Java: a Survival Guide and a bit about CC

1
Java a Survival Guide(and a bit about C/C)
THE US NATIONAL VIRTUAL OBSERVATORY

• Ray Plante

HelloWorld.java
javac HelloWorld.java java HelloWorld Hello
world!
2
Java a Survival Guide(and a bit about C/C)
THE US NATIONAL VIRTUAL OBSERVATORY

• Ray Plante

HelloWorld.java
javac HelloWorld.java java HelloWorld Hello
world!
Let's try Exercise 0
3
Goals

• Be able to read summer school Java code and
  understand what it does.
• Understand enough Java syntax to make simple
  changes or extensions to existing code
• Know how to build and run Java code.
• Recognize some common programming errors
• Warning some exercise instructions contain
  errors (on purpose)

4
Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Define one class per file.
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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Define one class per file.
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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Declare other classes well be using
Define one class per file.
8
Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Comments!
Declare other classes well be using
Define one class per file.
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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Comments!
Declare other classes well be using
Define one class per file.
Special commenting style for producing extractable
documentation
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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Comments!
Declare other classes well be using
Define one class per file.
Member fields hold a class internal data
Special commenting style for producing extractable
documentation
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Quick Tour of a Java Programsee Hello.java

• package nvoss.basicjava
• import java.io.Writer
• import java.io.
• /
• a Hello World class and application
• /
• public class Hello implements Cloneable
• private String who "world" // a default
  value is set
• /
• create a Hello instance
• _at_param toWho a name for who we are
  saying hello to
• /
• public Hello(String toWho)
• who toWho

Every class is part of a package.
Comments!
Declare other classes well be using
Define one class per file.
Member fields hold a class Internal data
Special commenting style for producing extractable
documentation
Constructor function is called to when a Hello
object is created.
12
Quick Tour of a Java Programsee Hello.java
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Quick Tour of a Java Programsee Hello.java
/ return the name of who is being
greeted / public String getWho()
return who / set the name
of who is being greeted / public void
setWho(String name) who name
/ say hello to the world /
public static void main(String args)
Hello greeter new Hello()
greeter.sayHello()
Methods!
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Quick Tour of a Java Programsee Hello.java
/ return the name of who is being
greeted / public String getWho()
return who / set the name
of who is being greeted / public void
setWho(String name) who name
/ say hello to the world /
public static void main(String args)
Hello greeter new Hello()
greeter.sayHello()
Methods!
A main method turns a class into an application.
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Quick Tour of a Java Programsee Hello.java
/ return the name of who is being
greeted / public String getWho()
return who / set the name
of who is being greeted / public void
setWho(String name) who name
/ say hello to the world /
public static void main(String args)
Hello greeter new Hello()
greeter.sayHello()
Methods!
A main method turns a class into an application.
Heres how you create a Hello object.
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Quick Tour of a Java Programsee Hello.java
/ return the name of who is being
greeted / public String getWho()
return who / set the name
of who is being greeted / public void
setWho(String name) who name
/ say hello to the world /
public static void main(String args)
Hello greeter new Hello()
greeter.sayHello()
Methods!
A main method turns a class into an application.
Heres how you create a Hello object.
Heres how you call a Hello method.
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Quick Tour of a Java Programsee Hello.java
/ return the name of who is being
greeted / public String getWho()
return who / set the name
of who is being greeted / public void
setWho(String name) who name
/ say hello to the world /
public static void main(String args)
Hello greeter new Hello()
greeter.sayHello()
Methods!
A main method turns a class into an application.
Heres how you create a Hello object.
Heres how you call a Hello method.

• Dont forget
• Every executable statement ends in a semicolon
  ()
• Methods and classes are enclosed in braces (
  )

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Packages and the CLASSPATH

• Every class belongs to a package
• Package names define a namespace
• Allows classes with the same name can come from
  different packages and used in the same program
• Package names are hierachical and imply a
  directory structure
• net.ivoa.adql net/ivoa/adql
• Class file must appear in a directory implied by
  package name
• Classpath
• A list of directories that Java searches to find
  classes
• Usually set with environment variable CLASSPATH
• Linux/MacOS/Unix colon separated
• Windows semi-colon separated
• Package directories must be relative to one of
  the classpath directories
• Instead of a directory, can also give path to a
  jar file
• Like a tar or zip file, but contains Java class
  files
• Executing a class as an application
• Remember class must contain a main() method
• Must provide full name of class, including
  package

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Packages and the CLASSPATH

• Every class belongs to a package
• Package names define a namespace
• Allows classes with the same name can come from
  different packages and used in the same program
• Package names are hierachical and imply a
  directory structure
• net.ivoa.adql net/ivoa/adql
• Class file must appear in a directory implied by
  package name
• Classpath
• A list of directories that Java searches to find
  classes
• Usually set with environment variable CLASSPATH
• Linux/MacOS/Unix colon separated
• Windows semi-colon separated
• Package directories must be relative to one of
  the classpath directories
• Instead of a directory, can also give path to a
  jar file
• Like a tar or zip file, but contains Java class
  files
• Executing a class as an application
• Remember class must contain a main() method
• Must provide full name of class, including
  package

Let's try Exercise 1
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Types, Classes, and Objects

• Primitive types
• boolean, float, double, int, long, short, byte,
  enum, void
• Class data functions on that data
• Java-speak data are members, functions are
  methods
• A complex type beyond primitive types
• Can create variables of the particular class type
• Variables have multi-faceted values
• Functions provide contract for accessing the
  data
• Class represents a logical concept
• e.g. Point, Image, VOTable, Stack, Class
• All classes implicitly inherit from the generic
  Object class
• Object class data values assigned to a variable
• Created with new
  Java-speak often called a class instance
• Hello greeting new Hello(world)
• Hello insult new Hello(dork)

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Constructors

• Special methods used to initialize data in an
  object
• Named after class name, no return type
• Called implicitly via new
• Executed after any initialization statements
• Constructors can be overloaded
• Multiple versions with different inputs
• Hello greeting new Hello() // calls
  public Hello()
• Hello insult new Hello(dork) // calls
  public Hello(String)

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Interacting with Objects

• Outside users of an Object can only access its
  public members methods
• marked with public modifier
• Use . operator
• Point pt new Point(4, 5)
• System.out.println(Position pt.x ,
  pt.y)
• pt.move(3, 3)
• Most classes hide their internal data
• private accessible only inside class
• protected accessible both internally and to
  subclasses
• Public access methods provided to manipulate
  internal data, if allowed
• Hello greeting new Hello()
• greeting.setWho(Dave)
• System.out.println(addressee is now
  greeting.getWho())
• If data is read-only, no set method is defined
• e.g., String is immutable, has no set methods

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Interacting with Objects

• Whats this?
• automatic variable referring to current object
• public void setWho(String who)
• this.who who
• Whats null?
• Special value for object variables that do not
  point to an instantiated object
• Default value for objects
• String name // Equivalent
• String name null // Equivalent
• if (name null)
• NullPointerException an error thrown if one
  attempts to use . on a object null
• if (name.length() 0) // Ouch! Throws an
  error if namenull

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Interacting with Objects

• Whats static?
• Data/method shared by all instances of a class
• Often used to declare constant data
• public class Hello
• public static final String DEFAULT world
• String who DEFAULT
• Do not need to create an instance to access
  static data/method.
• Use class name to access static data or method
• System.out.println( The default addressee
  is Hello.DEFAULT)
• String args new String0
• Hello.main(args) // call the static main()
  method directly
• Static method cannot access non-static data!

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Arrays

• Arrays can be made of a primitive or class type
• Declaring an array
• int a
• String words
• Instantiating initializing an array
• int b new int4 // length of 4, values
  set to 0
• words new String3 // length of 4 each
  value is null
• words new String the, quick, brown,
  fox
• An array index is zero-based a0 is the 1st
  element
• Arrays are objects
• Contain public member called length
• for(int i0 i
• bi i

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Arrays

• Arrays can be made of a primitive or class type
• Declaring an array
• int a
• String words
• Instantiating initializing an array
• int b new int4 // length of 4, values
  set to 0
• words new String3 // length of 4 each
  value is null
• words new String the, quick, brown,
  fox
• An array index is zero-based a0 is the 1st
  element
• Arrays are objects
• Contain public member called length
• for(int i0 i
• bi i

Let's try Exercise 2
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Java Packages

• Standard Java packages start with java
• java.lang the core classes
• Useful classes String, System, Object, Math
• Primitive wrapper classes Integer, Double,
  Boolean, Char, etc.
• Other useful standard packages
• java.util Vector, ArrayList, Hashtable,
  StringTokenizer
• java.io File, FileReader, FileWriter, etc.
• java.net URL
• Importing classes
• You can refer to a class by its full name
  java.net.URL
• If you want to refer to a class using the
  shortened name (e.g. URL), you must import it.
• E.g. import java.net.URL
• import java.net. imports everything from
  java.net
• You do not have to import
• Classes from the same package as the one be
  defined
• Classes from java.lang (import java.lang. is
  implicit done by compiler)

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Creating Your own API docs

• javadoc produces high-quality API documentation
• Extracts specially marked comments
• Class, data member, and method descriptions
• / \ 2 asterisks
• return the name of who is being greeted
• /
• public String getWho()
• / this is not extracted /
• Special tags within comments
• /
• create a Hello instance
• _at_param toWho a name for who we are saying
  hello to
• /
• public Hello(String toWho)
• Its incredibly easy!

29
Extending classes through inheritance

• Subclass an existing class modifications
• Subclass inherits data methods of parent class
• Subclass can add additional data methods
• Subclass replace or override inherited methods
• Single inheritance subclass can only extend one
  parent class, but
• Can be a chain of subclasses
• All classes inherit from Object
• Abstract class
• Cant be instantiated themselves must be
  subclassed
• Some functions are defined but dont have
  implementations
• Ex Writer -- how the writing is done depends
  on what is being written to
• Interface set of functions with no
  implementations, no data
• A class implements an interface
• A class can implement multiple interfaces
• a kind of multiple intheritance
• Whats the use of a class/interface that doesnt
  provide implementations for all its methods?

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How to extend a class through inheritance

• Use extends in the class definition
• public class SpiralGalaxy extends Galaxy
• Add any new data members
• Add constructors
• Constructors are not inherited however, you can
  call the parents constructor
• public SpiralGalaxy(String name)
• super(name) // will call Galaxy(String)
• If super constructor not called explicitly, the
  compiler inserts a super()
• Add new and/or overriding methods
• An overriding method may call parents version
• public Population getStellarPopulation()
• Population pop super.getStellarPopulation()

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Implementing interfaces

• List interface being supported with implements
• public class SpiralGalaxy extends Galaxy
• implements ClusterComponent
• Provide implementations of methods in interface
• public double getDarkMass()

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Exceptions an Intro

• Exception specialized objects that can be
  thrown when an error occurs
• if (greeter null)
• throw new NullPointerException(No Hello
  provided)
• Different types of exceptions are used for
  different types of errors
• When throw is used
• Execution of the current function stops
• The exception is passed to the caller of the
  function
• unless caught along the waymore on this later
• Exceptions travel up the call-stack until it is
  caught
• If it reaches the top of the call stack, Java
  will
• Print the exceptions error message
• Print the call stack it traveled through
• Exception in thread "main" java.lang.NullPointerEx
  ception No Hello provided
• at nvoss.basicjava.Hello.main(Hello.java9
  1)

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Exceptions an Intro

• Exception specialized objects that can be
  thrown when an error occurs
• if (greeter null)
• throw new NullPointerException(No Hello
  provided)
• Different types of exceptions are used for
  different types of errors
• When throw is used
• Execution of the current function stops
• The exception is passed to the caller of the
  function
• unless caught along the waymore on this later
• Exceptions travel up the call-stack until it is
  caught
• If it reaches the top of the call stack, Java
  will
• Print the exceptions error message
• Print the call stack it traveled through
• Exception in thread "main" java.lang.NullPointerEx
  ception No Hello provided
• at nvoss.basicjava.Hello.main(Hello.java9
  1)

Let's try Exercises 3 4
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Polymorphism

• Whats the use of a class/interface that doesnt
  provide implementations for all its methods?
• You can refer to a specific subclass instance
  using a more generic reference
• FileWriter out new FileWriter(file)
• or
• Writer out new FileWriter(file) // ok
• public void sayHello(Writer out)
• Hello greeter new Hello()
• greeter.sayHello(out)
• Okay because FileWriter is a type of Writer
• You can also refer to a class instance by its
  Interface name
• A Class or Interface is a contract
• It guarantees support for a set of methods

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Tips for debugging exceptions

• Recompile your code with g option
• Inserts line numbers into printed stack traces
• Find first line in stack trace that refers to a
  function in your code
• Is your function the first in the stack trace?
• Line number provides exact location where error
  occurred
• Root cause of the error may be somewhere else
• Is your function not the first in the stack
  trace?
• Line number gives location where you call another
  function
• There is probably something wrong with the
  objects you have passed to the function
• jdb, the Java Debugger, can be very useful

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Common Exceptions

• NullPointerException
• The usual explanation
• an object variable has been set to null later,
  an attempt is made to access its contents,
  usually via the . operator (e.g. var.get())
• How to fix it
• go to the designated line number and look for an
  variable using the . operator that will be the
  null variable
• If your class is not at the top of the call
  stack, look for variables that are being passed
  into the function one of these will be null.
• NoClassDefFoundError
• The usual explanation
• Your CLASSPATH does not include the indicated
  class or package
• How to fix it
• Adjust your CLASSPATH make sure it includes all
  the directories or jar files needed.
• ArrayIndexOutOfBoundsException
• The usual explanation
• While accessing array elements via , e.g. ai,
  the index i a.length
• How to fix it
• Look for the use of in the indicated line the
  exception message gives the value of the
  offending index.

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Handling Exceptions in code

• Use a try-catch block
• try
• // code that might generate an error
• catch (MalformedURLException ex)
• // ex holds the exception
• catch (ConnectException ex)
• catch (Exception ex)
• Each catch block handles a different type of
  failure
• When the exception type matches a catch block,
• The exceptions upward traversal halts
• The catch block is executed
• If block does not throw its own exception,
  execution outside the try-catch block continues

38
The throws clause

• A function must indicate what exceptions it might
  throw
• public void loadData(Reader in)
• throws IOException, ParseException
• Except for RuntimeExceptions
• Ex NullPointerException
• function user must either
• Enclose the method call in a try-catch block, or
• Add exception to calling functions throws clause

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The throws clause

• A function must indicate what exceptions it might
  throw
• public void loadData(Reader in)
• throws IOException, ParseException
• Except for RuntimeExceptions
• Ex NullPointerException
• function user must either
• Enclose the method call in a try-catch block, or
• Add exception to calling functions throws clause

Let's try Exercise 5
40
VO Software in C/C

• Development in C/C has lagged behind other
  languages
• Native libraries
• libVOTable http//terapix.iap.fr/cplt/docs/libVOT
  able/
• VOTable Parser in ANSI C
• VO-Indias VOTable Parsers in C
  http//vo.iucaa.ernet.in/voi/cplusparser.htm
• Streaming and non-streaming versions
• Wrappers Containers
• For leveraging legacy code or high-end resources
• IRAFs VO Services accessing IRAF capabilities
  via Web Serivces for VO applications
• NESSSI Gateway for deploying VO applications on
  the TeraGrid
• Starlink integrated support for VO formats,
  services
• VOclient in development