CODEWITHSUNDEEP
javagenerics
gfppaste
gfppaste

Reputation: 1121

Can someone explain basic Java Generics to me?

I have a problem set for my programming class which I'm working on. The problem set starts with:

Given the following definition of a linked list class:

1 public class Node<T> {
2 public T data;
3 public Node<T> next;
4 public Node(T data, Node<T> next) {
5     this.data = data; this.next = next;
6     }
7 }
8
9 public class LinkedList<T> {
10 private Node<T> front; 
11 ...
12 }

Now, I can't quite figure out what "Node< T >" means (line 1)... I found some literature on wildcards (I.E. List), but this is still not making much sense to me. I was wondering if anyone could explain why wildcards are used (specifically, in this case), or, if this isn't an example of a wildcard, I was wondering if someone could tell me what it is? The book I have ("Data Structures Outside In: Java") doesn't mention anything helpful, neither does the oracle tutorial site I was on (link below, in case anyone wants to see what oracle had to say)

(I was on this website: http://download.oracle.com/javase/tutorial/extra/generics/wildcards.html but I'm still having trouble understanding what wildcards are used for, and figuring out if this is an example of a wildcard)

thank you for your help!

Upvotes: 1

Views: 389

Answers (4)

bdparrish
bdparrish

Reputation: 2764

To answer your wildcard piece of the question...

Wildcards are constraints against the generic type. There are three types of wildcards:

<?>

<? extends T> or <? extends IRunnable>

<? super T>

A wildcard puts a restraint on what can go into either the class or a specific method in the generic. <?> means that any object can be used in the generic class or wherever this constraint is defined. <? extends T> means that the class coming into the generic must inherit from T or implement the interface T. <? extends IRunnable> means that the class coming into the generic must implement the IRunnable interface at some point in its hierarchy. Finally, <? super T> means that the incoming class to the generic must be of a super type of T (i.e., a parent in the hierarchy of T).

Upvotes: 1

joeslice
joeslice

Reputation: 3454

The power of generics in java -- you can now use this linked list in this way:

LinkedList<Pizza> pizzaList = new LinkedList<Pizza>();
// ...

Node<Pizza> p = pizzaList.front;
while (p != null) {
  eat(p.data);
  p = p.next;
}

Basically the generics take care of casting for you, where you otherwise would have been writing (Pizza)p.data in your code.

Upvotes: 2

Ryan Amos
Ryan Amos

Reputation: 5452

Wildcards, technically generics, can be any object. T can be String, Comparable, JList. The advantage is that everything in the list is the same type, so everything that comes out will be the same.

For example, I can make a Node and when I access Node.data, it will be in a String form, which means that I don't have to cast: String data = (String)node.data;

Upvotes: 1

Bozho
Bozho

Reputation: 597096

First, the Node represents a node of the linked list. With a singly-linked list each node holds a value and a reference to the next node in the list.

Then, the generics (T is generic type, not a wildcard) define that the type of value of each Node must be the same as the type defined for the List.

I'd recommend reading about java generics

Upvotes: 3

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