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 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".

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++.

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.printlnbut 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-by-

value.public void f(int n){...}

• All class types are automatically something very much like call-by-reference.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 CourseOff-the-Ground

Need some "magic formulas," but

Move to real classes quickly.

Do something about console input:add console input classuse JOptionPaneuse 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 SoftwareJava 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 MacGood (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.