Java is both a programming language as well as a well 
developed technology.It was found by Sun MicroSystems..
.Net is a framework found by Microsoft which supports 
multiple languages like VC#,VC++,VisualBasic,Web related 
applications such as ASP etc..In java source code is 
converted to ByteCode by JVM while in .NET code id 
converted to MSIL(Microsoft Intermediate Language Code)by 
the CLR supported by .NET..

Java is platform independent, only thing we need is jvm on 
the platform which comes in all operating systems by 
default,.net is also platform independent as it needs 
only .net framework,but since m$ owns the copyright to 
the .net framework api's,it becomes platform dependent.

Java uses a common language while in .net we can use many 
languages and it generates a platform specific code

In .NET due to disconnected data access through ADO.NET, 
has a high level performance against JDBC which requires 
multiple roundtrips to the data base
And Now a days MS people are saying that .NET can support 
more than 40 languages.Its Majaor advantage is the Visual 
studio.Net which is highly efficient than anyother IDE..

Java is a single language shared by multiple platforms 
nut .net is a framework for different languages but shared 
by single platform

the only main difference between java n .net is where .net 
is language independencies and java language dependency