Multiple cast in java

Question

I’m having a hard time doing an exercise because I don’t know how to solve this kind of casting:

The class D is the superclass.

The class C is the subclass.

 D d = (D) (C) new D();

I’d be grateful if someone could help me.

Answer 1

Let’s assume we have this code:

public class C {}

public class D extends C {}

That is, all that is a D, is also a C. But the opposite is not necessarily true.

And then this:

D d = (D) (C) new D();

Which is a concise way of writing what can be interpreted with these four steps:

D temp1 = new D();   // Etapa 1
C temp2 = (C) temp1; // Etapa 2
D temp3 = (D) temp2; // Etapa 3
D d = temp3;         // Etapa 4

What each step does:

1. new D() creates an object of the type D.

2. The cast for C checks if the object is of the type C. How does the compiler know that all D is a C, then this cast is valid. The resulting subexpression is of the type C.

3. The cast for D checks if the object is of the type C. How does the compiler know that not all C is a D, then a check will be done at runtime. If the object of type C is not a D, one ClassCastException will be launched. In this case, this check will always be successful. The resulting subexpression is of the type D.

4. The resulting value is assigned to the variable d. How does the compiler know what is right on the sign of = is the type D and the variable d is also the type D, then this assignment is valid.

Note that there are two types of Casts here. One of them (that of step 2) is a upcasting and the other (that of stage 3) is a downcasting. See more about this in that other question.