C＃和Java有什么主要区别？

Question

I just want to clarify one thing. This is not a question on which one is better, that part I leave to someone else to discuss. I don't care about it. I've been asked this question on my job interview and I thought it might be useful to learn a bit more.

These are the ones I could come up with:

• Java is "platform independent". Well nowadays you could say there is the Mono project so C# could be considered too but I believe it is a bit exaggerating. Why? Well, when a new release of Java is done it is simultaneously available on all platforms it supports, on the other hand how many features of C# 3.0 are still missing in the Mono implementation? Or is it really CLR vs. JRE that we should compare here?
• Java doesn't support events and delegates. As far as I know.
• In Java all methods are virtual
• Development tools: I believe there isn't such a tool yet as Visual Studio. Especially if you've worked with team editions you'll know what I mean.

Please add others you think are relevant.

Update: Just popped up my mind, Java doesn't have something like custom attributes on classes, methods etc. Or does it?

Answer 1

Comparing Java 7 and C# 3

(Some features of Java 7 aren't mentioned here, but the using statement advantage of all versions of C# over Java 1-6 has been removed.)

Not all of your summary is correct:

• In Java methods are virtual by default but you can make them final. (In C# they're sealed by default, but you can make them virtual.)
• There are plenty of IDEs for Java, both free (eg Eclipse, Netbeans) and commercial (eg IntelliJ IDEA)

Beyond that (and what's in your summary already):

• Generics are completely different between the two; Java generics are just a compile-time "trick" (but a useful one at that). In C# and .NET generics are maintained at execution time too, and work for value types as well as reference types, keeping the appropriate efficiency (eg a List<byte> as a byte[] backing it, rather than an array of boxed bytes.)
• C# doesn't have checked exceptions
• Java doesn't allow the creation of user-defined value types
• Java doesn't have operator and conversion overloading
• Java doesn't have iterator blocks for simple implemetation of iterators
• Java doesn't have anything like LINQ
• Partly due to not having delegates, Java doesn't have anything quite like anonymous methods and lambda expressions. Anonymous inner classes usually fill these roles, but clunkily.
• Java doesn't have expression trees
• C# doesn't have anonymous inner classes
• C# doesn't have Java's inner classes at all, in fact - all nested classes in C# are like Java's static nested classes
• Java doesn't have static classes (which don't have any instance constructors, and can't be used for variables, parameters etc)
• Java doesn't have any equivalent to the C# 3.0 anonymous types
• Java doesn't have implicitly typed local variables
• Java doesn't have extension methods
• Java doesn't have object and collection initializer expressions
• The access modifiers are somewhat different - in Java there's (currently) no direct equivalent of an assembly, so no idea of "internal" visibility; in C# there's no equivalent to the "default" visibility in Java which takes account of namespace (and inheritance)
• The order of initialization in Java and C# is subtly different (C# executes variable initializers before the chained call to the base type's constructor)
• Java doesn't have properties as part of the language; they're a convention of get/set/is methods
• Java doesn't have the equivalent of "unsafe" code
• Interop is easier in C# (and .NET in general) than Java's JNI
• Java and C# have somewhat different ideas of enums. Java's are much more object-oriented.
• Java has no preprocessor directives (#define, #if etc in C#).
• Java has no equivalent of C#'s ref and out for passing parameters by reference
• Java has no equivalent of partial types
• C# interfaces cannot declare fields
• Java has no unsigned integer types
• Java has no language support for a decimal type. (java.math.BigDecimal provides something like System.Decimal - with differences - but there's no language support)
• Java has no equivalent of nullable value types
• Boxing in Java uses predefined (but "normal") reference types with particular operations on them. Boxing in C# and .NET is a more transparent affair, with a reference type being created for boxing by the CLR for any value type.

This is not exhaustive, but it covers everything I can think of off-hand.

Answer 2

The following is a great in depth reference by Dare Obasanjo on the differences between C# and Java. I always find myself referring to this article when switching between the two.

http://www.25hoursaday.com/CsharpVsJava.html

Answer 3

C＃具有非常方便的自动属性，它们还有助于保持代码清洁，至少在getter和setter中没有自定义逻辑时。

Answer 4

Features of C# Absent in Java • C# includes more primitive types and the functionality to catch arithmetic exceptions.

• 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.

• Event handling is a "first class citizen"—it is part of the language itself.

• Allows the definition of "structs", which are similar to classes but may be allocated on the stack (unlike instances of classes in C# and Java).

• C# implements properties as part of the language syntax.

• C# allows switch statements to operate on strings.

• C# allows anonymous methods providing closure functionality.

• C# allows iterator that employs co-routines via a functional-style yield keyword.

• C# has support for output parameters, aiding in the return of multiple values, a feature shared by C++ and SQL.

• C# has the ability to alias namespaces.

• C# has "Explicit Member Implementation" which allows a class to specifically implement methods of an interface, separate from its own class methods. This allows it also to implement two different interfaces which happen to have a method of the same name. The methods of an interface do not need to be public; they can be made to be accessible only via that interface.

• C# provides integration with COM.

• Following the example of C and C++, C# allows call by reference for primitive and reference types.

Features of Java Absent in C#

• Java's strictfp keyword guarantees that the result of floating point operations remain the same across platforms.

• Java supports checked exceptions for better enforcement of error trapping and handling.

Answer 5

Another good resource is http://www.javacamp.org/javavscsharp/ This site enumerates many examples that ilustrate almost all the differences between these two programming languages.

About the Attributes, Java has Annotations, that work almost the same way.

Answer 6

Generics:

With Java generics, you don't actually get any of the execution efficiency that you get with .NET because when you compile a generic class in Java, the compiler takes away the type parameter and substitutes Object everywhere. For instance if you have a Foo<T> class the java compiler generates Byte Code as if it was Foo<Object> . This means casting and also boxing/unboxing will have to be done in the "background".

I've been playing with Java/C# for a while now and, in my opinion, the major difference at the language level are, as you pointed, delegates.

Answer 7

请查看下面给出的链接msdn.microsoft.com/en-us/library/ms836794.aspx它涵盖了C＃和java之间的相似之处和区别