The class contain three instance variable. Two of the variable are the List variable which reference ListNode(firstNode And lastNode) and the last variable is of type string. The code has two constructors, which are the no-argument constructor and the one-argument constructor. we also declare six methods that perform different tasks and functions.
The first method addToTheFront takes one parameter of a generic object and returns a void. We check if the list is empty if (isEmpty()). firstNode = lastNode = new ListNode<T>(insertItem) we assign the reference of the first last node to the first node and the lastNode both hold a reference to a new ListNode object. And if the list is not empty, we assign the reference of the new ListNode to firstNode. This action is performed with this expression else firstNode = new ListNode<>(insertItem, firstNode);.
The second method addToTheBack takes one parameter of a generic object and returns a void. We check if the list is empty if (isEmpty()). firstNode = lastNode = new ListNode<T>(insertItem); we assign the reference of the lastnode.nextNode to lastNode and the lastnode.nextNode holds a reference to a new ListNode object. And if the list is not empty, we assign the reference to a new ListNode. This is done with this expression else firsstNode = new ListNode<>(insertItem, firstNode);. The third and fourth methods are removeFromFront and removeFromBack respectively. Both methods have similar functions and working details but were repeated for the clarity of the reader.
The method returns a type of generic. The method checks if the list is empty and throws exceptions. The methods declare a new variable of type generic and assign firstNode.data to it. The code checks if the first and last node is the same both nodes should be null. if it is not empty firstNode.nextNode should be assigned firstNode and return the remove removeItem.
The fifth method checks if the list is not empty and prints the elements of the list. The last method is a predicate method to check if the list is empty. This method returns true or false.

Implementing List Test

ListTest is a class that allows us to test and debug our code by creating an object of the List generic class. Use the object of the class to perform different tasks and actions. The class serves as the entry point of our code with the help of the main method.

import excepton.EmptyListException;
public class ListTest {
public static void main(String[] args) {
List<Integer> list = new List<>();
list.addToTheFront(0);
list.print();
list.addToTheFront(2);
list.print();
list.addToTheBack(-1);
list.print();
list.addToTheBack(1);
list.print();
try{
int removedItem = list.removeFromFront();
System.out.printf("%n%d removed%n", removedItem);
list.print();
removedItem = list.removeFromFront();
System.out.printf("%n%d removed%n", removedItem);
list.print();
removedItem = list.removeFromBack();
System.out.printf("%n%d removed%n", removedItem);
list.print();
removedItem = list.removeFromBack();
System.out.printf("%n%d removed%n", removedItem);
list.print();
} catch (EmptyListException e) {
e.printStackTrace();
}
}
}

ListTest main method create List object from the generic List class. And start by adding elements to the list by using a list object to call the two list.addToTheFront(); and list.addToTheBack(); which was explain above. Using list.print() to print space between the list element. The method contains a try-and-catch block that declares int variable removeItem. list.removeFromFront() and list.removeFromBack() to remove the item from front and back. System.out.printf("%n%d removed%n", removedItem) the method uses string formatting in printing out the result. catch (EmptyListException e) { e.printStackTrace();}. This method used customized exceptions to catch the error or abnormality of the system.

Stack Data Structure

Stack is a sequence data collection that uses the LIFO principle in the manipulation of data. LIFO means Last In First Out, this process gives higher priority to the last element. This is also called FILO(First In Last Out) approach. A stack is an abstract data type structure that only allows a way to add and removing of elements from the stack.

Stack Basic operation

Push() - Adding or storing elements in the collection.
Pop() - removing or de-initializing elements in the collection.
Peek() - checking the last element in the collection.
isFull()- check if the stack is full.
isEmpty() - check if the stack is empty.
Implementing Stack Data Structure
In this tutorial, stack implementation will be explained in two major object-oriented ways. There are inheritance and composition.

Implementing Stack Data Structure with Inheritance

import excepton.EmptyListException;
public class StackInheritance<T> extends List<T> {
public StackInheritance() {
super("stack");
}
public StackInheritance(String listName) {
super(listName);
}
public void push(T insertItem){
super.addToTheBack(insertItem);
}
public T pop() throws EmptyListException {
return super.removeFromFront();
}
@Override
public void print() {
super.print();
}
@Override
boolean isEmpty() {
return super.isEmpty();
}
}

From the above code, import exceptions.EmptyListException. This declaration is used to import from our customise exception class. Class StackInheritance<T> extends List<T> was created to inherit from the List class. This class inherit two constructors from the parent. The two constructors are the no-argument constructor and the argument constructor. The no-argument constructor public StackInheritance() {super("stack");} take a string "stack". Also, the One argument constructor takes in one argument of String listName. The class has four methods. Two of these methods are declared, and the others are overridden. The declared methods push and pop are called the superclass method addToTheBack and removeFromTheBack. The overridden method print() and isEmpty() also call the superclass method print() and isEmpty() .

Stack Inheritance Test

import excepton.EmptyListException;
public class StackInheritanceTest {
public static void main(String[] args) {
StackInheritance<Integer> stack = new StackInheritance<>();
stack.push(-1);
stack.print();
stack.push(0);
stack.print();
stack.push(5);
stack.print();
stack.push(1);
stack.print();
try
{
int removedItem;
while (true)
{
removedItem = stack.pop(); // use pop method
System.out.printf("%n%d popped%n", removedItem);
stack.print();
}
}
catch (EmptyListException emptyListException)
{
emptyListException.printStackTrace();
}
}
}

ListTest class import exception from the customise exception class. The class has a main method that serves as the entry point of the program to the JVM. the class create a new stack object from the stackInherientance class. The object type is an Integer type wrapper, which represents an object integer.
The stack object call method from StackInherientance, is push() and print(). The method contains a try-and-catch block that declares int variable removeItem. removedItem = stack.pop(); the method assigns element of stack.pop() to removeItem variable

Stack Implementation with composition

import excepton.EmptyListException;
public class StackComposition<T>{
private final List<T> stackList;
public StackComposition() {
stackList = new List<T>("stack");
}
public void push(T object)
{
stackList.addToTheFront(object);
}
public T pop() throws EmptyListException
{
return stackList.removeFromFront();
}
public boolean isEmpty()
{
return stackList.isEmpty();
}
public void print()
{
stackList.print();
}
}

From the above code, import exceptions.EmptyListException. This declaration was used to import from our customise exception class. Class StackComposition<T> compose List<T> class. This class inherit a constructor which is a no-argument constructor. The no-argument constructor public StackComposition() {super("stack");} takes a string stack. The class has four declared methods. The declared method push and pop both call methods from List addToTheBack and removeFromTheBack . The method print() and isEmpty() also call the method print() and isEmpty() of the List class respectively.

Stack Inheritance Test

import excepton.EmptyListException;
public class StackInheritanceTest {
public static void main(String[] args) {
StackInheritance<Integer> stack = new StackInheritance<>();
stack.push(-1);
stack.print();
stack.push(0);
stack.print();
stack.push(5);
stack.print();
stack.push(1);
stack.print();
try
{
int removedItem;
while (true)
{
removedItem = stack.pop(); // use pop method
System.out.printf("%n%d popped%n", removedItem);
stack.print();
}
}
catch (EmptyListException emptyListException)
{
emptyListException.printStackTrace();
}
}
}

StackInheritanceTest class imports exception from the customise exception class. The class has a main method that serves as the entry point of the program to the JVM. the class create a new stack object from the stackComposition class. The object type is an Integer type wrapper, which represents an object integer.
The stack object call method from StackComposition, is push() and print(). The method contains a try and catch block that declares int variable removeItem. removedItem = stack.pop(); the method assigns element of stack.pop() to removeItem variable

Queue Data Structure

Queue collection is a data structure that uses the FIFO(first-in-first-out) principle in collecting, arranging and organising data. This principle only allows the addition of elements from the back of the data structure. And the removal of elements in the data structure is only possible from the front. Similarly, the first element that enters the queue is the first element that leaves the queue.

Basic Operations of Queue

A queue is an abstract data structure that allows the following operations:
Enqueue: Add an element or object to the end of the queue
Dequeue: Remove or delete an element from the front of the queue
IsEmpty: Check if the queue is empty
IsFull: Check if the queue is full
Peek: Get the value of the front of the queue without deleting the element

Implementing Queue

import excepton.EmptyListException;
public class Queue<T> {
private final List<T> queueList;
public Queue()
{
queueList = new List<T>("queue");
}
public void enqueue(T object){
queueList.addToTheBack(object);
}
public T dequeue() throws EmptyListException {
return queueList.removeFromFront();
}
public boolean isEmpty(){
return queueList.isEmpty();
}
public void print(){
queueList.print();
}
}

From the code above, we import exceptions from the custom exception class. The class has a constructor with no argument. The class also declares four methods enqueue(), dequeue(), isEmpty() and print(). Each function has a reference to the List class by calling appropriate methods in the List class. The first method public void enqueue(T object) takes an argument object with Type generic. This method returns void and the method uses List reference and calls the List method addToTheBack(object) which types in argument objects. public T dequeue() throws EmptyListException . This method returns a generic data type and throws an exception to take care of the unplanned event. The method reference object is called the List method removeFromFront(), to remove an element from the front. The other two methods isEmpty() and print() both calling methods of reference object which is, isEmpty() and print() respectively. public boolean isEmpty() this method is called a predicate method. It returns only the boolean type while public void print() returns void.

Queue Testing

import excepton.EmptyListException;
public class QueueTest {
public static void main(String[] args) {
Queue<Integer> queue = new Queue<>();
queue.enqueue(-1);
queue.print();
queue.enqueue(0);
queue.print();
queue.enqueue(1);
queue.print();
queue.enqueue(5);
queue.print();
try
{
int removedItem;
while (true)
{
removedItem = queue.dequeue(); // use dequeue method
System.out.printf("%n%d dequeued%n", removedItem);
queue.print();
}
}
catch (EmptyListException emptyListException)
{
emptyListException.printStackTrace();
}
}
}

QueueTest class imports exceptions from the customized exception class. The class has a main method that serves as the entry point of the program to the JVM. the class creates a new stack object from the Queue class. The object type is an Integer type wrapper, which represents an object integer.
The stack object call method from Queue, which is enqueue() and print(). The enqueue method is used to add elements to the queue. The method contains a try-and-catch block that declares int variable removeItem. removedItem = queue.dequeue; the method assigns an element of queue.dequeue to removeItem variable. The dequeue method is used to remove or dequeue elements from the queue

TREE Data Structure

A tree data structure is a non-linear, recursive two-dimensional collection with special properties. In a tree data structure, each node consists of data and references to a list of nodes. Unlike other data structures e.g. arrays, Linked List, Stack and queues which are linear data and store data sequentially. In a tree data structure, each node element is connected by an edge of another element.

Terminology in Tree Data Structure

Path- The path is a list of sequence nodes along the edge of the tree.
Root - is the first node along any path. Each tree contained only one root at the top
Parent - any that has a node upward is except the root
Child - the node that is connected to a parent node downward is called the chard node
Leaf - A leaf is a node that has no child node
Subtree- subtree is a node that consists of a node descendant.
Visiting- visiting means checking the value of a node when the node is active
Traversing- moving through the node in a particular order.
Level - refers to the generation that a node belongs to.
Keys- The key is a value of the node on which the search operation is based.
The basic operation of Tree
Insert - insert an element in a tree
Search - Searching for an element in a tree
Preorder Traversal- walking through a tree in a pre-order manner
Postorder Traversal- walking through a tree in a post-order manner
Inorder Traversal- walking through a tree in an orderly manner

Implementing a Generic Tree

In the implementation of tree data structure with generic. We separate the implementation into two classes which are TreeNode and Tree. The TreeNode declare only one method which is inserted () method while TreeNode name method for tree basic operations.

Implementing TreeNode class

// class TreeNode definition

class TreeNode<T extends Comparable<T>>{
// package access members
TreeNode<T> leftNode;
T data; // node value
TreeNode<T> rightNode;
// constructor initializes data and makes this a leaf node
public TreeNode(T nodeData){
data = nodeData;
leftNode = rightNode = null; // node has no children
}
// locate insertion point and insert new node; ignore duplicate values
public void insert(T insertValue){
// insert in the left subtree
if (insertValue.compareTo(data) < 0){
// insert new TreeNode
if (leftNode == null){
leftNode = new TreeNode<>(insertValue);
}else { // continue traversing left subtree recursively
leftNode.insert(insertValue);
}
// insert in the right subtree
}else if (insertValue.compareTo(data) > 0){
// insert new TreeNode
if (rightNode == null){
rightNode = new TreeNode<>(insertValue);
}else{ // continue traversing right subtree recursively
rightNode.insert(insertValue);
}
}
}
}

From the code above, class TreeNode was declared as a package identifier. We declare three instance variables two are references to TreeNode. data is the third variable with generic type T. We have a constructor that set the data variable to nodeData. The constructor also set firstNode and lastNode to null with this expression leftNode = rightNode = null;. This means that the node has no children at the setting time. We create a method insert. This method used comparable interface to check if the insert value is data. We check if the leftNode is null. Using the expression to insert new value leftNode = new TreeNode<>(insertValue). And if the leftNode is not null, we continue to element to the left using this leftNode.insert(insertValue).

Implementing Tree class

public class Tree<T extends Comparable<T>> {
   private TreeNode<T> root;
   public Tree()
   {
       root = null;
   }
   // inserting a new node in the search tree
   public void insertNode(T insertValue){
       if (root == null)
           root = new TreeNode<T>(insertValue); // create root node
       else
           root.insert(insertValue); // call the insert method
   }
   //Begin preorder traversal
   public void preorderTraversal()
   {
       preorderHelper(root);
   }
   // recursive method to perform preorder traversal
   private void preorderHelper(TreeNode<T> node) {
       if (node == null)
           return;
       System.out.printf("%s ", node.data); // output node data
       preorderHelper(node.leftNode); // traverse left subtree
       preorderHelper(node.rightNode); // traverse right subtree
   }
   // begin inorder traversal
   public void inorderTraversal()
   {
       inorderHelper(root);
   }
   // recursive method to perform inorder traversal
   private void inorderHelper(TreeNode<T> node)
   {
       if (node == null)
           return;
       inorderHelper(node.leftNode); // traverse left subtree
       System.out.printf("%s ", node.data); // output node data
       inorderHelper(node.rightNode); // traverse right subtree
   }
   // begin postorder traversal
   public void postorderTraversal()
   {
       postorderHelper(root);
   }
   // recursive method to perform postorder traversal
   private void postorderHelper(TreeNode<T> node)
   {
       if (node == null)
           return;
       postorderHelper(node.leftNode); // traverse left subtree
       postorderHelper(node.rightNode); // traverse right subtree
       System.out.printf("%s ", node.data); // output node data
   }
} // end class Tree

We declare a class called Tree which extends a comparable method. We also declare a private variable of root with reference to TreeNode class. We declare a constructor and the Constructor initializes the node with the null value. We declare a method insertNode which take a parameter of insertValue. The parameter is of type generic. The method checks if the root is null on initialization. If the root is null, new TreeNode(insertValue) is assigned to the root. If the root is not null root.insert(insertValue). We declare six other methods three of which make a recursive call to itself, using it to traverse to left or right as the case may be. The three methods are preorderHelper, inorderHelper and postorderHelper. The other three methods are called to reference the last three respectively.

Conclusion

In this article, we learn about how to customize data structure with generic and we discussed the motivation of generic, overloading of generic, memory allocation and control in generic. We also explained different data structure and their implementation in generic. The data structure discussed is as follows List and LinkedList, Stack, Queue and tree. We also Make sure all implementations are explained in detail.
Generic in Java is an important part of the programming language giving you the power to manipulate data of different data types.
The code for this project is available on the GitHub repository.

References

• Java Version 16 documentation on stream -Tutorialspoint -Geeksforgeeks -Section.io
• Java - How to Program 10th Ed - Early Objects Version by Pual Deitel and Harvey Deitel
• Introduction to Java Programming, Comprehensive Version by Y. Daniel Liang of Armstrong Atlantic State University