Nested types: interview questions & scenarios

Model answer

A static nested class is an ordinary class that lives in the outer class's namespace. It can see the outer class's private members, but it has no link to an outer instance, so you create it with new Order.Builder() and it can only reach instance fields through an explicit reference.

An inner class (non-static member) belongs to one outer instance. javac adds a hidden final Outer this$0 field and passes the outer object through the constructor, so you create it with outer.new Inner(), and it reads outer fields directly. That hidden reference keeps the outer object alive as long as the inner one is.

Model answer

Because the class or lambda gets a copy of the value, not the variable. The local lives in the method's stack frame, which is gone when the method returns, while the object may live on (stored, returned, run on another thread). So javac copies the value into a synthetic field such as val$percent:

Fig. 1

The stack frame of schedule holds percent and orderId; the heap object Scheduler$1 holds copies in val$percent and val$orderId plus this$0; after the method returns the frame is gone but the object and its copies remain

If you could reassign the local later, the copy and the original would silently disagree. Java forbids it: since Java 8 the variable needn't be declared final, but it must be effectively final (never reassigned).