Comparison of C Sharp and Java: Difference between revisions

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This is a comparison of the C# programming language with the Java programming language. As two modern garbage-collected runtime-compiled languages derived from C and C++, Java and C# are very similar. This page documents the strong general similarities of the languages and then points out those instances where the languages differ. Both languages were designed carefully, and if one language has a feature another lacks it is the result of a conscious design decision. Thus, the reader is advised to avoid the temptation to 'keep score,' and instead think about why the designers made each decision.

Language

Object handling

Both languages are object oriented from the ground up, with a syntax similar to C++. (C++ in turn is derived from C.) Neither language is a superset of C or C++, however. Both use garbage collection as a means of reclaiming memory resources, rather than formal deallocation of memory. And both include thread synchronization mechanisms as part of their language syntax.

Both Java and C# have strong and weak object references. Java allows registering a listener that will be notified when a reference is garbage collected, which allows for the good performance of the WeakHashMap that C# lacks. C# only supports this by using a finalizer (which is also available in Java). C# on the other hand allows the programmer to suppress the finalizer of a specific object (e.g., an SQL connection or a file stream that always needs to be properly closed). This can be very useful since finalizers are expensive in generational garbage collection (applied by both Java and .NET), and are often used only as a fail-safe for when the programmer does not close the object. An object with a finalizer will normally be promoted to an extra generation and kept alive longer before it is collected.

C# allows restricted use of pointers, which are considered unsafe by some language designers. C# addresses that concern by requiring that code blocks or methods that use the feature be marked with the unsafe keyword, so that all clients of such code can be aware that the code may be less secure than otherwise. The compiler requires the /unsafe switch to allow compilation of a program that uses such code. Generally, unsafe code is only used to allow better interoperability with unmanaged APIs or system calls (which are inherently "unsafe"), or sometimes as a small performance enhancement.

Data types

Both languages support the idea of primitive types (known as value types in C#). C# has more primitive types than Java, with unsigned as well as signed integer types being supported, and a special decimal type for decimal floating-point calculations. Java lacks unsigned types. In particular, Java lacks a primitive type for an unsigned byte. Strings are treated as (immutable) objects in both languages, but support for string literals provides a specialised means of constructing them. C# also allows verbatim strings for quotation without escape sequences, which also allow newlines.

Both allow boxing and unboxing to translate primitive data to and from their object form. Effectively, this makes the primitive types a subtype of the Object type. In C# this also means that the primitive types can define methods, such as an override of Object's ToString() method. In Java, separate primitive wrapper classes are provided for this, which means it requires a static call Integer.toString(42) instead of an instance call 42.ToString(). Another difference is that Java makes heavy use of such types in generics, and as such allows an implicit unboxing conversion (in C# this requires a cast). This conversion can potentially throw an (unchecked) null pointer exception, which may not be obvious by inspection of the code in Java.

C# allows the programmer to create user-defined value types, using the struct keyword. From the programmer's perspective, they can be seen as lightweight classes. Unlike regular classes, and like the standard primitives, such value types are allocated on the stack rather than on the heap. They can also be part of an object (either as a field or boxed), or stored in an array, without the memory indirection that normally exists for class types. Structs also come with a number of limitations. Because structs have no notion of a null value and can be used in arrays without initialization, they always come with an implicit default constructor that essentially fills the struct memory space with zeroes. The programmer can only define additional constructors with one or more arguments. This also means that structs lack a virtual method table, and because of that (and the fixed memory footprint), they cannot allow inheritance (but can implement interfaces).

Enumerations in C# are derived from a primitive integer type. Any value of the underlying primitive type is a valid value of the enumeration type, though an explicit cast may be needed to assign it. This allows to combine enumeration values together using the bitwise OR operator if they are bit flags. Enumeration values in Java, on the other hand, are objects. The only valid values in a Java enumeration are the ones listed in the enumeration. A special enumeration set object is needed to combine enumeration values together. Java enumerations allow differing method implementations for each value in the enumeration. Both C# and Java enumerations can be converted to strings, but only Java enumerations allow to customize such conversion.

Arrays

Array and collection types are also given significance in the syntax of both languages, thanks to an iterator-based foreach statement loop. In both languages an array corresponds to an object of the Array class, although in Java this class does not implement any of the collection interfaces. C# has true multi-dimensional arrays, as well as the arrays-of-arrays that are available in Java (and which in C# are commonly called jagged arrays). Multi-dimensional arrays can in some cases increase performance because of increased locality (as there is a single pointer dereference, instead of one for every dimension of the array as is the case for jagged arrays). Another advantage is that the entire multi-dimensional array can be allocated with a single application of operator new, while jagged arrays require explicit loops and allocations for every dimension.

Inner classes

Both languages allow inner classes, where a class is defined entirely within another class. In Java, these classes have access to both the static and non-static members of the outer class (unless the inner class is declared static, then it only has access to the static members). Local classes can be defined within a method and have read-only access to the method's local variables, and anonymous local classes allow to create class instances overriding some of their class methods.

C# only provides static inner classes, and requires an explicit reference to the outer class to its non-static members; no local inner classes are supported. Instead, C# provides anonymous delegates as a construct that can provide access to local variables and members (see Event handling).

Generics

Further information: Generic programming

Generics in Java are implemented by using type erasure. This means that information about the generic types is ignored at runtime, and only obtainable via reflection on the static class objects. In .NET 2.0, information on generic types is fully preserved at runtime. Java's approach requires additional run time type checks, does not guarantee that generic contract will be followed, and lacks reflection on the generic types. Java does not allow to specialize generic classes with primitive types, while C# allows generics for both reference types and value types, including primitive types. Java instead allows the use of boxed types as type parameters (e.g., List<Integer> instead of List<int>), but this comes at a cost since all such values need to be heap-allocated. In both Java and C#, generic specializations that use different reference types share equivalent underlying code, but for C# the CLR dynamically generates optimized code for specializations on value types. In .NET, type safety for generics is fully checked at compile time and enforced at load time by the CLR. In Java, it is only partially checked at compile time and needs to be enforced using cast operations at run time, as the Java VM is not aware of generic types.

Notation and special features

Special feature keywords

keyword	feature, example usage
get, set	C# implements properties as part of the language syntax, as an alternative for the accessor methods used in Java.
out, ref	C# has support for output and reference parameters. These allow returning multiple output values from a method, or passing values by reference.
switch	In C#, the switch statement also operates on strings and long. Java switch statement does not operate on long primitive type.
strictfp	Java uses strictfp to guarantee the result of floating point operations remain the same across platforms.
checked, unchecked	In C#, checked statement blocks or expressions can enable run-time checking for arithmetic overflow.
using	C#'s using forces the object created to be disposed or closed after the code block has run. // Create a small file "test.txt", write a string, and close it (even if an exception occurs) using (StreamWriter file = new StreamWriter("test.txt")) { file.Write("test"); }
goto	C# supports the goto keyword. This can occasionally be useful, but the use of a more structured method of control flow is usually recommended. A common use of the goto keyword in C# is to transfer control to a different case label within a switch statement. Both C# and Java allow qualified breaks and continues, which make up for many of the uses of goto. switch(color) { case Color.Blue: Console.WriteLine("Color is blue"); break; case Color.DarkBlue: Console.WriteLine("Color is dark"); goto case Color.Blue; // ... }

Event handling

Java requires the programmer to implement the observer pattern manually, though it provides some syntactic sugar in form of anonymous inner classes, which allow one to define the body of the class and create an instance of it in a single point in the code. This is typically used to create observers.

C# provides extensive support for event-driven programming at language level, including delegate types. These are type-safe references to methods and can be combined to allow multicasting. To support them there is a special syntax to define events in classes and operators to register, unregister or combine event handlers. Delegates support covariance and contravariance, and can be created as anonymous methods with full-featured closure semantics.

Closures have also been proposed as a new feature for Java SE 7. Like delegates in C#, such closures would have full access to all local variables in scope, not just read-only access to those marked final (as with anonymous inner classes).

Numeric applications

To adequately support applications in the field of mathematic and financial computation, several language features exist. In this category, Java provides the strictfp keyword, that enables strict floating-point calculations for a region of code. This will ensure that calculations return the exact same result on all platforms. C# provides no equivalent, but does provide the built-in decimal type, for accurate decimal floating-point calculations. This forgoes the problems that exist with binary floating-point representations (float, double). Such binary representations are not suited to accurately represent decimal numbers and hence introduce rounding errors. For financial applications, an accurate decimal type is essential. Since Java 5.0, the BigDecimal class also provides such characteristics for Java. BigDecimal and BigInteger are types provided with Java that allow arbitrary-precision representation of numbers. The .NET framework does not currently include such classes, although third party implementations exist as well as plans to include such types for the next framework version.

In Java there is no way to provide the same level of integration for library-defined types such as BigDecimal or complex numbers as there is for the primitive types. For this purpose, C# provides the following:

• Operator overloading and indexers providing convenient syntax (see below).
• Implicit and explicit conversions; allow conversions such as exist for the built-in int type that can implicitly convert to long.
• Valuetypes and generics based on valuetypes; in Java every custom type must be allocated on the heap, which is detrimental for performance of both custom types and collections.

In addition to this, C# can help mathematic applications with the checked and unchecked operators that allow to enable or disable run-time checking for arithmetic overflow for a region of code. It also offers rectangular arrays, that have advantages over regular nested arrays for certain applications.

Operator overloading

C# includes a large number of notational conveniences over Java, many of which, such as operator overloading and user-defined casts, are already familiar to the large community of C++ programmers. It also has "Explicit Member Implementation" which allows a class to specifically implement methods of an interface, separate to its own class methods, or to provide different implementations for two methods with the same name and signature inherited from two base interfaces.

C# includes indexers which can be considered a special case of operator overloading (like C++ operator[]), or parametrized get/set properties. An indexer is a property named this[] which uses one or more parameters (indexes); the indexes can be objects of any type:

myList[4] = 5;
string name = xmlNode.Attributes["name"];
orders = customerMap[theCustomer]; 

Java does not include operator overloading in order to prevent abuse of the feature, and to keep the language simpler. C# allows operator overloading (subject to certain restrictions to ensure logical coherence), which, when used carefully, can make code terser and more readable.

Methods

Methods in C# are non-virtual by default, and have to be declared virtual explicitly if desired. In Java, non-final methods are virtual by default. Virtualness guarantees that the most recent override for the method will always be called, but incurs a certain runtime cost on invocation as these invocations cannot be normally inlined, and require an indirect call via the virtual method table. Some JVM implementations, including the Sun reference implementation, implement inlining of the most commonly called virtual methods to reduce the runtime overhead from virtual method calls.

In Java there is no way to make methods non-virtual (although they can be "sealed" by using the final modifier to disallow overriding). This means that there is no way to let derived classes define a new, unrelated method with the same name. This can be a problem when a base class is designed by a different person, and a new version introduces a method with the same name and signature as some method already present in the derived class. In Java, this will mean that the method in the derived class will implicitly override the method in the base class, even though that was not the intent of the designers of either class. To prevent this versioning problem, C# requires explicit declaration of intent when overriding virtual methods in a derived class. If a method should be overridden, the override modifier must be specified. If overriding is not desired, and the class designer merely wishes to introduce a new method shadowing the old one, the new keyword must be specified.

To partially accommodate for these versioning problems, Java 5.0 introduced the @Override annotation, but to preserve backwards compatibility it could not be made compulsory, so it cannot prevent the above accidental overriding situation. Like the override keyword in C#, it can however help ensure that a method in the base class with the same signature exists and is correctly overridden.

Conditional compilation

Unlike Java, C# implements conditional compilation using preprocessor directives. It also provides a Conditional attribute to define methods that are only called when a given compilation constant is defined. This way, assertions can be provided as a framework feature with the method Debug.Assert(), which is only evaluated when the DEBUG constant is defined. Since version 1.4, Java provides a language feature for assertions that can be enabled and disabled at runtime.

Namespaces and source files

C#'s namespaces are similar to those in C++. Unlike package names in Java, a namespace is not in any way tied to location of the source file. While it's not strictly necessary for a Java source file location to mirror its package directory structure, it is the default behavior.

Both languages allow importing of classes (e.g., import java.util.* in Java), allowing a class to be referenced using only its name. Sometimes classes with the same name exist in multiple namespaces or packages. Such classes can be referenced by using fully qualified names, or by importing only selected classes with different names. To do this, Java allows importing a single classes (e.g., import java.util.List). C# allows importing classes under a new local name using the following syntax: using Console = System.Console. It also allows importing specializations of classes in the form of using IntList = System.Collections.Generic.List<int>.

Java has a static import syntax that allows using the short name of some or all of the static methods/fields in a class (e.g., allowing foo(bar) where foo() can be statically imported from another class). C# has a static class syntax (not to be confused with static inner classes in Java), which restricts a class to only contain static methods. C# 3.5 will introduce extension methods to allow users to statically add a method to a type (e.g., allowing foo.bar() where bar() can be an imported extension method working on the type of foo).

Java requires that a source file name must match the only public class inside it, while C# allows multiple public classes in the same file, and puts no restrictions on the file name. As of Version 2, C# allows a class definition to be split into several files, by using the partial keyword in the source code.

Exception handling

Java supports checked exceptions (in addition to unchecked exceptions). C# only supports unchecked exceptions. Checked exceptions enforce the programmer to declare all exceptions thrown in a method, and to catch all exceptions thrown by a method invocation.

Some would argue that checked exceptions are very helpful for good programming practice, ensuring that all errors are dealt with. Others, including Anders Hejlsberg, chief C# language architect, argue that they were to some extent an experiment in Java and that they haven't been shown to be worthwhile except for in small example programs. One criticism is that checked exceptions encourage programmers to use an empty catch block, resulting only in code bloat: catch (Exception e) {}. Another criticism of checked exceptions is that a new implementation of a method may cause unanticipated checked exceptions to be thrown, which is a contract-breaking change. This can happen in methods implementing an interface that only declares limited exceptions, or when the underlying implementation of a method changes. To allow for such unanticipated exceptions to be thrown, some programmers simply declare the method can throw any type of exception ("throws Exception"), which defeats the purpose of checked exceptions. In some cases however, exception chaining can be applied instead; re-throwing the exception in a wrapper exception. For example, if an object is changed to access a database instead of a file, an SQLException could be caught and re-thrown as an IOException, since the caller may not need to know the inner workings of the object.

There are also differences between the two languages in treating the try-finally statement. The finally is always executed, even if the try block contains control-passing statements like throw or return. This may result in unexpected behavior, especially if try-code and finally-code return different values. C# resolves this problem by prohibiting any control-passing statements like return or break in the finally block.

Lower level code

The Java Native Interface (JNI) feature allows Java programs to call non-Java code. However, JNI does require the code to be called to follow several conventions and impose restrictions on types and names used. This means that an extra adaption layer between legacy code and Java is often needed. This adaption code must be coded in a non-Java language, often C or C++.

In addition, third party libraries provide for Java-COM bridging, e.g. JACOB (free), and J-Integra for COM (proprietary).

.NET Platform Invoke (P/Invoke) offers the same capability by allowing calls from C# to what Microsoft refers to as unmanaged code. Through metadata attributes the programmer can control exactly how the parameters and results are marshalled, thus avoiding the need for extra adaption code. P/Invoke allows almost complete access to procedural APIs (such as Win32 or POSIX), but no direct access to legacy C++ class libraries.

In addition, .NET Framework also provides a .NET-COM bridge, allowing access to COM components as if they were native .NET objects.

C# also allows the programmer to disable the normal type-checking and other safety features of the CLR, which then enables the use of pointer variables. When this feature is used, the programmer must mark the code using the unsafe keyword. JNI, P/Invoke, and "unsafe" code are equally risky features, exposing possible security holes and application instability. An advantage of unsafe, managed code over P/Invoke or JNI is that it allows the programmer to continue to work in the familiar C# environment to accomplish some tasks that otherwise would require calling out to unmanaged code. A program or assembly using unsafe code must be compiled with a special switch and will be marked as such. This enables runtime environments to take special precautions before executing potentially harmful code.

History

Popularity and evolution

Java is older than C# and has built up a large and highly active user base, becoming the lingua franca in many modern branches of computer science, particularly areas which involve networking. Java dominates programming courses at high school and college level in the United States, and there are currently more Java than C# books [2]. Java's maturity and popularity have ensured more third party Java API and libraries (many of them open source) than C# [3].

By contrast, C# is a relatively new language. Microsoft has studied existing languages such as Java and Delphi, and has changed some aspects of the language and runtime environment in response to perceived failures and difficulties with its predecessors.

An occasionally voiced criticism of the Java language is that it evolves slowly, lacking some features which make fashionable programming patterns and methodologies easier. An occasionally voiced criticism of the C# language is that its designers are perhaps too quick to pander to current trends in programming - lacking focus and simplicity. Certainly Java's designers seem to have taken a more conservative stand on adding major new features to their language syntax than other current languages - perhaps not wanting to tie the language too tightly with trends which may in the medium/long term be dead ends. With the Java 5.0 release, this trend may have been broken, as it introduced several new major language features: foreach statement, autoboxing, methods with variable number of parameters (varargs), enumerated types, generic types, and annotations. It should be noted that all of those, except generic types, were already present in C# by that time, some under different names.

C# has evolved rapidly to attempt to streamline development for problem-specific features. C# 3.0 adds a SQL-like language integrated queries suited for querying data from collections, databases or XML documents. It however builds upon general-purpose language features; lambda expressions and extension methods features, that also allow such queries to be expressed and optimized for user types. Problem-specific language additions to Java have been considered and, for now at least, rejected. This approach, along with a number of other new languages and technologies which address themselves specifically at current programming trends, has sparked a renewed debate within the Java camp about the future direction of the Java language and whether its 'conservative evolution' is right.

Marketplace

Since C#'s inception the two languages have been compared and contrasted. It is undeniable that C# and the CLR (Common Language Runtime) managed code environment in which most C# applications run owe a lot to Java and the JRE (Java Runtime Environment). However C# also accommodates constructs more commonly found in languages such as C++, Delphi (designing which was Anders Hejlsberg's principal job when he was at Borland), and, in recent C# versions, borrowing from dynamic scripting languages such as Ruby.

It can be argued that Microsoft developed C# in some part as a result of recognizing that the managed code environment which Java champions has many virtues in an increasingly networked world, particularly as the internet moves out onto devices other than personal computers, and security becomes ever more important. Before creating C#, Microsoft implemented a modified Java environment, called J++, adding new features in a manner which was in direct contravention to the standards and conventions ensuring the platform neutrality which lies at the heart of Java. This violated the license agreement Microsoft had signed, requiring that standards and specifications be strictly adhered to in return for using the Java name and brand logos. Sun Microsystems sued, and won, thus preventing Microsoft from further production of J++. With the release of the .NET framework (and C#), the project was revived in the form of J#.

Despite this fractious start, as time has passed it has become apparent that the two languages rarely compete one-on-one in the marketplace. Java dominates the mobile sector, and has a large following within the web based applications market. C# is well recognized in the Windows desktop market, and is being pushed by Microsoft as the language for Windows applications. C# is also a player in the web application market, thanks to ASP.NET.

See also

• Comparison of C# and VB.NET
• Comparison of Java and C++
• Java programming language
• C#
• Comparison of the Java and .Net platforms

External links

• Contrasting C# and Java Syntax
• Java vs. C# - Code for Code Comparison
• Nine Language Performance Round-up
• Java and C-Sharp Compared
• MSDN: The C# Programming Language for Java Developers