Beginning Java for .NET developers

Beginning Java for .NET
developers

Why learn Java?
• Complement your developer skills
• Be able to develop Android apps
• Target Linux OS

Where do I get Java?
• Oracle (official vendor, acquirer of Sun)
• IBM (no Windows support – at least in
Nov.2013)
• OpenJDK (Ubuntu)
• many others

IDEs
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Eclipse
Netbeans (Oracle)
IntelliJ IDEA (JetBrains, makers of ReSharper)
many others

Common features
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Object-oriented
Compiled, not interpreted
Statically typed
Type safe
Runtime (JVM) – based (available on many platforms)
Garbage collected
Allows native interoperability
Runs on mobile devices (smartphones, tablets, even
feature-phones)
• JLS / CLS – multiple languages on the same platform

Similar projects and technologies
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Hibernate – NHibernate
log4j – log4net
Play – ASP.NET MVC
JUnit – NUnit
JavaFX – WPF
Swing/AWT – Winforms
JSP/JSF – ASP.NET (Webforms)

Subtle and not-so-subtle differences
• Language differences
(calls, conventions, execution, syntactic
sugar, code organization, syntax)
• Platform differences (architecture, classsystem, execution, operations, data types and
others)

Language - Calling methods
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No out or ref parameter modifiers
No optional parameters
No named parameters
params written as “…”
public void doSomething(Object… args) { … }

• No extension methods

Language - Coding conventions
• Methods are camelCased and not PascalCased
• The opening brace is to be put on the same line :
public void doSomething() {
}

• Interfaces are not prefixed with a capital I :
Throwable, Runnable etc.

• Enum values are all-uppercase
• Abbreviation words in compound names can be
all-caps : URLParser (as opposed to .NET’s
UrlParser)

Language - Code execution
• Switch allows fall-through by design
• Convenient multicatch statement :
try { … } catch(IOException | NetworkException e) { … }

• Override return in finally
• No #Ifdef
• Try-with-resources as using equivalent :
try(IOStream s = new IOStream()) { … }

Language – Convenience features
• No as feature. Test with instanceOf and then
cast
• No lambdas yet. Promised in Java 8. Use
anonymous inner classes
• Static method import. Like extensions
methods but the other way around.
import static com.something.Otherclass.someMethod;
…
someMethod(..);

• No explicit interface implementation

Language – Organization
• No nested packages in a single file.
namespace Outer
{
namespace Inner
{
…
}
}

• Only one public class in a .java file
• The public class name must match case-wise the
filename
• No partial classes
• No partial methods

Language – Syntax
• protected means ‘protected internal’

• implements or extends instead of colon (‘:’)
• No var facility
•

foreach syntax

: for(Type

variable : collection){…}

• instanceOf instead of is
• SomeClass.class instead of typeof(SomeClass)

• Annotations are prefixed with @ and not ‘[‘, ‘]’
• .. can also be applied to assignment statements

Language – Syntax (cont’d)
• No indexers

public int this[int index] { get { … } }

• No #regions
• Binary numeric literals :

int n217 = 0b11011001;

• Underscore allowed (and ignored) in numeric literals :
int cardNo = 1234_5678_9012_3456;

• The base class is called superclass, the derived class is
called subclass
• Calling the superclass constructor is done within the
constructor not outside, it is optional, but if done, must
be the first statement of the constructor

Language – data types
• Default access modifier (i.e. not specifying one)
means package (kind of internal), for methods
or fields.
• String must be written with capital S. Seems
irrelevant but it will be the most common typo.
• Interfaces can have static final fields (“constants”)
– before enums this was a way to simulate
enums.
• No operator overloading
• No implicit or explicit conversions can be defined
on types

Language – Enums
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Reference type – like classes
Enums can have fields and constructor (though it must be private)
You can override methods. Typically toString()
Lazily created. Each enum value is instantiated at its first use.
Abstract methods, overridable in specific values
Allows inheritance. Although it has the semantic value of a
superset, not subset.
valueOf(String name) instead of Enum.Parse
values() for enumerating all values
name() – gets the enum value name; typically toString() does the
same but the latter can get overriden
ordinal() – gets the order index of the value

Platform – Architecture
• @Override annotation is optional. Overload instead

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of override can occur.
Cloning is awkard, non-intuitive. Cloneable marker
interface and protected clone().
No static classes. Make it final, create a private
constructor and throw an exception inside the
constructor
Return type covariance : Override a method and return
a subclass of the original method’s return type class.
Typical use : overriding clone().
You can alter a collection while iterating it without
getting an exception, but only if done through the
current iterator.

Platform – Classes
• A method is virtual by default – not like in .NET where
it’s sealed (final) by default. This is how Hibernate
strived and NHibernate struggles.
• Generics are implemented using type erasure. No
List<primitive> and no arrays of generic types.
• No (known) way to create a generic type, by
reflection, at runtime.
• Inner classes have outer class reference by default.
Except static inner classes.
• No method generators. i.e. yield
• Type inference by ‘diamond’ style constructors.
List<Integer> = new ArrayList<>();

Platform – Execution
• Checked exceptions. Each method must
declare what exceptions it can throw or catch
them.
• RuntimeExceptions are exempt
• Errors are not catchable

Platform – Convenience features
• No properties
• No events
• Three ways to simulate events :
– Nested anonymous classes
– Implementing interfaces and passing this
– Lambdas (promised in Java 8)

Platform – Operations
• Primitives have reference-types peers (wrappers)
• Do not confuse them to their primitive
counterparts. == operator will compare instances
instead of values
• Strings also should not be compared with ==
operator. Although there is string interning this
will usually fail you
• No checked mode. Numeric wrap-around always.

Platform – Data types
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No unsigned numeric data types
No multi-dimensional arrays
No structs or other user-defined value-types
No dynamic infrastructure. Only in non-Java
languages compiled to bytecode.
• The primitives are not part of the type
hierarchy. Only their wrappers are.

Platform – others
• Upon deployment desktop apps don’t have an
EXE or other prepared entry point, executable
file
• The Garbage Collector does not have a
(separate) Large Object Heap, nor its
associated issues (fragmentation)

Further reading
• Enums : http://docs.oracle.com/javase/tutorial/java/javaOO/enum.html
• JLS : http://docs.oracle.com/javase/specs/
• Java 8 Release date : http://openjdk.java.net/projects/jdk8/

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