Updates. Office B116E Office Hours : Friday 1 p.m. 2 p.m. Also by appointment Office Hours Purpose:

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Rose Terry
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1 CS 180 Amit Gupta

2 Updates Office B116E Office Hours : Friday 1 p.m. 2 p.m. Also by appointment gupta75@purdue.edu Office Hours Purpose: Course Material and Recitation No project questions

3 Quizzes I will give quizzes on the material covered in recitation and lecture at my discretion. Account for 5% for your grades. Mostly to get feedback about the material covered in class. Legitimate Absence: With proof, you will be assigned the average score from your section.

4 Project 1 Should be posted by this weekend on the website. Will be available to work from coming Monday. Deals with encoding and decoding of secret messages. More details will be available next week.

5 Review Life of a class Object Multiple Instantiation of a class Concurrency/ Multithreading Examples of Multithreaded Code

6 Memory How are variable stored in memory? Is there a difference in how primitive and class types are stored?

7 Life of a class object In Java, programmers care about two memory areas - Stack and Heap. Heap - Memory area where all Java objects reside Stack - Memory area where all the method invocations and local variables reside.

8 Instantiation of Class Following is a Dog class. public class Dog(){ private String name; private String type; public void setname(string name) this.name = name; } public String getname(){ return name; } }

9 Instantiation of a class Three steps to instantiate a class object Declare a reference variable mydog o Dog mydog = new Dog(); o Above statement allocates space for reference variable mydog on stack memory area. Create a Java Object Dog mydog = new Dog(); Above statement allocates space for a new Dog object (instance variables) on the heap.

10 Instantiation of a class Assign the new Dog object to the mydog reference variable Dog mydog = new Dog() ; Reference variable on Stack Dog Object on heap

11 Multiple Instantiation of a class Book b = new Book(); Book c = new Book(); Two different class objects are created on heap with each of them having their own instance variables. Reference variables b and c are allocated on Stack memory area. Heap

12 Examples b = c; Reference variable now points the object pointed by reference variable c. Book Object 1 becomes eligible for garbage collection as there are no reference variables point to this object.

13 Concurrency Motivation: Modern computer facilitates concurrency through multiple processors. Rather than increase clock speed or operations per second of an individual processor, we simply add more processors. How does this change in hardware design affect the way we program?

14 Concurrency When tasks execute in sequence, they are executed one after the other. When tasks execute in parallel, they are executed simultaneously. By writing code that can execute multiple tasks in parallel, we gain a significant advantage by having multiple processors. Sequential tasks are unable to use the processing power available.

15 Concurrency/ Multithreading Java has multithreading built into the language Concurrency is needed so as to perform multiple things at same point of time. A thread is a separate thread of execution which means a separate call stack for every thread.

16 Separate Call Stacks?? Every thread has its own call stack. Thus, there are multiple stacks in case of multiple threads. In reality, only a true multiprocessor system can do more than one thing at a time. A uniprocessor system can run only a single thread at a point of time. Multiple call stacks give the appearance of multiple things happening at the same time as execution can move back between different stacks very rapidly.

17 Thread class in Java Thread class present in java.lang package. Important Methods: o void join() - Wait for this thread to die o void start() - Causes this thread to begin execution. JVM calls the run method of this thread. o static void sleep(int millis) - Causes the currently executing thread to sleep for specified amount of time.

18 Using a Threaded Class Instantiate a new instance of your class. Call the start() method This method calls the run() method in your class. Why does start() call run()? Why not call() run directly?

19 Starting a Thread in Java

20 Program Output The result of program depend on JVM thread scheduler switches between the main thread and myrunnable thread. Running the program will give different results every time. Output of program : o top o' the stack back in main OR o back in main o top o' the stack

21 Euclidean distance Example

22 Multithreaded Version

23 Euclidean Distance:Multithreaded Version Problem is divided into two subproblems. Each of the subproblem is solved by a separate thread. On a multiprocessor machine, these threads will run at same time and hence time taken to solve the problem will be reduced to half. But remember, on a uniprocessor machine there would not be much difference as atmost a single thread can run at a point of time.

24 Questions

Programming II (CS300)

1 Programming II (CS300) Chapter 03: Creating Classes MOUNA KACEM mouna@cs.wisc.edu Spring 2019 Creating Classes 2 Constructors and Object Initialization Static versus non-static fields/methods Encapsulation