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

Question

class Shape { }
class Square extends Shape { }

Shape shape1 = null;
Square square1 = null;

Options

• Ashape1 = (Square) new Square();
• Bshape1 = new Square();
• Csquare1 = (Square) new Shape();
• Dsquare1 = new Shape();
• Esquare1 = new Square();
• Fshape1 = square1;
• Gshape1 = new Shape();
• Hsquare1 = shape1;

• A
  85% (29)
• D
  9% (3)
• F
  3% (1)
• H
  3% (1)

Explanation

Note: The stated answer (A, B, C, E) appears to contain an error - options F and G also compile by standard Java rules, and a well-formed version of this question would likely ask "which six compile?" or exclude F and G from the choices.

Here is the accurate breakdown:

Compiles (A, B, C, E, F, G):

• B, E, G are straightforward: assigning to a reference of the same type (E, G) or a parent type (B) is always valid - Square IS-A Shape.
• F (shape1 = square1) also compiles for the same reason as B: assigning a Square reference to a Shape variable is a widening conversion requiring no cast.
• A compiles because explicitly casting a Square to Square before assigning to a Shape is redundant but legal.
• C (square1 = (Square) new Shape()) compiles because the compiler permits explicit downcasts when the target type is in the same hierarchy - however, it throws a ClassCastException at runtime since the actual object is a Shape, not a Square.

Does not compile (D, H):

• D and H both attempt to assign a parent-type reference to a child-type variable without an explicit cast, which the compiler rejects.

Memory tip: "Widening (up the tree) is free; narrowing (down the tree) needs a cast to compile, but may still explode at runtime."

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