Java Review Session

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Java Review Session

• CS 402
• Melissa Carroll
• Berk Kapicioglu
• Most slides by Matt Hibbs

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First Off

• Slides available on course website
• This presentation doesnt get into very fine
  detail, so for more info check out Java in a
  Nutshell, which is available online to Princeton
  students for free. You can search/find it at
• catalog.princeton.edu

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Why Java?

• Cross Platform
• Bytecode and the JVM run on most platforms
• Security
• JVM can restrict access to local machine
• Safety
• Garbage Collection (fewer memory leaks)
• Ease of Coding
• Many available packages

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Java Basics

• http//java.sun.com (SDK)
• Development Environments
• Eclipse
• Emacs
• Notepad and Command Line

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Hello World

• In C
• int main (int argc, char argv)
• printf(Hello World!\n)
• return 0
• / end main /

• In Java
• public class HelloWorld
• public static void main (String args)
• System.out.println(Hello World!)
• / end main /
• //end HelloWorld

• Things to notice
• Similar syntax
• Classes
• System.out.println()
• main is void public static void main

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How Java Differs from C

• Exclusively Object-Oriented Language
• EVERYTHING must live in a class (mostly)
• No Global Variables
• No Pointers
• Also, no -gt or operators
• Blessing and a Curse
• Garbage Collection
• Loss of Power

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How Java Differs from C

• No Preprocessor (no include, define, etc.)
• No goto statement
• Declare/Define Variables Methods anywhere
  (within a class)
• No struct, enum, or typedef
• Cant overload Operators
• Use new rather than malloc()

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Java Data Types

• Primitives
• boolean, char, byte, short, int, long, float,
  double
• char is Unicode (16 bits)
• boolean is true/false (not 1/0)
• int i 1
• if (i) //BAD
• if (i 1) //GOOD

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Java Data Types

• Primitives
• Type conversions
• Cant convert boolean
• boolean b false
• int i b //BAD
• int j b ? 1 0 //GOOD
• Converting up is automatic
• Converting down requires a cast
• double f 37.5 // it used to be float but //
  gave compiler error
• int i f //BAD
• int j (int) f //GOOD (truncates)

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Java Data Types

• References (by reference vs. by value)
• Everything thats not primitive (classes and
  arrays)
• Think of these as hidden pointers
• Foo a new Foo()
• Foo b a
• a and b now reference the same object

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Java Arrays

• Mostly what you would expect
• String textLines
• String textLines2 new String15
• textLines23 Hi, how are you today?
• int fib 1, 1, 2, 3, 5, 8, 13, 21
• System.out.print(fib5) // 8
• textLines is just a Reference, no array was
  actually made
• textLines2 is an un-initialized array
• fib is an initialized array

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Java Development
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Java Platform

• Lots of built-in objects and functions for
  various purposes
• Graphics (java.awt)
• Math Functions (java.math)
• Networking (java.net)
• Databases (java.sql)
• Well mostly use the platform for data structures
  (java.util) and I/O (java.io)

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Java Packages

• Java organizes classes into larger groups called
  packages
• Both the Java Platform and any classes that you
  write are organized into packages
• For code that you write, dont worry so much
  about what package youre in (the default is fine)

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Using the Java Platform

• http//java.sun.com/j2se/1.4.2/docs/api/index.html
  
• You can always use the full package path name to
  access classes
• java.io.FileReader fr new java.io.FileReader
  (test.txt)
• But thats pretty annoying, so you can use import
  statements at the beginning of your code to avoid
  this
• import java.io.FileReader
• //...
• FileReader fr new FileReader(test.txt)
• //...

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Object Orientation
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Java - Classes

• Similar to C classes in many ways
• Heres a simple Java class that well break down
• public class Circle
• private float radius
• public static final double PI 3.14159
• public Circle(float r)
• radius r
• public float area()
• return PI radius radius

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Java - Classes

• Naming
• public class Circle
• The basic form is
• ltmodifiersgt class ltnamegt
• Typically need an access modifier
• public, protected, private
• Can have additional modifiers
• static, final, abstract
• If a Class is public, it MUST be the only public
  class in its file, and this file MUST be called
  ltnamegt.java

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Java Access Modifiers

• Access modifiers control who has access to a
  variable or class
• public anything can access it
• protected only classes in same package can
  access (not important for 402)
• private only accessible within the class that
  defines it

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Java - Classes

• Instance Field
• private float radius
• Any variable declared in a class is a field
• Typically have an access modifier
• In this case the field is uninitialized, so it
  will get the default value (0).

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Java - Classes

• Class fields and methods
• Use static modifier
• Are instance-independent
• Refer to using class name e.g. Circle.PI
• No need to include class name if using within
  class
• If only used static methods, would not be OO

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Java - Classes

• Constant class field
• public static final double PI 3.14159
• Again, typically have an access modifier
• If you want to have a constant variable, also use
  the static and final modifiers
• static means that there is only one PI no matter
  how many instances of Circle we make
• final means that PI cannot be changed

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Java - Classes

• Constructors
• public Circle(float r) radius r
• This is what gets called when an instance of your
  class is created
• Typically used to just initialize fields
• If you dont write one, you get a default
  constructor for free, which does nothing
• Refer to other constructors with this()

Name of the class
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Java - Classes

• Methods
• public float area()
• return PI radius radius
• Again, need an access modifier
• Need a return type (void is a valid choice)
• Can optionally pass in arguments

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Java - Classes

• Heres the whole thing again
• public class Circle
• private float radius
• public static final double PI 3.14159
• public Circle(float r)
• radius r
• public float area()
• return PI radius radius

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Java - Objects

• So, weve made a class, now what?
• We can start making circles
• public Circle c1
• public Circle c2 new Circle()
• public Circle c3 new Circle(2.5)
• c1 didnt actually make a circle, it just made a
  circle reference
• c2 used the default constructor, and has radius 0
• c3 used our constructor, and has radius 2.5

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Java - Objects

• Now that we have circles, we can call methods on
  them
• public float f c3.area()
• System.out.print(f) //19.6349

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Java - Interfaces

• Interfaces are one of the ways that Java allows
  for Inheritance (the other is with subclasses)
• For example, we could make an interface called
  Shape that our Circle class can implement
• public interface Shape
• public float area ()
• // end Shape
• public class Circle implements Shape
• //...
• //end Circle
• Now, since we know the Circle implements Shape,
  we know that Circle must contain an area() method
• Classes can implement one interface, no
  interfaces, or multiple interfaces (many do)

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Java Reference Oddities

• Since we only deal with references to objects,
  some behavior may seem odd
• Circle hulaHoop new Circle (3)
• Circle CD hulaHoop
• CD.radius 1 //we cant actually do this (lets
  pretend)
• System.out.print(hulaHoop.radius) // 1

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Packages to Look At

• Have a look at java.util, especially
  java.util.HashMap and java.util.LinkedList
• Both are what they sound like and have fairly
  straightforward functions
• These may or may not be useful in upcoming
  assignments

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What to Focus on for 402

• Do focus on
• Basic syntax like control flow, declarations
• Syntax for declaring classes and methods,
  instantiating objects
• Writing output to stdout, stderr (for debugging)
• Basic OO like interfaces
• Java.util
• Dont worry so much about
• Reading input from stdin and files
• Misc packages like java.security

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Example Demo
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Suggestions

• Play around with Java
• Find an IDE your comfortable with, and just write
  some programs to get a feel for it.
• Use the Java API specification at
  http//java.sun.com/j2se/1.4.2/docs/api/index.html
  
• If you have any problems, e-mail Berk
  (bkapicio_at_cs) or Melissa (mkc_at_cs) or feel free to
  ask your friends and/or classmates for help

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Suggested Programs

• A rcenet rsesercah sduty fnuod taht it deosnt
  mttaer in waht oredr the ltteers of a wrod are,
  so lnog as the frist and lsat letetrs are in the
  rgiht pclae. The rset can be a toatl mses and you
  can siltl raed it whtuoit a porbelm.

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Suggested Programs

• A recent research study found that it doesnt
  matter in what order the letters of a word are,
  so long as the first and last letters are in the
  right place. The rest can be a total mess and you
  can still read it without a problem.
• As a toy, write a program that garbles text like
  this it would get you familiar with I/O and
  text manipulation.

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Suggested Programs

• Using the built in HashMap, write a tester as
  though you had written this class yourself
  (possibly in CS 226)
• Maybe you could help out the TAs by writing a
  program that uses student names as a key and maps
  them to student ID numbers (You dont really need
  to do this, we can just look it up)

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Questions?