Why Final variable doesn't require initialization in main method in java?

For instance variable level

• A final variable can be initialized only once.

• A final variable at class level must be initialized before the end of the constructor.

For local (method) level

• A final variable at method level can be initialized only once.
• It must be initialized before it is used

So basically if you don't use a local final variable you can also skip it's initialization.

If the variable is at instance level you have to initialize it in the definition or in the costructor body.

In your code you have an instance variable final int b that is never initialized so you have an error.

You have also a local variable final int a that is never used. So you haven't an error for that variable.

The answer is provided in JLS.

A variable can be declared final. A final variable may only be assigned to once. It is a compile-time error if a final variable is assigned to unless it is definitely unassigned immediately prior to the assignment.

What is definitive assignment

Now, in case of a local variable, it's scope is valid inside the block it's declared. And flow will be linear (from top to bottom). So the compiler can identify easily where the variable will be initialized at first.

But in case of a field, it's impossible to find which method will assign the variable first unless it's assignment in a Constructor.

So, you have to assign final fields during declaration or in a Constructor.