CS18000: Programming I Final Exam 8 May 8am-10am Lilly Functional Abstraction. Functional Specification. CS18000: Programming I 4/28/2010

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1 CS18000: Programming I Final Exam 8 May 8am-10am Lilly 1105 Review 26 April 2010 Prof. Chris Clifton What You ve (hopefully) Learned Functional Abstraction Recursion Data Abstraction Classes as data types Polymorphism, Inheritance I/O as Abstract Data Concurrent Execution Threads Exception Handling Synchronization Putting it all together: Projects involving Visualization Interfaces Networking 4/28/2010 CS Abstraction: How Programming Scales Functional Abstraction Clear specification of what a function does Don t sweat the details of How something is done Data Abstraction Clear specification of what something is Don t worry about how it is represented Functional Abstraction Function provides a capability We use that capability We don t care how it is done! What do we care about? Input required Output produced 4/28/2010 CS /28/2010 CS Input Number of inputs Data type Permissible values Preconditions Meaning Functional Specification Output Number of outputs (is input changed)? Data type Permissible values Postconditions Meaning Data Abstraction: What makes a data type? Semantics: What is being represented not how What we can do with it Functional abstractions Constraints Can all operations be applied to any object of the type? Can an object be modified? Special values constants 4/28/2010 CS /28/2010 CS Chris Clifton 1

2 Type: (semi)formal Definition Domain: Set of possible values Infinite? Operations Constructor Field Access Methods Accessor, Mutator Invariants Must be true at completion of every operation Type: Implementation as Class Representation Must represent all possible domain values Operations Define Constructor(s) Define methods Invariants Ensure invariants hold after execution of Constructor(s), methods 4/28/2010 CS /28/2010 CS Goal: Separate Abstraction and Implementation Using an Abstract Data Type Know what it is / what it does How isn t important Once defined, can use Doesn t need to be implemented (okay, we can t run the program but we can write a program using it) Java interface Defines an (abstract) data type Javaspeak: reference type Declares Constants (static final variables) Method type, name, and parameters Assumed to be public No constructor(s) Cannot construct an instance of an interface 4/28/2010 CS /28/2010 CS Inheritance Define a new class that extends an existing class 1. Same interface (just like using an Interface) 2. Same methods 3. Same representation Three separate concepts But unfortunately not independent 4/28/2010 CS Extending the Interface Child class must support all features of parent Just like implementing an interface Key difference: Some features already implemented Can be used anywhere parent class can be used Just like an interface Can define new fields and methods But can t be used if the type is the parent class Just like an interface 4/28/2010 CS Chris Clifton 2

3 Extending the Methods (name) Overloading Overriding methods If instance of child, use child (even if you think you have parent) Gate g = new Not(); g.getvalue() will use method from Not class Caveat: Class methods (static) not overridden The one you get depends on the type of object 4/28/2010 CS Two Java methods can share a name Which is which? Signature: Method identified by type (class), name, and type/number of arguments Can do for any method Use sparingly, can be confusing 4/28/2010 CS Using methods from Parent Non-overridden methods easy Just use example in Not Overridden methods accessed using super keyword kind of like this Constructor of parent must be called in constructor Exception: Default parent constructor (no arguments) called automatically before child constructor if you don t Extending the Representation Child class gets all fields of parent class Can access those not declared private or protected Best to access through parent methods Can add new fields Also hide existing by using same name Bad idea works like class methods: Which you get depends on type 4/28/2010 CS /28/2010 CS Putting it all together: Inheritance Three concepts: 1. Inherit the interface 2. Inherit the methods 3. Inherit the representation Use inheritance (extends) when you want all three If you just want interface, use interface/implements If you just want methods, create as an instance variable What if we don t know the class? Class getclass() defined for all objects Every class (automatically) inherits from Class object Class has several useful methods String getname(); Class getsuperclass(); boolean isinstance(object obj); if(g.getclass().getname().equals( UnaryOperator )) power( (UnaryOperator) g ); 4/28/2010 CS /28/2010 CS Chris Clifton 3

4 Caveats Solution: Generics getclass() can be used as a crutch for bad design It should be used only when you can t find a better way Try to compare Class objects, not names In a large system, same name can be used multiple places (class defined inside a class) What if we want to support many types? But only one at a time Generics: Parameter attached to a type Queue<Integer>, Queue<String> instead of IntegerQueue, StringQueue One class (Messy) alternative: ObjectQueue use getclass() to enforce single type 4/28/2010 CS /28/2010 CS Error Handling Handling an Exception 0 Let it crash 1. Preconditions forbid misuse May be okay for programmers But what about user input? 2. Error return code Check if empty If it is, return special value Only works if the return type has room for error codes bs = Stack<boolean>; 3. Exceptions End function without return ing Method can t return normally You won t be able to used a returned result Instead, continue someplace else try { catch So what is an EmptyPop? A type (Class) that extends Exception Stack<boolean> s; try { boolean result = s.pop(); if (result) { else { catch (EmptyPop e) { /* Whatever you want to do if the stack was empty */ 4/28/2010 CS /28/2010 CS Declaring an Exception Generating an Exception To use an exception you must pick one or define your own Can extend Exception without doing anything Gives a unique name for your exception Or can add information Reason it was generated If specific to a class, make it a nested class Stack.EmptyPop interface Stack<E> { public class EmptyPop extends Exception { ; void push (E item); E pop() throws EmptyPop; /* Precondition:!isEmpty(); */ boolean isempty; First, create an instance of the exception May contain error message Sample of the bad input Then throw it public E pop() throws EmptyPop { if (isempty()) throw new EmptyPop(); E result = top; top = rest.top; rest = rest.rest; return result; 4/28/2010 CS /28/2010 CS Chris Clifton 4

5 Do use In unexpected situations No meaningful return value Can t recover/process inside a method Doesn t make sense to continue after failed method call When to use exceptions (guidelines) Don t use Under normal conditions Possible to return something sensible Possible to recover and complete method The next steps are the same regardless of the exception Concurrency: Model program as Tasks Multiple (independent) tasks Each is its own program All run (apparently) simultaneously Programming in Java: Thread class Task becomes an instance of a Thread Number of threads matches number of tasks Need not match number of processors/cores In practice: task is an instance of a class inheriting from thread 4/28/2010 CS /28/2010 CS Running a task: Steps Initialize the task Constructor Method calls Do as little work as possible Start the task Begins concurrent execution Thread has to know what to do (no arguments passed in) Make results available run() cannot return results but instance (and its fields/methods) stays around 4/28/2010 CS What happens t.start() causes two programs to run The program calling t.start() the run() method inside instance t What if we don t have enough processors? Scheduler chooses which to run Periodically stops one to let another run We have little control over what runs 4/28/2010 CS Communicating with Threads So far: Task is initialized, runs, results available Nice model for functions Problem: What if task needs to know things in the middle? Video game: move other objects task needs to know if player has moved Solution: Synchronization Allow concurrency But limit when needed Once account has given one ATM the balance, it doesn t respond to the other until the balance is updated Why isn t this easy? At least when we don t deal with the real world 4/28/2010 CS /28/2010 CS Chris Clifton 5

6 Why is this hard? Synchronization Theory Concurrent system architecture Two things really can happen at once! How do you control which gets to memory? CPU Cache Operations: Read or Write Bus Memory CPU Cache Semaphores Test value and set in one operation if (! semaphore) semaphore=true; Only use resource after testing and setting semaphore Full version: counter if (semaphore > 0) semaphore -= 1; Monitors Protected block of code Can t be executed concurrently Perform critical operations in monitor Don t leave until done Different threads can call the monitor But won t start until previous one done. 4/28/2010 CS /28/2010 CS Doing this in Java: synchronized A synchronized method is a monitor public synchronized void updatebalance (int amount) throws insufficientfunds { if (amount > balance) throw new insufficientfunds(); balance = balance amount; Synchronization is on a per-object basis Can run same method concurrently for different objects Cannot run two synchronized methods concurrently on the same object Also synchronize block of code on object of choice synchronized(semaphore) { Concurrency: Coordination Synchronized blocks others until we are done What if we need another task to finish before we can complete? Example: Waiting for ATM to dispensecash(); Solution: wait(); Used in synchronized code Allows others to use that object but we stop They call notify() or notifyall() when done 4/28/2010 CS /28/2010 CS Wait Method in the Object class Can call wait() from any object, or on any object. Call wait() only in synchronized block for that object Synchronized method of that instance Synchronized(theInstance) { theinstance.wait(); Synchronized static method of class Gives up synchronization Other threads can use that object, even though we are still in synchronized block 4/28/2010 CS Wait Current thread will stop until notify() is called on that object Thread resumes when nobody else is in synchronized block for object Can also interrupt thread that is waiting interrupt() is method in Thread class causes wait() to throws InterruptedException Wait with timeout wait(long milliseconds); wait(long milliseconds, int nanoseconds); Returns on notify OR timeout 4/28/2010 CS Chris Clifton 6

7 Notify Method in the Object class Call notify() only in synchronized block for that object Synchronized method of that instance Synchronized(theInstance) { theinstance.notify(); Synchronized static method of class Otherwise you ll get an IllegalMonitorStateException Waiting thread resumes when notifying thread leaves Synchronized block/method NotifyAll all compete to start Recommendations on use Remember that concurrency / synchronization doesn t hold while you wait Make sure it is okay for someone else to be using / modifying the waiting instance. Make sure the wait is on the right object Synchronized(lock) { wait(); waits for notify on instance of current method, not on lock Make sure you SHOULD be awake Synchronized(lock) { while (! lock.readytocontinue() ) lock.wait(); 4/28/2010 CS /28/2010 CS Common Errors: Concurrency Race conditions Two threads try to change the same data Challenge: May not happen the same way every time! Make sure you use synchronized properly Deadlock/Livelock Thread in a synchronized block calls another synchronized block But that is in use by a thread trying to use the first Sequential execution Over-use of synchronized blocks EntryForm Done Name Number Event-Based Programming (Example: GUIs) Jbutton Done JTextField Name JTextField Number Panel JLabel Name 4/28/2010 CS /28/2010 CS Components with actions Component isn t just about looking good Needs to accept (user) interaction Acting on the component generates Event MouseEvent button press, release; enter object; leave object KeyEvent Key Pressed, Key Released But how do we find out about the event? Event-Based Programming Component has listener waits for event to occur executes method when it does Easy to use simply define appropriate methods But need to be careful Think like concurrent programs 4/28/2010 CS /28/2010 CS Chris Clifton 7

8 ActionListener Using a listener Listen for event When event occurs, method in listener will be executed This method can do whatever is needed ActionListener is an Interface Requires one method: actionperformed(actionevent e); You define class with appropriate actionperformed 4/28/2010 CS Must add listener to the object What events cause action set by the object Listener can also look at the event to decide what action should be Some objects may have multiple listeners JButton text disabled? NoticeEvent name a NoticeEvent name b ActionListener 4/28/2010 CS Question: Are GUIs really concurrent? It seems like things are just happening Similar to concurrent threads Program doesn t end without window finishing Similar to concurrent threads Is it really the same? Exercise: Test and find out Files: Stream of Data Values written sequentially Write the value of name Write the value of address Write the next name Read in same order Read name Read address 4/28/2010 CS /28/2010 CS History First storage devices were tape drives Inherently sequential Does this still make sense? Direct Access Storage Device Can move to anyplace But faster in sequence Why sequentially? 4/28/2010 CS Stream as an Abstraction Many devices good at sequential access Tape drive Telephone line Satellite link Fewer good at truly random access Disks read a block, not individual byte Even then, sequential blocks faster USB flash drive seems random access But designed to give a block of sequential data per request Best if you need the whole thing And then there is concurrent access 4/28/2010 CS Chris Clifton 8

9 (Generic) Stream Operations Buffered I/O open Make ready for reading (at beginning), writing (at beginning or at end) Generally done by a Stream constructor in Java read / write Get data from stream or put data on stream skip Move ahead (or back) some distance in stream Equivalent to reading and throwing away what is read reset Move to beginning of stream flush Make sure data sent to stream Used only when writing close Make sure data sent to stream Stream can t be used without open ing again Others may use stream Stream reads an entire block (or more) Saves it in memory When you ask for the next byte, you get it When you ask for a byte that isn t there, get the next block Dramatic performance improvements But not always invisible to program 4/28/2010 CS /28/2010 CS Network I/O Network connection is a stream One end writes data (OutputStream) Other end reads data (InputStream) Some network connections have both Can use higher-level abstractions Scanner Printwriter Network I/O: Socket Endpoint of network stream Analogous to File class (but not quite) FileInputStream constructed using a File Socket has getinputstream(), getoutputstream() new Socket( 80); Creates a (2-way) connection to given address 4/28/2010 CS /28/2010 CS Network I/O: Server What if we don t know who will connect? E.g., web server Solution: Listener new ServerSocket(80); // Listen on port 80 accept() method waits returns a Socket when someone connects Linked Data Structures Array: List of pointers to contained objects lset: aset: n: abc i: 3 n: hjk i: 2 n: dc i: 3 n: abc i: 25 n: def i: 7 Alternative: Object has pointer to next contained object! 4/28/2010 CS /28/2010 CS Chris Clifton 9

10 What goes in a Linked Data Structure? Contents of one cell Instance variables for object Pointer to an object Data Pointer to next object Instance of your own class Data Null value for this variable means end Can even have multiple next objects Data Data Recursion Idea: break a problem down into small, similar sub-problems Write a method to solve first Call that method to solve next Sounds similar to a loop Same basic effect But often an easier way to conceptualize the solution 4/28/2010 CS /28/2010 CS Save what is left to do Call (recursive) method public static int factorial( int n ) { if ( n==1 ) return 1; else return n * factorial( n-1 ); factorial(3) Visualizing Recursion: Call Stack factorial(3) return 3 * factorial(2) factorial(2) return 2 * factorial(1) factorial(1) return Exceptions as a Programming Technique Difficulties with Dynamic Data Structures: Removing self Solution 1: Violate Encapsulation Operate on your child Move child to self Solution 2: Exceptions LinkedSet s = item5 item4 item2 item1 item3 4/28/2010 CS /28/2010 CS Idea: Inform Parent Tell parent to eliminate us Parent has called us with recursive call Return new head void remove(item); Should it be LinkedSet<E> remove(e item);? Does this make sense if nothing changed? What if we want to return (for example) the removed item? Solution: throw exception Exception has information parent needs to know 4/28/2010 CS Loops Off-by-one Infinite Zero Arrays Out-of-bounds Uninitialized objects Scope int i=0; void func() { for (int i=0; i<5; i++) i+=1; // What is i? Testing and Debugging: Common Errors Equivalence Testing == tests if same object.equals() tests if objects same Failure to initialize Null pointer exception Numeric Errors Precision Overflow/Underflow Casting 4/28/2010 CS Chris Clifton 10

11 Types of Testing Black-Box Tests Black-Box Testing Based purely on program specifications White-Box Testing Exercise the code (as written) Regression Testing Re-run tests when changes made 4/28/2010 CS Boundary Value Testing If known ranges of inputs, test at ends of ranges Both inside and outside of bounds Equivalence Partitioning Choose one (or more) from a group of functionally similar values All-Pairs Testing Test different pairs of input values Fuzz Testing Random choices of (potentially unlikely) values 4/28/2010 CS White-Box Tests Branch Testing Make sure every statement is exercised Ensure each side of a decision (if-then or loop) is taken Loop Testing Like boundary case testing for loops All-Paths testing Make sure every path through the program is tested Gets to be very expensive Exponential growth in number of cases Testing Scope System Testing Test the entire program And the environment the system runs in Unit Testing Test individual modules Use skeletons / stub procedures, Driver code Integration Testing Test interfaces between units 4/28/2010 CS /28/2010 CS Package: Namespace Class names are unique in a package Different packages can use the same class names real class name is package.class datastructures.set<recipe>.add(r); Within a package, don t need package name Assumed to refer to current package 4/28/2010 CS Declaring a package Put package name at top of every file package datastructures; Naming Conventions All lowercase Companies use domain name in reverse Use _ in place of things that don t work or after reserved words, e.g. int_. has no special meaning java.awt and java.awt.event separate packages Just a convention that they have something in common 4/28/2010 CS Chris Clifton 11