LinkList Drawings

LinkList maintains a linked-list of simple graphics commands, and displays the shapes during the paint method.

Use the [Add] button to add a new command at the end of the list.  Use [Undo] to remove the last command from the end of the list.  The [List] button displays all the commands.

Explanation

Java has no extra construct for creating "record" types.  Instead, a group of "fields" can be encapsulated in an inner-class (Node).  

Java has no pointers like C++.  Java has references (similar to pointers), but you cannot manipulate them directly - e.g. you cannot do arithmetic on the reference values.  The references can only be used to retrieve values.  

So a linked-list is formed by creating a class called Node, instantiating lots of Node objects via the new command, and linking these together using the next reference variable.

Linked-lists seem complicated to program, and might not seem more useful than an array (which is easier to program).  But it is possible for a linked-list Node to contain any quantity or any type of data, whereas arrays are type-specific and occupy a fixed size.  Thus linked-lists provide more flexibility than arrays.

Important Commands

class Node
A Node is declared in an inner-class - that is a non-public class defined inside the current class.  It is very simple, containing only 2 variables (properties) :   command and next.

Node next = null;
The next variable is a reference (pointer) connecting one Node to the next Node.

Node head = null; Node tail = null;
head and tail will point to the first and last nodes in the linked-list.  At the beginning, the point nowhere, as no Nodes exist yet.

tail.next = new Node();
Create a new Node, and attach it to the end of the linked-list.

head.command = "s 100 150 100";
Copies a String into the command field in the first node in the list.

Node temp = head;
Make a new temporary pointer, and assign it to point at the first node in the linked-list.

temp = temp.next;
Move temp to point at the next node in the linked-list.

while (temp != null)
Loop until temp goes past the end of the linked-list.

Comments

This program is not fully error-trapped.  A NullPointerException will occur if  temp.command  is accessed when temp is pointing at null.

The deleteTail method avoids NullPointerExceptions by never deleting if there is only one Node in the linked-list.

A "real" program needs to allow inserting and deleting at any position in the list.  

Linked-lists exist in RAM, and disappear when the program stops running.  It would be useful to save the Linked-List in a file.

Although this might not seem more useful than an array, it is possible for a linked-list Node to contain any quantity or type of data, whereas arrays are type-specific and occupy a fixed size.