Given the class definitrions: class Shape { } class Square extends Shape { } Given the variable declarations: Shape shape1 = null; Square square1 = null; Which four compile?

Assigning `new Square()` to `shape1` is a valid upcast since `Square` extends `Shape`; the explicit `(Square)` cast is redundant but harmless and compiles successfully.

Assigning `new Square()` to `shape1` is a valid implicit upcast because a `Square` object is also considered a `Shape` object due to inheritance.

Casting `new Shape()` to `(Square)` and assigning it to `square1` is syntactically valid at compile time, as `Square` extends `Shape`, though it would result in a `ClassCastException` at runtime because `new Shape()` does not create an object that is truly an instance of `Square`.

This is a compile-time error because assigning `new Shape()` directly to a `Square` reference (`square1`) is an implicit downcast, which is not allowed without an explicit cast as a `Shape` is not guaranteed to be a `Square`.

Assigning `new Square()` to `square1` is a direct and valid assignment because both the object being created and the reference variable are of the same `Square` type.

While `shape1 = new Shape();` is syntactically correct when classes are in the same package, it would fail to compile if `Shape` and the code instantiating it were in different packages, as `Shape`'s default constructor is package-private.