CODEWITHSUNDEEP
javaoopcasting
dscer
dscer

Reputation: 228

Custom Object Casting Method

If you have two different classes A and B, and B is a subclass of A, you cannot cast as follows:

A a = new A();
B b = new B();
A newA = (A)b;

Is there a way to enable the above code to work (no alterations to the above code) without the JVM throwing a ClassCastException?

------------EDIT----------

Sorry, I made a mistake in the code in the above question. The correct version is below:

A a = new A();
B b = new B();
B newB = (B)a;

Upvotes: 2

Views: 4322

Answers (4)

hvgotcodes
hvgotcodes

Reputation: 120268

B already has an is-a relationship to A. You don't need to cast it....You can throw a B at any method or reference that expects/points to an A.

Based on your edit -- there is something wrong with your design if you want to do this. While a B is-a A, the opposite is NOT true. An A is not a B. In other words, since B extends A, it probably has methods/properties on it that are NOT defined on A. If you cast an A to a B, then methods that accept that reference might try to invoke a method it believes is on the instance, since you told the compiler that it got a B, when in reality the underlying A does not have the required method.

Casting here will only lead to pain and failure.

Upvotes: 4

Tom Hawtin - tackline
Tom Hawtin - tackline

Reputation: 147164

With new code, no you can't do that. You'd have to create a new object:

B newB = new B(a);

or

B newB = B.of(a);

A non-abstract non-leaf class should generally be avoided anyway. Also, since 1.5 (released 2004), there shouldn't be much of the casting syntax about.

Upvotes: 1

Óscar López
Óscar López

Reputation: 236114

If B is a subclass of A the above should work, and the cast would be unnecessary:

A a = new A();
B b = new B();
A newA = b; // no need to cast!

Upvotes: 2

Alejandro B.
Alejandro B.

Reputation: 5092

I think you can simply assign:

A newA = b;

Upvotes: 2

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