Java程序辅导

Compsci 201
Java and Simulation
Owen Astrachan
ola@cs.duke.edu
September 7, 2018
9/7/18 Compsci 201, Fall 2018, Java and Simulation 1
Reminder about class links
• Compsci/Internet: http://bit.ly/201fall18
• Sakai: https://sakai.duke.edu/portal/site/201fall18
9/7/18 Compsci 201, Fall 2018, Java and Simulation 2
D is for …
• Debugging
• A key skill in making your programs run
• Digital
• All about the 1s and 0s
9/1/17 Compsci 201, Fall 2017, Java Basics 3
PFF
• How to succeed in finishing and submitting APTs, 
and Assignment P0
• Java Knowledge
• Git, Submission knowledge
• Applying these concepts to P1: Nbody simulation
• Solving a problem that requires computational
simulation, not only mathematical analysis
9/7/18 Compsci 201, Fall 2018, Java and Simulation 4
Assignment P0 and P1
• Simple Java class with driver program
• How is a program run? What is main?
• See documentation and create methods
• Example of 201 work-flow
• GitHub (and classroom)
• Using SSH and GitHub together
• Submitting via Gradescope
9/7/18 Compsci 201, Fall 2018, Java and Simulation 5
Class encapsulates state and behavior
• Class is a template, object has characteristics
• Dogs have fur, speed, temperament, size, …
• Typically we don’t use examples like this, but they 
can help build intuition and understanding
• Class dog, retriever extends dog, method bark()
9/7/18 Compsci 201, Fall 2018, Java and Simulation 6
Classes in Java
• Define class Foo in 
Foo.java
• Create object by 
calling new Foo(..)
• Access object by 
calling methods: 
obj.doSomething()
• Some methods return 
values, use them!
9/7/18 Compsci 201, Fall 2018, Java and Simulation 7
State
Constructor
Methods
(behavior)
Classes in Java
• State: instance 
variables: private
• Constructors: initialize 
instance variables and 
other state
• Methods: functions 
aka behavior
• Documentation: 
Javadoc and other 
comments
9/7/18 Compsci 201, Fall 2018, Java and Simulation 8
Overloaded Methods/Constructors
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• Same name, different parameters
• Constructor has no return type: class name
• Methods must have return type, can be void
• Which one called?
• Parameters determine
• Can one call other?
• Yes: parameters
Constructor
• Same name as class
• No return type
• Overload with different parameters
• Each should initialize all instance variables
• Factor out common code into helper method if 
lengthy
• Can call another constructor using this(…) 
9/7/18 Compsci 201, Fall 2018, Java and Simulation 10
What is this?
• An object instance refers to itself
• Method or constructor: object references itself
• Every reference to an instance variable myVar
could be written as this.myVar
• Code for an object to pass itself:
• callMethod(this,”hello”);
• Constructor can call other constructor
• this(“hello”);
9/7/18 Compsci 201, Fall 2018, Java and Simulation 11
Project P1
• See course website for details
• Not quite this, but …
• Read assignment before starting to code
• Think before fingers on keys
• Use Piazza and Helper Hours
9/7/18 Compsci 201, Fall 2018, Java and Simulation 12
What is a static method?
• Objects are instances of a class
• Thus objects have instance variables
• Typically private, accessed in methods
• Static method belongs to class, not object
• No instance variables
• Accesses static variables and methods
• More on this later, for now? …
9/7/18 Compsci 201, Fall 2018, Java and Simulation 13
Projects in 201: Briefly
• Start with GitHub classroom, fork repository
• Now you have starter code and a place to store 
your work. 
• Clone repo to your machine: you need Git for this
• You use GitHub. Only ola uses GitHub classroom
• You manage your code/projects on GitHub
• Make changes, complete project
• Push changes frequently using Git (what?)
9/7/18 Compsci 201, Fall 2018, Java and Simulation 14
What is Git?
• Git is a free and open source distributed version 
control system designed to handle everything 
from small to very large projects with speed and 
efficiency. https://git-scm.com/
• In teams, huge win. Individually, huge win
• Git is complicated when it doesn’t work, lots of 
commands
• Git relies on SSH to be secure
9/7/18 Compsci 201, Fall 2018, Java and Simulation 15
Submission and Grading
• After completing assignment (or before)
• Push code using Git 
• Submit code on Gradescope. Wait a minute
• Connect to GitHub repository, grade
• Can always upload Zip file
• Submit before deadline!
• Autograder runs and tells you what happened!
• Don’t submit until you think code works!
9/7/18 Compsci 201, Fall 2018, Java and Simulation 16
Working on P0 (Redux) and P1
• Make sure you can start and finish before coding
• What does that mean?
9/7/18 Compsci 201, Fall 2018, Java and Simulation 17
WOTO (redux)
http://bit.ly/201fall18-sept5-2
9/7/18 Compsci 201, Fall 2018, Java and Simulation 18
Charles Isbell
9/7/18 Compsci 201, Fall 2018, Java and Simulation 19
http://www.pbs.org/newshour/bb/online-graduate-programs-offer-
degrees-significant-savings/
For me, the differences are simple to state: Computationalists grok that 
models, languages and machines are equivalent.
• Context matters: Threads
• Machine learning researcher
• Systems that interact intelligently 
with many other intelligence agents
• Exec. Assoc. Dean @ Georgia Tech 
• Rethinking education: Online
Masters in Computer Science
N-Body Simulation
• Class Body represents Celestial Body
• Planet, Sun, Asteroid
• Models an object in 2D space, not 3D
• Position, Velocity, Mass, Image for display
• Class NBody drives the simulation
• Compute gravitational forces: physics
• Time-step simulation: compute all forces, 
update ,display
9/7/18 Compsci 201, Fall 2018, Java and Simulation 20
Body.java and class Body
• Illustrates stand Java idioms
• Constructors, Methods, Instance Variables
• State is private: six instance variables
• myXPos, … using my convention - this object
• Initialized by constructors
• Methods are public
• Include accessors aka getters for state
• No setters, cannot change myXPos other than 
via the update method, a mutator
9/7/18 Compsci 201, Fall 2018, Java and Simulation 21
NBody Project and Body.java
• No class provided, but client code uses Body
• Eclipse and Java don’t like this 
• Instance variables, constructors, getter methods
• A Body has x, y, xv, yv, mass, image file
• Some of these are used by client code
• The image file only used by Body.draw()
• Getter methods getX(), getXVel(), etc.
9/7/18 Compsci 201, Fall 2018, Java and Simulation 22
WOTO
http://bit.ly/201fall18-sept5-2
9/7/18 Compsci 201, Fall 2018, Java and Simulation 23