Roadmap for Java developers learning C#

C# and Java have many similarities. As you learn C#, you can apply much of the knowledge you already have from programming in Java:

1. Similar syntax: Both Java and C# are in the C family of languages. That similarity means you can already read and understand C#. There are some differences, but most of the syntax is the same as Java, and C. The curly braces and semicolons are familiar. The control statements like if, else, switch are the same. The looping statements of for, while, and do...while are same. The same keywords for class and interface are in both languages. The access modifiers from public to private are the same. Even many of the builtin types use the same keywords: int, string, and double.
2. Object-oriented paradigm: Both Java and C# are object-oriented languages. The concepts of polymorphism, abstraction, and encapsulation apply in both languages. Both added new constructs, but the core features are still relevant.
3. Strongly typed: Both Java and C# are strongly typed languages. You declare the data type of variables, either explicitly or implicitly. The compiler enforces type safety. The compiler catches type-related errors in your code, before you run the code.
4. Cross-platform: Both Java and C# are cross-platform. You can run your development tools on your favorite platform. Your application can run on multiple platforms. Your development platform isn't required to match your target platform.
5. Exception handling: Both Java and C# throw exceptions to indicate errors. Both use try - catch - finally blocks to handle exceptions. The Exception classes have similar names and inheritance hierarchies. One difference is that C# doesn't have the concept of checked exceptions. Any method might (in theory) throw any exception.
6. Standard libraries: The .NET runtime and the Java Standard Library (JSL) have support for common tasks. Both have extensive ecosystems for other open source packages. In C#, the package manager is NuGet. It's analogous to Maven.
7. Garbage Collection: Both languages employ automatic memory management through garbage collection. The runtime reclaims the memory from objects that aren't referenced. One difference is that C# enables you to create value types, as struct types.

You'll be productive in C# almost immediately because of the similarities. As you progress, you should learn features and idioms in C# that aren't available in Java:

1. Pattern matching: Pattern matching enables concise conditional statements and expressions based on the shape of complex data structures. The is statement checks if a variable "is" some pattern. The pattern-based switch expression provides a rich syntax to inspect a variable and make decisions based on its characteristics.
2. String interpolation and raw string literals: String interpolation enables you to insert evaluated expressions in a string, rather than using positional identifiers. Raw string literals provide a way to minimize escape sequences in text.
3. Nullable and non-nullable types: C# supports nullable value types, and nullable reference types by appending the ? suffix to a type. For nullable types, the compiler warns you if you don't check for null before dereferencing the expression. For non-nullable types, the compiler warns you if you might be assigning a null value to that variable. Non-nullable reference types minimize programming errors that throw a System.NullReferenceException.
4. Extension methods: In C#, you can create methods that extend a class or interface. Extension methods extend the behavior of a type from a library, or all types that implement a given interface.
5. LINQ: Language integrated query (LINQ) provides a common syntax to query and transform data, regardless of its storage.
6. Local functions: In C#, you can nest functions inside methods, or other local functions. Local functions provide yet another layer of encapsulation.

There are other features in C# that aren't in Java. You'll see features like async and await, and using statements to automatically free nonmemory resources.

There are also some similar features between C# and Java that have subtle but important differences:

1. Properties and Indexers: Both properties and indexers (treating a class like an array or dictionary) have language support. In Java, they're naming conventions for methods starting with get and set.
2. Records: In C#, records can be either class (reference) or struct (value) types. C# records can be immutable, but aren't required to be immutable.
3. Tuples have different syntax in C# and Java.
4. Attributes are similar to Java annotations.

Finally, there are Java language features that aren't available in C#:

1. Checked exceptions: In C#, any method could theoretically throw any exception.
2. Checked array covariance: In C#, arrays aren't safely covariant. You should use the generic collection classes and interfaces if you need covariant structures.

Overall, learning C# for a developer experienced in Java should be smooth. You'll find enough familiar idioms to quickly be productive, and you'll learn the new idioms quickly.