Difference between namespace in C# and package in Java

From: http://www.javacamp.org/javavscsharp/namespace.html

---

Java

Packages are used to organize files or public types to avoid type conflicts. Package constructs can be mapped to a file system.

system.security.cryptography.AsymmetricAlgorithm aa;

may be replaced:

import system.security.Crypography; 
class xxx { ...
AsymmetricAlgorithm aa;

There is no alias for packages. You have to use import statement or fully-qualified name to mention the specific type.

package n1.n2;
    class A {}
    class B {}

or

package n1.n2;
   class A {}

Another source file:

package n1.n2;
   class B {}

Package cannot be nested. One source file can only have one package statement.

C#

Namespaces are used to organize programs, both as an "internal" organization system for a program, and as an "external" organization system.

System.Security.Cryptography.AsymmetricAlgorithm aa;

may be replaced:

using System.Security.Crypography; 
AsymmetricAlgorithm aa;

Alternatively, one could specify an alias for the the namespace, eg

using myAlias = System.Security.Crypography; 

and then refer to the class with

myAlias.AsymmetricAlgorithm 

namespace N1.N2
{
    class A {}
    class B {}
}

or

namespace N1
{
    namespace N2
    {
        class A {}
        class B {}
    }
}

There are a few details that differ.

In Java the directory structure should match the package structure. No such restriction in C#.

In C# you can have multiple namespaces in one file. In Java one file belongs to one package (see previous).

Java has default/package accessibility. C# internal accessibility goes in assemblies.

If you use VS and Eclipse and let them structure the project, then you will not feel the differences much.

There's no such term as "namespace" in Java - a package acts as a namespace in Java though, in terms of providing a scope for names. It's also part of the accessibility model.

From section 7 of the Java Language Specification:

Programs are organized as sets of packages. Each package has its own set of names for types, which helps to prevent name conflicts. A top level type is accessible (§6.6) outside the package that declares it only if the type is declared public.

EDIT: Okay, after the clarification: a Java package is similar to a C# namespace - except that it has an impact on accessibility, whereas in C# namespaces and accessibility are entirely orthogonal.