Announcements  If you need more review of Java…  I have lots of good resources – talk to me  Use “Additional Help” link on webpage  Weekly assignments problems due before class  Remove rust from summer and get back into coding  Problems designed to help learn new material, too

Primitives vs. References Primitive VariablesReference Variables  Variables are copiers holding a value  Assignment copies value  Update variable being assigned only  Aliased with assignments  Now refer to same instance  Both see updates to object  Re-assigning variable does not update aliases

Opening Question  Are array variables primitives or references?

Array Variables  Arrays are special type of object  Array variables are references in own right  Like all variables, must be instantiated before use: int[] bob = new int[30]; Car[] parkingLot = new Car[300];  Array instances aliased by assignments  Array variables like any reference variable

Memory Trace Example int[] data = new int[3]; int[] al = data; for (int i = 0; i < data.length; i++) { data[i] = i * 2; } al[0] = 5; al = new int[1]; al[0] = 7;

Entries Are Variables, Too!  Within an array entries like individual variables int[] bob = new int[30]; Car[] parkingLot = new Car[300];  bob ’s entries would be primitive variables  parkingLot ’s entries refer to instances  Entries initialized like variable when array created  bob ’s entries would all be set to 0  null stored in all entries within parkingLot

Memory Trace Example Car[] data = new Car[3]; Car[] al = data; for (int i = 0; i < data.length; i++) { data[i] = new Car(); } al[0] = new Car(); al = new Car[1]; al[0] = data[0];

Higher-Dimension Arrays  Arrays instantiated with any dimension desired  2-d array is table

Higher-Dimension Arrays  Arrays instantiated with any dimension desired  2-d array is table identifying entries by row & column int[][] pixels = new int[20][100]; pixels[1][45] = 32;

Higher-Dimension Arrays  Arrays instantiated with any dimension desired  2-d array is table identifying entries by row & column int[][] pixels = new int[20][100]; pixels[1][45] = 32;  Entries stored in virtual cube with 3-d array int[][][] bob = new Car[60][4][10]; bob[45][3][7] = new Car();  Rare to use higher dimensions, but are possible

Computer Professor MemoryHuge (> 250GB)Can’t remember 12 names Computing speed Fast (> 2 billion/second)Slow (needs fingers & toes) Takes direction Does exactly as toldLeaves toilet seat up Speed of updates Nearly instantaneousWears t-shirts from 1989 Ability to plan & reason Cannot make plans; No reasoning skills; Lacks common sense Makes (semi-)complicated plans; Good reasoning skills; Lacks common sense Computer vs. Professor

 People are very good at reasoning & analyzing  Slow computing outcome of each potential decision  Considering every possible data combinations hard  Trouble when lots of data available to be examined  Computers can perform simple tasks quickly  Maintain billions of data items within memory  Very good when at performing simple operations  Cannot create plan since lacks concept of future  To do any task, must be given specific instructions People vs. Computers

 Computers best processing huge data sets  Perform specific tasks and output results: College’s students’ transcripts available Compute & print receipt for groceries  Filter & provide few items for user to choose from Personalized recommendations from Amazon Google’s list of matching web pages  Most of year examines how to organize data  How data used determines structures selected Computers Use Collections

How Could We Do This?  Know simple way to organize data: array  Entries refer to another array to create matrices  Many limitations arise when using arrays  Must specify unchangeable size when created Pirate[] rum = new Pirate[1]; Pirate[] h2o = new Pirate[ variable ]; rum = new Pirate[60]; // old rum was lost! h2o = rum; // h20 now alias to rum’s instance  Waste memory requiring maximum size for array /* Each Amazon.com customer uses 400+MB of RAM! */ Rating[] hertz = new Rating[ ];

 Often need to keep values ordered or organized  Alphabetical list of names & phone numbers  Keep list of job bids from smallest to largest  Web pages ordered from most to least important  Desired index’s not just assigned new value  Lose current value stored at that index  Instead, must first shift values within the array Inserting into an Array 12 n e 0

 Still need to keep values ordered or organized  Removing a value presents own set of problems  Could simply assign null, but that creates a gap  Finding start & end of data hard once gaps allowed  Instead, must shift values to fill in all gaps that exist Removing From an Array 0 12 n

Your Turn  Get into your groups and complete activity

For Next Lecture  Reading from AF 7.1 – 7.10 for Friday  What are fields and methods again?  How do they get used?  What is the point of a constructor?  There is weekly assignment problem on Angel  Due before Friday’s lecture (via )  Get back into the swing of writing Java code