Transition from C to Java

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Transition from C to Java

• Walt Savitch
• University of California, San Diego
• wsavitch_at_ucsd.edu

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Java even a simple program is not simple.

• public class Program1
• public static void main(String arg)
• System.out.println("Hello World")

3
Some Similarities betweenC and Java

• Simple (primitive) types int, double, char
• Control Structures if-else, switch, while, for
• Arithmetic expressions
• Both have a string type
  C string, Java String.
• Arrays
• Both have classes.
• Both have a "main".

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Some Differences betweenC and Java

• Java has automatic garbage collection. C does
  not.
• C has operator overloading. Java
  does not.
• C says "function".
  Java says "method".
• These require no explanation, unless students
  already know C.

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More Differences

• C classes can be avoided. Java classes cannot
  reasonably be avoided.
• C has built in console I/O. Java has no
  standard console input (but does have standard
  console output.)
• C and Java divide a program into pieces (for
  separate compilation) in different ways.
• These require some explanation.

6
C classes can be avoided.Java classes cannot
reasonably be avoided.
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Every compilation unit in Java is a class. A
program is a class with a method named
mainpublic class Program1 public static
void main(String arg)
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In Java, every method is a member of some
class.You cannot have a freestanding (global)
function in Java.
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You can fake a "no classes" program in Java by
making all methods static.

• But dont do it!

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A Sample Java Class
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public class PetRecord private String name
private int age//in years public
PetRecord(String initName,
int initAge) name initName if
((initAge lt 0)) System.out.println("Error"
) else age initAge
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public void writeOutput()
System.out.println("Name "
name) System.out.println("Age "
age " years")
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C has built in console I/O.Java has no
standard console input(but Java does have
standard console output.)
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C has cin, cout, cerrJava has
System.out.print and System.out.printlnbut
NO console input.Solutions?
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Solutions

• AP does not require console input.
• There are classes for console input that are not
  part of Java but written in Javae.g.,
  SavitchIn.readInt()
• JOptionPane, simple GUI I/O

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C and Java divide a program into pieces (for
separate compilation) in different ways.
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C Traditionally has an interface (header)
file, implementation file(s), application
(driver) file.C Can confine a program to a
single file if you want.
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• Java A compilation unit is always a class
  definition.
• Every class is in a separate file (except for
  some special cases).
• No header files.
• Normally, you have no one file programs in Java.

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More Subtle Differences

• C has pointer types.
• Java has no pointer types .
• Assignment () and equality comparison () have
  minor differences.
• C gives a choice of parameter types.
• Java No choice of parameter types.
• Exception handling can be avoided in C
• Exception handling is needed for some fundamental
  things in Java, e.g. file I/O.

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Java has no pointer types

• But Java does have "pointers".
• In Java class (and array) types are REFERENCE
  TYPES.
• A reference is a "pointer". All class values in
  Java are handled as references, but it is all
  automatic.
• In Java primitive types are just like in C.

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• In Java a primitive type variable holds values,
  just as in C. int n 42
• Java a class type variable contains a reference
  ("pointer") to the object (value).
• However, this is all automatic. There are no
  pointer types as such in Java.
• PetRecord myDog new PetRecord("Fido", 3)
• Note that all class objects are created
  dynamically.

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Assignment () and equality comparison () have
minor differences.

• On primitive (simple) types, and are the
  same in C and Java.
• In Java, and on classes (or arrays) are
  comparing references ("pointers"),
• and you cannot overload (redefine) and in
  Java.

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Assignment () and equality comparison () have
minor differences.

• If (n 0) .
• In C this is probably an error with no error
  message, assuming you meant to use .
• In Java this generates a compiler error.
• In Java ints neither are nor can they be type
  cast to Booleans

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C a choice of parameter types.Java no choice
of parameter types.

• C Call-by-value
• void f(int n)
• C Call-by-reference
• void f(int n)
• Other C variants
• void f(const int n)
• void f(const int n)

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C a choice of parameter types.Java no choice
of parameter types.

• Java all parameters are call-by-value.
• But, it is almost like there are different
  parameter types for primitive types and classes.

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Java no choice of parameter types,but

• All primitive type
• parameters are automatically call-by-value.
• public void f(int n)
• ...
• All class types are automatically something very
  much like call-by-reference.
• public void f(String n)
• ...

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C a choice of parameter types.Java no choice
of parameter types.

• Java Full Story
• In Java primitive types are just like in C.
• In Java class (and array) types are REFERENCE
  TYPES.
• A reference is a "pointer". All class values in
  Java are handled as references, but it is all
  automatic.
• All parameters are call-by-value of a reference.

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C a choice of parameter types.Java no choice
of parameter types.

• Java Full Story
• In Java all parameters are call-by-value.
• Parameter is a local variable initialized to the
  value of the argument.
• Primitive types no surprises.
• Class type (local) variables hold references.
• Class parameters are call-by-value of a reference.

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Java no choice of parameter types.

• public void change(PetRecord r)
• r.name "FooFoo"
• This really changes its PetRecord argument.
• public void change(int n)
• n 42
• This does not change its int argument.

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Java no choice of parameter types.

• public void change(int n)
• n 42
• This does not change its int argument.
• There is no way to write a Java method that has a
  parameter for an int variable and that changes
  the value of an argument variable.

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There is no way to write a Java method that has a
parameter for an int variable and that changes
the value of an argument variable.So, how do you
manage to cope?

• int n computeNewValue()
• OR use class objects.

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• public class Stuff
• private int n
• ....
• public void changeTheN(Stuff s)
• s.n 42

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Exception handling can be avoided in
CException handling is needed for some
fundamental things in Java, e.g. file I/O.

• Solutions
• AP requirements do not include file I/O.
• Teach exception handling.
• Fake it with "magic formulas"

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AP Exception Requirements

• Covers exceptions as error messages.
• Does not cover try/throw/catch.
• Does not cover throws clause (declaring
  exceptions).

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Exception handling in Java

• Fake it with "magic formulas" approach
• public class TextFileOutputDemo
• public static void main(String arg)
• throws IOException
• PrintWriter outputStream
• new PrintWriter())
• outputStream.println("To file")

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• public class TextFileOutputDemo
• //without magic formula
• public static void main(String arg)
• PrintWriter outputStream null try
• outputStream new PrintWriter(
• new FileOutputStream("out.txt"))
• catch(FileNotFoundException e)
• outputStream.println("To file")

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Style Comparison C/Java

• Java uses loooong names
• e.g. FileNotFoundException
• while C uses some abbreviations
• Java spelling conventions
• ClassName, variableName, methodName,LITERAL_NAME
• Java has an official commenting style
• javadoc

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javadoc

• Extracts an interface from a class definition.
• May not need full blown details for AP course,
  but be consistent with javadoc.
• Comments before method headings/
• javadoc comment style.
• /

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Getting a Java CourseOff-the-Ground

• Need some "magic formulas," but
• Move to real classes quickly.
• Do something about console inputadd console
  input classuse JOptionPaneuse magic formulas

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"Magic Formulas"

• public class ProgramName
• public static void main(String arg)
• means "begin".
• Use this to explain simple flow of control
  then quickly move to classes and explain what
  this means.

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Console Input

• You need to do something.
• Use SavitchIn or some other console input class
  or
• Use a very messy magic formula or
• Explain the formula (still messy) or
• Use JOptionPane.

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GUIs (Graphical User Interfaces, i.e., Windowing
Interfaces)
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GUIs

• Not part of the AP requirements.
• Applets Designed to be used over the internet.
  Can be used for ordinary programs, but have some
  problems and no easier than regular windowing
  systems.
• "Regular Windowing Systems"
• Swing Library is the latest version.

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Java Software

• Java is well standardized. SDK (aka JDK) Java
  compiler is free.
• java.sun.com
• Works well with Windows and Unix
• Want Java 2, version 1.4 or higher
• (Standard Edition is enough)
• Mac users have traditionally had limited choices,
  but things are better now.
• JJ works with all operating systems.

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Java Software for Mac

• Good (free?) Java compiler for Mac OS X (Im
  told)
• http//developer.apple.com/java/
• Some of the good IDEs for Mac Code Warrior,
  BlueJ.
• JJ Works for any operating system.

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IDEs

• Windows
• TextPad (shareware) www.textpad.com
• use with Sun SDK
• Forte (free) java.sun.com
• Borland www.borland.com
• Mac
• BlueJ (free) www.bluej.org
• CodeWarrior www.metrowerks.com
• JJ Works with all operating systems.
• www. .LearnJavaNow.org/

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Text Books

• Lots to choose from.
• For example,
• Walter Savitch Java An Introduction to Computer
  Science and Programming,
• Prentice-Hall.