School of Engineering and Computer Science, Victoria University of Wellington COMP 103 Linked Structures

2 RECAP-TODAY RECAP  Reviewing collections – is there a better implementation ? TODAY  A whole new way to implement Collections: Linked Structures

3 Better Implementations for Collections  We’ve looked at various common collections: lists, stacks, queues, sets, bags, maps.  When and how to use them.  Implementations of collection, arrays as underlying data structure to ArrayList, ArrayQueue, SortedArrayBag, ArrayPriorityQueue….  Arrays have some inherent limitations for implementing collections:  Need contiguous chunks of memory  Inserting and removing may mean moving everything up or down  Better implementation ?  Faster insertion and deletion: Linked structures….  Faster lookup: hashing  Other kinds of collections: trees and graphs

4 How can we insert faster ?  Fast lookup in array ⇒ items must be sorted  Arrays stored in contiguous memory. ⇒ inserting new items will be slow  You can’t insert fast with an sorted array!  To make insert faster, we would need each item to be in its own chunk of memory  But, how do we keep track of the order ? BAJHC MXP K B A J H C M X P K

5 Linked Structures  Put each value in an object with a field for a link to the next  Traverse the list by following the links  Insert by changing links. No need to shift everything up.  Remove by changing links No need to shift things down. B A J H C M X P K

6 Linked List Structures  Can think of this as Nodes inside Nodes:  Or as Nodes “pointing” to Nodes:  Each node contains a reference to the next node  reference = memory location of or “pointer” to object  Can view a node in two ways:  an object containing two fields  the head of a linked list of values JMACXP JMA C XP shorthand for "null": end of the list

7 Memory allocation  What are references/pointers ?  A pointer/reference is an address of a chunk of memory, where the data can be stored.  How do you get this memory allocated ?  You’ve been doing it since the start of Comp102, using new: creating an object allocates some chunk memory for the object. new returns the address of the chunk of memory copying the address does not copy the chunk of memory.  Memory should be recycled after use:  In Java, you don’t have to worry about freeing memory. The garbage collector automatically frees up any memory chunks that no longer have anything pointing/referring to them.  In languages without a garbage collector (eg C, C++), you have to do this yourself. OK to ignore in small programs, but requires special care in large programs!

8 A Linked Node class: public class LinkedNode { private E value; private LinkedNode next; public LinkedNode(E item, LinkedNode nextNode) { value = item; next = nextNode; } public E get() { return value; } public LinkedNode next() { return next; } public void set(E item) { value = item; } public void setNext(LinkedNode nextNode) { next = nextNode; } } thing

9 Using Linked Nodes LinkedNode colours = new LinkedNode (“red”, null); colours.setNext(new LinkedNode (“blue”, null)); colours = new LinkedNode (“green”, colours); System.out.printf(“1st: %s ”, colours.get() ); System.out.printf(“2nd: %s ”, colours.next().get() ); System.out.printf(“3rd: %s ”, colours.next().next().get() );  1 st : green 2 nd : red 3 rd : blue colours “red”“blue”“green”

10 Using Linked Nodes  Remove the second node: colours.setNext( colours.next().next() );  Copy colours, then remove first node LinkedNode copy = colours; colours = colours.next(); “red” colours “green”“blue” copy No references. Garbage collector will get it eventually

11 Using Linked Nodes  Making a big list: LinkedNode squares = null; for (int i = 1; i (i*i, squares);  This builds the list backwards -  Exercise: Build the list working forwards squares /

12 Iterating through a linked list LinkedNode rest = squares; while (rest != null){ System.out.printf("%6d \n", rest.get()); rest = rest.next(); } or for (LinkedNode rest=squares; rest!=null; rest=rest.next()){ System.out.printf("%6d \n", rest.get()); } or we could do it with recursion squares rest /

13 T wo ways to print a linked list /** Print the values in the list starting at a node */ public void printList(LinkedNode list){ if (list == null) return; System.out.printf("%s, ", list.get()); printList(list.next()); } or public void printList(LinkedNode list){ for (LinkedNode rest=list; rest!=null; rest=rest.next() ) System.out.printf("%s, ", rest.get()); } “dog”“cat” “cow” list recursive iterative as per previous slide