Suppose that we have classes A, B, C and D. Suppose that B is a subclass of A, t

Question

Suppose that we have classes A, B, C and D. Suppose that B is a subclass of A, that C is a subclass of B, and D is a subclass of A. Suppose that we make the following declarations.

A a1 = new A();

A a2 = new C();

D d1 = new D();

For each part below, explain what, if any, errors would be caused by the statement in that part. Be sure to consider both compile time and run time errors.

(a) A a3 = new B();

(b) B b1 = new A();

(c) B b2 = (B) a1;

(d) B b3 = (B) a2;

(e) B b4 = (B) d1;

(f) B b5 = (C)(A)new D();

Explanation / Answer

(a) A a3 = new B();

This is valid statement. We can create object for child class with parent class.

(b) B b1 = new A();

This will throw compilation error because we can not create parent class object with child class.

(c) B b2 = (B) a1;

This statement is syntactically correct. It will now throw any compilation error but it will throw run time exception. Because we can not explicitly cast parent class as child.

(d) B b3 = (B) a2;

This is valid statement because ultimately a2 is an instance of C which is inherited by B. We can explicitly cast sub class object into parent class.

(e) B b4 = (B) d1;

It will throw compilation error because there is no relation between B and D. So we can not convert D type object into B

(f) B b5 = (C)(A)new D();

This is syntactically correct but it will throw runtime exception because D type object can not be converted into A type object as they don't have relation.

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