A non-static nested class in Java contains an implicit reference to an instance of the parent class. Thus to instantiate a Node
, you would need to also have an instance of Stack
. In a static context (the main method), there is no instance of Stack
to refer to. Thus the compiler indicates it can not construct a Node
.
If you make Node
a static class (or regular outer class), then it will not need a reference to Stack
and can be instantiated directly in the static main method.
See the Java Language Specification, Chapter 8 for details, such as Example 8.1.3-2.