Xamarin for Java developers

Статия
03/13/2018

If you are a Java developer, you are well on your way to leveraging your skills and existing code on the Xamarin platform while reaping the code reuse benefits of C#. You will find that C# syntax is very similar to Java syntax, and that both languages provide very similar features. In addition, you'll discover features unique to C# that will make your development life easier.

Overview

This article provides an introduction to C# programming for Java developers, focusing primarily on the C# language features that you will encounter while developing Xamarin.Android applications. Also, this article explains how these features differ from their Java counterparts, and it introduces important C# features (relevant to Xamarin.Android) that are not available in Java. Links to additional reference material are included, so you can use this article as a "jumping off" point for further study of C# and .NET.

If you are familiar with Java, you will feel instantly at home with the syntax of C#. C# syntax is very similar to Java syntax – C# is a "curly brace" language like Java, C, and C++. In many ways, C# syntax reads like a superset of Java syntax, but with a few renamed and added keywords.

Many key characteristics of Java can be found in C#:

Class-based object-oriented programming
Strong typing
Support for interfaces
Generics
Garbage collection
Runtime compilation

Both Java and C# are compiled to an intermediate language that is run in a managed execution environment. Both C# and Java are statically-typed, and both languages treat strings as immutable types. Both languages use a single-rooted class hierarchy. Like Java, C# supports only single inheritance and does not allow for global methods. In both languages, objects are created on the heap using the new keyword, and objects are garbage-collected when they are no longer used. Both languages provide formal exception handling support with try/catch semantics. Both provide thread management and synchronization support.

However, there are many differences between Java and C#. For example:

In Java, you can pass parameters only by value, while in C# you can pass by reference as well as by value. (C# provides the ref and out keywords for passing parameters by reference; there is no equivalent to these in Java).
Java does not support preprocessor directives like #define.
Java does not support unsigned integer types, while C# provides unsigned integer types such as ulong, uint, ushort and byte.
Java does not support operator overloading; in C# you can overload operators and conversions.
In a Java switch statement, code can fall through into the next switch section, but in C# the end of every switch section must terminate the switch (the end of each section must close with a break statement).
In Java, you specify the exceptions thrown by a method with the throws keyword, but C# has no concept of checked exceptions – the throws keyword is not supported in C#.
C# supports Language-Integrated Query (LINQ), which lets you use the reserved words from, select, and where to write queries against collections in a way that is similar to database queries.

Of course, there are many more differences between C# and Java than can be covered in this article. Also, both Java and C# continue to evolve (for example, Java 8, which is not yet in the Android toolchain, supports C#-style lambda expressions) so these differences will change over time. Only the most important differences currently encountered by Java developers new to Xamarin.Android are outlined here.

Going from Java to C# Development provides an introduction to the fundamental differences between C# and Java.
Object-Oriented Programming Features outlines the most important object-oriented feature differences between the two languages.
Keyword Differences provides a table of useful keyword equivalents, C#-only keywords, and links to C# keyword definitions.

C# brings many key features to Xamarin.Android that are not currently readily available to Java developers on Android. These features can help you to write better code in less time:

Properties – With C#'s property system, you can access member variables safely and directly without having to write setter and getter methods.
Lambda Expressions – In C# you can use anonymous methods (also called lambdas) to express your functionality more succinctly and more efficiently. You can avoid the overhead of having to write one-time-use objects, and you can pass local state to a method without having to add parameters.
Event Handling – C# provides language-level support for event-driven programming, where an object can register to be notified when an event of interest occurs. The event keyword defines a multicast broadcast mechanism that a publisher class can use to notify event subscribers.
Asynchronous Programming – The asynchronous programming features of C# (async/await) keep apps responsive. The language-level support of this feature makes async programming easy to implement and less error-prone.

Finally, Xamarin allows you to leverage existing Java assets via a technology known as binding. You can call your existing Java code, frameworks, and libraries from C# by making use of Xamarin's automatic binding generators. To do this, you simply create a static library in Java and expose it to C# via a binding.

Note

Android programming uses a specific version of the Java language that supports all Java 7 features and a subset of Java 8.

Some features mentioned on this page (such as the var keyword in C#) are available in newer versions of Java (e.g. var in Java 10), but are still not available to Android developers.

Going from Java to C# development

The following sections outline the basic "getting started" differences between C# and Java; a later section describes the object-oriented differences between these languages.

Libraries vs. assemblies

Java typically packages related classes in .jar files. In C# and .NET, however, reusable bits of precompiled code are packaged into assemblies, which are typically packaged as .dll files. An assembly is a unit of deployment for C#/.NET code, and each assembly is typically associated with a C# project. Assemblies contain intermediate code (IL) that is just-in-time compiled at runtime.

For more information about assemblies, see the Assemblies and the Global Assembly Cache topic.

Packages vs. namespaces

C# uses the namespace keyword to group related types together; this is similar to Java's package keyword. Typically, a Xamarin.Android app will reside in a namespace created for that app. For example, the following C# code declares the WeatherApp namespace wrapper for a weather-reporting app:

namespace WeatherApp
{
    ...

Importing types

When you make use of types defined in external namespaces, you import these types with a using statement (which is very similar to the Java import statement). In Java, you might import a single type with a statement like the following:

import javax.swing.JButton

You might import an entire Java package with a statement like this:

import javax.swing.*

The C# using statement works in a very similar way, but it allows you to import an entire package without specifying a wildcard. For example, you will often see a series of using statements at the beginning of Xamarin.Android source files, as seen in this example:

using System;
using Android.App;
using Android.Content;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using System.Net;
using System.IO;
using System.Json;
using System.Threading.Tasks;

These statements import functionality from the System, Android.App, Android.Content, etc. namespaces.

Generics

Both Java and C# support generics, which are placeholders that let you plug in different types at compile time. However, generics work slightly differently in C#. In Java, type erasure makes type information available only at compile time, but not at run time. By contrast, the .NET common language runtime (CLR) provides explicit support for generic types, which means that C# has access to type information at runtime. In day-to-day Xamarin.Android development, the importance of this distinction is not often apparent, but if you are using reflection, you will depend on this feature to access type information at run time.

In Xamarin.Android, you will often see the generic method FindViewById used to get a reference to a layout control. This method accepts a generic type parameter that specifies the type of control to look up. For example:

TextView label = FindViewById<TextView> (Resource.Id.Label);

In this code example, FindViewById gets a reference to the TextView control that is defined in the layout as Label, then returns it as a TextView type.

For more information about generics, see the Generics topic. Note that there are some limitations in Xamarin.Android support for generic C# classes; for more information, see Limitations.

Object-oriented programming features

Both Java and C# use very similar object-oriented programming idioms:

All classes are ultimately derived from a single root object – all Java objects derive from java.lang.Object, while all C# objects derive from System.Object.
Instances of classes are reference types.
When you access the properties and methods of an instance, you use the "." operator.
All class instances are created on the heap via the new operator.
Because both languages use garbage collection, there is no way to explicitly release unused objects (i.e., there is not a delete keyword as there is in C++).
You can extend classes through inheritance, and both languages only allow a single base class per type.
You can define interfaces, and a class can inherit from (i.e., implement) multiple interface definitions.

However, there are also some important differences:

Java has two powerful features that C# does not support: anonymous classes and inner classes. (However, C# does allow nesting of class definitions – C#'s nested classes are like Java's static nested classes.)
C# supports C-style structure types (struct) while Java does not.
In C#, you can implement a class definition in separate source files by using the partial keyword.
C# interfaces cannot declare fields.
C# uses C++-style destructor syntax to express finalizers. The syntax is different from Java's finalize method, but the semantics are nearly the same. (Note that in C#, destructors automatically call the base-class destructor – in contrast to Java where an explicit call to super.finalize is used.)

Class inheritance

To extend a class in Java, you use the extends keyword. To extend a class in C#, you use a colon (:) to indicate derivation. For example, in Xamarin.Android apps, you will often see class derivations that resemble the following code fragment:

public class MainActivity : Activity
{
    ...

In this example, MainActivity inherits from the Activity class.

To declare support for an interface in Java, you use the implements keyword. However, in C#, you simply add interface names to the list of classes to inherit from, as shown in this code fragment:

public class SensorsActivity : Activity, ISensorEventListener
{
    ...

In this example, SensorsActivity inherits from Activity and implements the functionality declared in the ISensorEventListener interface. Note that the list of interfaces must come after the base class (or you will get a compile-time error). By convention, C# interface names are prepended with an upper-case "I"; this makes it possible to determine which classes are interfaces without requiring an implements keyword.

When you want to prevent a class from being further subclassed in C#, you precede the class name with sealed – in Java, you precede the class name with final.

For more about C# class definitions, see the Classes and Inheritance topics.

Properties

In Java, mutator methods (setters) and inspector methods (getters) are often used to control how changes are made to class members while hiding and protecting these members from outside code. For example, the Android TextView class provides getText and setText methods. C# provides a similar but more direct mechanism known as properties. Users of a C# class can access a property in the same way as they would access a field, but each access actually results in a method call that is transparent to the caller. This "under the covers" method can implement side effects such as setting other values, performing conversions, or changing object state.

Properties are often used for accessing and modifying UI (user interface) object members. For example:

int width = rulerView.MeasuredWidth;
int height = rulerView.MeasuredHeight;
...
rulerView.DrawingCacheEnabled = true;

In this example, width and height values are read from the rulerView object by accessing its MeasuredWidth and MeasuredHeight properties. When these properties are read, values from their associated (but hidden) field values are fetched behind the scenes and returned to the caller. The rulerView object may store width and height values in one unit of measurement (say, pixels) and convert these values on-the-fly to a different unit of measurement (say, millimeters) when the MeasuredWidth and MeasuredHeight properties are accessed.

The rulerView object also has a property called DrawingCacheEnabled – the example code sets this property to true to enable the drawing cache in rulerView. Behind the scenes, an associated hidden field is updated with the new value, and possibly other aspects of rulerView state are modified. For example, when DrawingCacheEnabled is set to false, rulerView may also erase any drawing cache information already accumulated in the object.

Access to properties can be read/write, read-only, or write-only. Also, you can use different access modifiers for reading and writing. For example, you can define a property that has public read access but private write access.

For more information about C# properties, see the Properties topic.

Calling base class methods

To call a base-class constructor in C#, you use a colon (:) followed by the base keyword and an initializer list; this base constructor call is placed immediately after the derived constructor parameter list. The base-class constructor is called on entry to the derived constructor; the compiler inserts the call to the base constructor at the start of the method body. The following code fragment illustrates a base constructor called from a derived constructor in a Xamarin.Android app:

public class PictureLayout : ViewGroup
{
    ...
    public PictureLayout (Context context)
           : base (context)
    {
        ...
    }
    ...
}

In this example, the PictureLayout class is derived from the ViewGroup class. The PictureLayout constructor shown in this example accepts a context argument and passes it to the ViewGroup constructor via the base(context) call.

To call a base-class method in C#, use the base keyword. For example, Xamarin.Android apps often make calls to base methods as shown here:

public class MainActivity : Activity
{
    ...
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

In this case, the OnCreate method defined by the derived class (MainActivity) calls the OnCreate method of the base class (Activity).

Access modifiers

Java and C# both support the public, private, and protected access modifiers. However, C# supports two additional access modifiers:

internal – The class member is accessible only within the current assembly.
protected internal – The class member is accessible within the defining assembly, the defining class, and derived classes (derived classes both inside and outside the assembly have access).

For more information about C# access modifiers, see the Access Modifiers topic.

Virtual and override methods

Both Java and C# support polymorphism, the ability to treat related objects in the same manner. In both languages, you can use a base-class reference to refer to a derived-class object, and the methods of a derived class can override the methods of its base classes. Both languages have the concept of a virtual method, a method in a base class that is designed to be replaced by a method in a derived class. Like Java, C# supports abstract classes and methods.

However, there are some differences between Java and C# in how you declare virtual methods and override them:

In C#, methods are non-virtual by default. Parent classes must explicitly label which methods are to be overridden by using the virtual keyword. By contrast, all methods in Java are virtual methods by default.
To prevent a method from being overridden in C#, you simply leave off the virtual keyword. By contrast, Java uses the final keyword to mark a method with "override is not allowed."
C# derived classes must use the override keyword to explicitly indicate that a virtual base-class method is being overridden.

For more information about C#'s support for polymorphism, see the Polymorphism topic.

Lambda expressions

C# makes it possible to create closures: inline, anonymous methods that can access the state of the method in which they are enclosed. Using lambda expressions, you can write fewer lines of code to implement the same functionality that you might have implemented in Java with many more lines of code.

Lambda expressions make it possible for you to skip the extra ceremony involved in creating a one-time-use class or anonymous class as you would in Java – instead, you can just write the business logic of your method code inline. Also, because lambdas have access to the variables in the surrounding method, you don't have to create a long parameter list to pass state to your method code.

In C#, lambda expressions are created with the => operator as shown here:

(arg1, arg2, ...) => {
    // implementation code
};

In Xamarin.Android, lambda expressions are often used for defining event handlers. For example:

button.Click += (sender, args) => {
    clickCount += 1;    // access variable in surrounding code
    button.Text = string.Format ("Clicked {0} times.", clickCount);
};

In this example, the lambda expression code (the code within the curly braces) increments a click count and updates the button text to display the click count. This lambda expression is registered with the button object as a click event handler to be called whenever the button is tapped. (Event handlers are explained in more detail below.) In this simple example, the sender and args parameters are not used by the lambda expression code, but they are required in the lambda expression to meet the method signature requirements for event registration. Under the hood, the C# compiler translates the lambda expression into an anonymous method that is called whenever button click events take place.

For more information about C# and lambda expressions, see the Lambda Expressions topic.

Event handling

An event is a way for an object to notify registered subscribers when something interesting happens to that object. Unlike in Java, where a subscriber typically implements a Listener interface that contains a callback method, C# provides language-level support for event handling through delegates. A delegate is like an object-oriented type-safe function pointer – it encapsulates an object reference and a method token. If a client object wants to subscribe to an event, it creates a delegate and passes the delegate to the notifying object. When the event occurs, the notifying object invokes the method represented by the delegate object, notifying the subscribing client object of the event. In C#, event handlers are essentially nothing more than methods that are invoked through delegates.

For more information about delegates, see the Delegates topic.

In C#, events are multicast; that is, more than one listener can be notified when an event takes place. This difference is observed when you consider the syntactical differences between Java and C# event registration. In Java you call SetXXXListener to register for event notifications; in C# you use the += operator to register for event notifications by "adding" your delegate to the list of event listeners. In Java, you call SetXXXListener to unregister, while in C# you use the -= to "subtract" your delegate from the list of listeners.

In Xamarin.Android, events are often used for notifying objects when a user does something to a UI control. Normally, a UI control will have members that are defined using the event keyword; you attach your delegates to these members to subscribe to events from that UI control.

To subscribe to an event:

Create a delegate object that refers to the method that you want to be invoked when the event occurs.
Use the += operator to attach your delegate to the event you are subscribing to.

The following example defines a delegate (with an explicit use of the delegate keyword) to subscribe to button clicks. This button-click handler starts a new activity:

startActivityButton.Click += delegate {
    Intent intent = new Intent (this, typeof (MyActivity));
    StartActivity (intent);
};

However, you also can use a lambda expression to register for events, skipping the delegate keyword altogether. For example:

startActivityButton.Click += (sender, e) => {
    Intent intent = new Intent (this, typeof (MyActivity));
    StartActivity (intent);
};

In this example, the startActivityButton object has an event that expects a delegate with a certain method signature: one that accepts sender and event arguments and returns void. However, because we don't want to go to the trouble to explicitly define such a delegate or its method, we declare the signature of the method with (sender, e) and use a lambda expression to implement the body of the event handler. Note that we have to declare this parameter list even though we aren't using the sender and e parameters.

It is important to remember that you can unsubscribe a delegate (via the -= operator), but you cannot unsubscribe a lambda expression – attempting to do so can cause memory leaks. Use the lambda form of event registration only when your handler will not unsubscribe from the event.

Typically, lambda expressions are used to declare event handlers in Xamarin.Android code. This shorthand way to declare event handlers may seem cryptic at first, but it saves an enormous amount of time when you are writing and reading code. With increasing familiarity, you become accustomed to recognizing this pattern (which occurs frequently in Xamarin.Android code), and you spend more time thinking about the business logic of your application and less time wading through syntactical overhead.

Asynchronous programming

Asynchronous programming is a way to improve the overall responsiveness of your application. Asynchronous programming features make it possible for the rest of your app code to continue running while some part of your app is blocked by a lengthy operation. Accessing the web, processing images, and reading/writing files are examples of operations that can cause an entire app to appear to freeze up if it is not written asynchronously.

C# includes language-level support for asynchronous programming via the async and await keywords. These language features make it very easy to write code that performs long-running tasks without blocking the main thread of your application. Briefly, you use the async keyword on a method to indicate that the code in the method is to run asynchronously and not block the caller's thread. You use the await keyword when you call methods that are marked with async. The compiler interprets the await as the point where your method execution is to be moved to a background thread (a task is returned to the caller). When this task completes, execution of the code resumes on the caller's thread at the await point in your code, returning the results of the async call. By convention, methods that run asynchronously have Async suffixed to their names.

In Xamarin.Android applications, async and await are typically used to free up the UI thread so that it can respond to user input (such as the tapping of a Cancel button) while a long-running operation takes place in a background task.

In the following example, a button click event handler causes an asynchronous operation to download an image from the web:

downloadButton.Click += downloadAsync;
...
async void downloadAsync(object sender, System.EventArgs e)
{
    webClient = new WebClient ();
    var url = new Uri ("http://photojournal.jpl.nasa.gov/jpeg/PIA15416.jpg");
    byte[] bytes = null;

    bytes = await webClient.DownloadDataTaskAsync(url);

    // display the downloaded image ...

In this example, when the user clicks the downloadButton control, the downloadAsync event handler creates a WebClient object and a Uri object to fetch an image from the specifed URL. Next, it calls the WebClient object's DownloadDataTaskAsync method with this URL to retrieve the image.

Notice that the method declaration of downloadAsync is prefaced by the async keyword to indicate that it will run asynchronously and return a task. Also note that the call to DownloadDataTaskAsync is preceded by the await keyword. The app moves the execution of the event handler (starting at the point where await appears) to a background thread until DownloadDataTaskAsync completes and returns. Meanwhile, the app's UI thread can still respond to user input and fire event handlers for the other controls. When DownloadDataTaskAsync completes (which may take several seconds), execution resumes where the bytes variable is set to the result of the call to DownloadDataTaskAsync, and the remainder of the event handler code displays the downloaded image on the caller's (UI) thread.

For an introduction to async/await in C#, see the Asynchronous Programming with Async and Await topic. For more information about Xamarin support of asynchronous programming features, see Async Support Overview.

Keyword differences

Many language keywords used in Java are also used in C#. There are also a number of Java keywords that have an equivalent but differently-named counterpart in C#, as listed in this table:

Java	C#	Description
`boolean`	bool	Used for declaring the boolean values true and false.
`extends`	`:`	Precedes the class and interfaces to inherit from.
`implements`	`:`	Precedes the class and interfaces to inherit from.
`import`	using	Imports types from a namespace, also used for creating a namespace alias.
`final`	sealed	Prevents class derivation; prevents methods and properties from being overridden in derived classes.
`instanceof`	is	Evaluates whether an object is compatible with a given type.
`native`	extern	Declares a method that is implemented externally.
`package`	namespace	Declares a scope for a related set of objects.
`T...`	params T	Specifies a method parameter that takes a variable number of arguments.
`super`	base	Used to access members of the parent class from within a derived class.
`synchronized`	lock	Wraps a critical section of code with lock acquisition and release.

Also, there are many keywords that are unique to C# and have no counterpart in the Java used on Android. Xamarin.Android code often makes use of the following C# keywords (this table is useful to refer to when you are reading through Xamarin.Android sample code.

C#	Description
as	Performs conversions between compatible reference types or nullable types.
async	Specifies that a method or lambda expression is asynchronous.
await	Suspends the execution of a method until a task completes.
byte	Unsigned 8-bit integer type.
delegate	Used to encapsulate a method or anonymous method.
enum	Declares an enumeration, a set of named constants.
event	Declares an event in a publisher class.
fixed	Prevents a variable from being relocated.
`get`	Defines an accessor method that retrieves the value of a property.
in	Enables a parameter to accept a less derived type in a generic interface.
object	An alias for the Object type in the .NET framework.
out	Parameter modifier or generic type parameter declaration.
override	Extends or modifies the implementation of an inherited member.
partial	Declares a definition to be split into multiple files, or splits a method definition from its implementation.
readonly	Declares that a class member can be assigned only at declaration time or by the class constructor.
ref	Causes an argument to be passed by reference rather than by value.
set	Defines an accessor method that sets the value of a property.
string	Alias for the String type in the .NET framework.
struct	A value type that encapsulates a group of related variables.
typeof	Obtains the type of an object.
var	Declares an implicitly-typed local variable.
value	References the value that client code wants to assign to a property.
virtual	Allows a method to be overridden in a derived class.

Interoperating with existing java code

If you have existing Java functionality that you do not want to convert to C#, you can reuse your existing Java libraries in Xamarin.Android applications via two techniques:

Create a Java Bindings Library – Using this approach, you use Xamarin tools to generate C# wrappers around Java types. These wrappers are called bindings. As a result, your Xamarin.Android application can use your .jar file by calling into these wrappers.
Java Native Interface – The Java Native Interface (JNI) is a framework that makes it possible for C# apps to call or be called by Java code.

For more information about these techniques, see Java Integration Overview.

Summary

This article provided an introduction to the Xamarin.Android C# programming environment from a Java developer's perspective. It pointed out the similarities between C# and Java while explaining their practical differences. It introduced assemblies and namespaces, explained how to import external types, and provided an overview of the differences in access modifiers, generics, class derivation, calling base-class methods, method overriding, and event handling. It introduced C# features that are not available in Java, such as properties, async/await asynchronous programming, lambdas, C# delegates, and the C# event handling system. It included tables of important C# keywords, explained how to interoperate with existing Java libraries, and provided links to related documentation for further study.

Споделяне чрез

Xamarin for Java developers

Overview

Going from Java to C# development

Libraries vs. assemblies

Packages vs. namespaces

Importing types

Generics

Object-oriented programming features

Class inheritance

Properties

Calling base class methods

Access modifiers

Virtual and override methods

Lambda expressions

Event handling

Asynchronous programming

Keyword differences

Interoperating with existing java code

Further reading

Summary

Допълнителни ресурси

Споделяне чрез

Xamarin for Java developers

Overview

Going from Java to C# development

Libraries vs. assemblies

Packages vs. namespaces

Importing types

Generics

Object-oriented programming features

Class inheritance

Properties

Calling base class methods

Access modifiers

Virtual and override methods

Lambda expressions

Event handling

Asynchronous programming

Keyword differences

Interoperating with existing java code

Further reading

Summary

Related links

Допълнителни ресурси