construct object from parent class

A extends B, may i reference B suboject in A's constructor?

public A(B b)
{
  /*(B)this = b*/
}

If not, how do i construct an A object with a B object?, i mean other that using Cloneable to create some other instance or solutions out of the JLS (dozer, beanutils, etc.). In a nutshell, how would the A constructor look like to make this code to run?

B b = new B("hi");
A a = new A(b);
assertequals(a.getSalute(), "h开发者_StackOverflow中文版i");//ok so far

a.setSalute("hola");
assertequals(b.getSalute(), "hola");//A maintains a reference to B

f(a);//prints hola
f(b);//prints hola as well

Thanks in advance

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It looks like you're trying some kind of decorator pattern. I'm not the biggest fan of "program to an interface" because I think it's good advice that's over-used, but I think this is a good situation for it.

interface Wrapable { // replace Wrapable with something more appropriate
   // common methods here?
}

class B implements Wrapable {

}

class A implements Wrappable {
    Wrappable whatThisWraps;

    A(Wrappable w) { whatThisWraps = w; }
}

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Yes. Java does not disallow forward references to concrete classes the way C++ does.

You do have to be aware though of static class initializers.

From DCL12-J. Prevent class initialization cycles:

According to §12.4, "Initialization of Classes and Interfaces" of the Java Language Specification [JLS 2005]

Initialization of a class consists of executing its static initializers and the initializers for static fields (class variables) declared in the class.

In other words, the presence of a static field triggers the initialization of a class. However, a static field could depend on the initialization of a class, possibly creating an initialization cycle. The Java Language Specification also states in §8.3.2.1, "Initializers for Class Variables" [JLS 2005]

...at run time, static variables that are final and that are initialized with compile-time constant values are initialized first.

This statement can be misleading because it is inapplicable to instances that use values of static final fields that are initialized at a later stage. Declaring a field to be static final is insufficient to guarantee that it is fully initialized before being read.

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So long as your constructors are appropriately constructed, you can reference B in A's constructor as you described.

If A extends B, any A Object can be considered an B Object, but the reverse is not true.

To better answer your original question, creating a B from a A should be straightforward - change the A in the constructor to an B (since any A is also a B), call the appropriate getter methods and assign their values to the instance variables held by A. This type of constructor public B(B b)... is a copy-constructor. If you decide to keep A in the constructor, then just apply the call/copy/assign logic mentioned by calling all methods inherited by A from B.