You can create a generic class or interface by placing the generic type parameter between the smaller and the greater signs (< and >) after name of the class. The example code below creates a generic class called ListNode. T is the generic type parameter. When you define a generic method or variable, you can place the generic type parameter as you do with a normal data type. This generic type parameter must be the same as the generic type parameter defined with the class.
Example:
class ListNode<T>
{
public T val;
public ListNode<T> next;
public ListNode(T val) { this.val = val; next = null; }
}
When you use a generic class to store value by creating an object and pass the value to it, you should specify the specific data type of the value.
Example:
ListNote<Integer> node=new ListNode<Integer>(123);
You can use many generic type parameters in a class, interface, or methods. You will need to separate the type parameters by comma. See the example below.
Example:
class MyMap<T1,T2>{
private T1 key;
private T2 value;
MyMap(T1 key, T2 value){
this.key=key;
this.value=value;
}
}
Here is the complete LinkedList program that uses the Generics feature.
class ListNode<T>
{
public T val;
public ListNode<T> next;
public ListNode(T val) { this.val =val; next = null; }
}
interface ListOperations<T>
{
void insertNode(T val, int post);
void deleteNode(int post);
void printList();
}
class LinkedList<T> implements ListOperations<T>
{
protected ListNode<T> pfirst; //pfirst points to the first element of the linked list
protected ListNode<T> plast;//plast points to the last element of the linked list
protected int NumItems;
public LinkedList() { pfirst = plast = null; NumItems = 0; }
public void insertNode(T val,int post)
{
ListNode<T> newNode = new ListNode<T>(val);
if (pfirst == null)
{
pfirst = newNode;
plast = newNode;
NumItems++;
}
else
{
if (post == 1)
{
newNode.next = pfirst;
pfirst =newNode;
NumItems++;
}
else if(post>1 && post<=NumItems){
ListNode<T> ta = pfirst;
for (int i = 1; i < post - 1; i++) ta = ta.next;
newNode.next = ta.next;
ta.next = newNode;
NumItems++;
}
else if (post == (NumItems + 1))
{
plast.next = newNode;
plast = newNode;
NumItems++;
}
else
{
System.out.println("Invalid position");
}
}
}
public void deleteNode(int post)
{
if (pfirst == null)
{
System.out.println("Empty list");
return;
}
if (post == 1)
{
if (NumItems == 1)
{
pfirst = null;
plast = null;
}
else
{
ListNode<T> temp = pfirst;
pfirst = pfirst.next;
temp = null;
}
}
else if(post>=1 && post<=NumItems){
ListNode<T> temp, ta;
ta = pfirst;
for(int i=0;i<post-1;i++) ta=ta.next;
temp = ta.next;
ta.next = temp.next;
if (temp.next == null) plast = ta;
temp = null;
}
}
public void printList()
{
ListNode<T> ta = pfirst;
while (ta != null)
{
System.out.println(ta.val);
ta = ta.next;
}
}
public Object[] toArray(){
Object[] arr=new Object[NumItems];
ListNode<T> ta = pfirst;
int i=0;
while (ta != null)
{
arr[i]=ta.val;
ta = ta.next;
i++;
}
return arr;
}
}
public class GenericsJava {
public static void main(String[] args){
//Creating linked list to store text
LinkedList<String> lstString = new LinkedList<String>();
lstString.insertNode("Item1", 1);
lstString.insertNode("Item2", 2);
lstString.insertNode("Item3", 3);
lstString.printList();
//creating linked list to store numbers
LinkedList<Integer> lstNumber = new LinkedList<Integer>();
lstNumber.insertNode(10, 1);
lstNumber.insertNode(13, 2);
lstNumber.insertNode(13, 3);
lstNumber.printList();
}
}
