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Java2C#
Antonio Cisternino
Part I
Premessa
C# si pronuncia circa come “see sharp”!!!
 Non C cancelletto
 Non C gratella
 Non C diesis
 Gioco di parole:

C# = vedi definito
C# = C ++++
C# = C ++
Do diesis
Visual Studio .NET
Gli studenti di informatica e di ingegneria
informatica hanno diritto ad una copia di
VS.NET
 Distribuzione (modello biblioteca):

 Firmare
alla consegna
 Prendere i dischi
 Riportare i dischi!
 Firmare la restituzione
Foreword: .NET and Java

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.NET comes after Java and many features have been
borrowed from it as well from other systems
Java has done the same in the past borrowing from C++,
Smalltalk, and many others (thus try to understand the
differences and improvements!)
Frameworks tend to collect a set of techniques
developed by researchers and organize them in a
meaningful way
With .NET the computer science onion grows with an
additional layer: on top of the OS lies the abstract
machine and the framework
Outline
Introduction to .NET and CLR
 Overview of C# wrt Java
 Java as subset of C#
 CLR type system

Outline
 Introduction
to .NET and CLR
Overview of C# wrt Java
 Java as subset of C#
 CLR type system

Microsoft .NET


‘Dot NET’ is a brand name to indicate a set of
technologies. It is a platform to develop
applications.
Two essential elements of the platform are:
 The
Common Language Runtime (CLR)
 A class library (Framework) which complements the
CLR

Framework and CLR offer a set of services and
an execution environment to .NET programs
Common Language Runtime
in different
languages
issupport
like composing
Programming
Goal
of
.NET
initiative
is
to
pieces in different keys, particularly if you work at the
program
atwritten
all levels
keyboard. interoperability
If you have learned or
pieces in many
keys,is
each
key will have
its own designed
special emotional
aura.
 CLR
a
runtime
support
to
be
Also, certain kinds of figurations “lie in the hand” in one
used
byaremany
languages
key but
awkward
in another. So you are channeled
by your
choice of supported
key. In some ways,
even enharmonic
 Main
services
by
.NET
are:
keys, such as C-sharp and D-flat, are quite distinct in
feeling.
A common
type how
system
This shows
a notational system can play a
rolecollection
in shaping system
the final product.
significant
A garbage
 An
(Gödel, Escher, Bach: an eternal golden braid, Hofstadter, 1980, Chapter X)
execution environment
Common Language Runtime




CLR supports the execution of programs in
Common Intermediate Language (CIL) format
CIL is a language of a stack based machine
The core of CLR is the Execution Engine (EE)
EE is responsible of compiling CLI code into
machine code using a JIT
Common Language Runtime


CLR could be roughly seen as a shared backend of multiple compilers
The overall model is:
 compile
a language into CIL (compiler)
 compile CIL into executable (runtime)
 Execute the compiled program

The model is different from Java where an
interpreter is needed and JIT an optimization
How CLR works
C#
Unmanaged
GC
x86
C++
Managed x86
Security
Managed
ML
CIL
VB
BCL
Loader
JIT
CLR
…
CLR and progr. languages

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

If a language compiles to CIL may offer types to
other programs
A language may expose CIL types written in
other languages
Together with CLR Microsoft deploys a new
language called C#
Both CLI and C# are ECMA standards and a
reference implementation has been released by
Microsoft (http://msdn.microsoft.com)
Outline

Introduction to .NET and CLR
 Overview
of C# wrt Java
Java as subset of C#
 CLR type system

C# Language
C# is a language derived from C++
 Java has influenced the language
although most influence come from CLR
 It has been designed together with CLR
thus it exploits all its services
 It is more expressive than Java
 Interoperability is built-in into language

C# Overview

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It is object oriented
It supports interfaces + single inheritance
Type system is common rooted (in object)
Objects on the heap are garbage collected
It allows stack-based allocation of quasi-objects
It exposes metadata extensions as attributes
It introduces delegate type and events
C# Overview (cont.)
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It allows to control over virtual methods
Methods parameters could be labeled in/out/ref
Classes may have a destructor like C++
Explicit release of resources is handled through
interfaces
Better control on name clashes
It supports properties
It allows overloading of operators
C# Overview (cont.)
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Arrays can be multidimensional
Alias for types are allowed
XML documentation is supported
Access to memory through pointers in unsafe
mode
A preprocessor-like syntax is supported
Through platform invoke it is possible to access
external functions stored in dlls
C# and Java
‘Italic’ features are shared with Java
 Java is essentially a subset of C# with four
important exceptions:

 Throws
clause on methods
 Inner classes
 Thread synchronization
 Class loading
C# and Java (cont.)

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Inner classes in C# are allowed only as namespace,
state must be explicitly passed as arguments
Threads are exposed through the Monitor class
Class loading model has same expressivity on both
sides
IMPORTANT: C# and CLR have been designed together
but are separate thus functionality are better separated
than in Java/JVM
Thus understanding C# benefits from some
understanding of the underlying CLR
C# and Java (cont.)


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C# allows defining multiple classes in the same
file: the unit of deployment is the Assembly (very
different from JAR!)
Types are organized in namespaces that are
similar to Java packages
Namespaces are defined as blocks rather than
statements at the beginning of the file
A single file may contain definitions related to
different namespaces
Outline
Introduction to .NET and CLR
 Overview of C# wrt Java

 Java

as subset of C#
CLR type system
Java in C#
The syntax of C# is rather closed to Java
because both derive from C++
 The most noticeable syntax difference in
the shared constructs are extends and
implements keywords not present in C#
 package nearly maps into namespace
 C# adopts Pascal convention (capital first)

Hello world
import
using System;
java.lang;
package
namespace it.unipi.di.hello;
it.unipi.di.hello {
public class Hello {
public static
void main(String[]
Main(string[] args) {
System.out.println("Hello
Console.WriteLine ("Hello world");
}
}}
}
Inheritance and interfaces
Java
class A {}
interface B {}
interface C {}
class D extends A
implements B, C
{}

C#
class A {}
interface B {}
interface C {}
class D : B, C, D
{}

Methods
Java
String f(int i){
while (true) {
for (int j = 0;
j < 10;
j++) …;
}
return "f";
}

C#
string f(int i) {
while (true) {
for (int j = 0;
j < 10;
j++) …;
}
return "f";
}

Exception handling
Java
try {
foo();
}catch (IOException
f) {
} catch(Exception e){
… // Not use e
} finally {
}

C#
try {
foo();
} catch (IOException
f) {
} catch {
… // General handler
} finally {
}

Other stuffs
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Local variable declarations used in Java can be
used in C#
Protection mechanisms (private, protected,
public, internal/package) are the same
Java values are included in C# (C# allows
custom defined values): numbers and
references
Separation between CLR and C# is better
defined than Java and JVM: there is the notion
of “standard library” defined within the standard
Outline
Introduction to .NET and CLR
 Overview of C# wrt Java
 Java as subset of C#

 CLR
type system
Type system
One way to address interoperability
(perhaps the most important) within CLR is
sharing the same type representation for
multiple languages
 C# exposes most of the CLR type system
 CLR type system is really sophisticated
with respect to the one exposed by JVM

Java type system
T[]
Object
Class
interface T
String
class T
int
Base types
CLR type system
Array
Object
Type
interface T
String
ValueType
T[]
Struct T
int
Base types
class T
Enum
Enum T
Delegate
Delegate T
Type system features
Type system is common rooted! The class
System.Int32 is inherited from Object
 The trick to avoid inefficiencies in treating
values as objects is called
boxing/unboxing
 Value types can be defined although with
some restriction: i.e. no inheritance

Type system features
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There are several type constructors: array,
delegates, enumerations
Enumerations are equivalent to integer but type
information is preserved at runtime
Type descriptions are accessible by programs
through reflection
Through class Type and related classes it is
possible to inspect type’s structure
C# and the CLR type system

C# standard defines many base types (int,
string, object, and so on) that are mapped
into CLR types; for instance:
 System.Int32
 double  System.Double
 string  System.String
 int

All CLR types are accessible from C# and
vice-versa.
Type constructors in C#

Type constructors are:
of type T  T[]
 Delegate  delegate ...
 Enum  enum { … }
 Value types  struct T { … }
 Classes  class T { … }
 Interfaces  interface T { … }
 Array
Next lecture
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Array types
Enum types
Value types


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Delegate types



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differences from classes
boxing and unboxing
Base structure
Multicast delegates
Event model using delegates
Event type