Transition from C++ to Java
Walt Savitch
University of California,
San Diego
wsavitch@ucsd.edu
Java: even a simple program is
not simple.
public class Program1
{
public static void main(String[] arg)
{
System.out.println("Hello World");
}
}
•
•
•
•
Some Similarities between
C++ 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".
Some Differences between
C++ 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++.
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.
C++ classes can be avoided.
Java classes cannot reasonably be
avoided.
Every compilation unit in Java is a
class. A program is a class with a
method named main:
public class Program1
{
public static void main(String[] arg)
{
In Java,
every method is a member of
some class.
You cannot have a freestanding
(global) function in Java.
You can fake a "no classes"
program in Java by making all
methods static.
But don’t do it!
A Sample Java Class
public class PetRecord
{
private String name;
private int age;//in years
public PetRecord(String initName,
int initAge)
{
name = initName;
if ((initAge < 0))
System.out.println("Error");
else
age = initAge;
}
public void writeOutput()
{
System.out.println("Name: "
+ name);
System.out.println("Age: "
+ age + " years");
}
}
C++ has built in console I/O.
Java has no standard console input
(but Java does have standard console
output.)
C++: has cin, cout, cerr
Java has:
System.out.print and
System.out.println
but NO console input.
Solutions?
Solutions:
• AP does not require console input.
• There are classes for console input
that are not part of Java but written
in Java:
e.g., SavitchIn.readInt()
• JOptionPane, simple GUI I/O
C++ and Java divide a program
into pieces (for separate
compilation) in different ways.
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.
• 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.
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.
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++.
• 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.
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.
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
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);
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.
Java: no choice of parameter types,
but
• All primitive type
parameters are automatically call-byvalue.
public void f(int n)
{...}
• All class types are automatically
something very much like call-byreference.
public void f(String n)
{...}
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.
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.
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.
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.
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.
public class Stuff
{
private int n;
....
public void changeTheN(Stuff s)
{
s.n = 42;
}
}
Exception handling can be avoided in C++
Exception 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"
AP Exception Requirements
• Covers exceptions as error messages.
• Does not cover try/throw/catch.
• Does not cover throws clause
(declaring exceptions).
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");
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");
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
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.
*/
Getting a Java Course
Off-the-Ground
Need some "magic formulas," but
Move to real classes quickly.
Do something about console input:
add console input class
use JOptionPane
use magic formulas
"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.
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.
GUIs
(Graphical User Interfaces,
i.e., Windowing Interfaces)
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.
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.
Java Software for Mac
Good (free?) Java compiler for Mac OS X
(I’m told):
http://developer.apple.com/java/
Some of the good IDE’s for Mac
Code Warrior, BlueJ.
JJ Works for any operating system.
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/
Text Books
Lots to choose from.
For example,
Walter Savitch
Java: An Introduction to Computer
Science and Programming,
Prentice-Hall.