class A {}
class B extends A {}
Hi, I'm learning Java and I'm trying to understand why this works:
A[] tab = new A[10];
tab[0] = new B();
and not this:
A[] tab = new B[10];
tab[0] = new A();
this line A[] tab = new B[10]; means that the compiler is booking 10 places for B in memory. tab[0] = new A() is setting tab[0] equal to a new A object that is smaller (?) than B (B extends A).
Why is there an ArrayStoreException: A error ? How does it work ?
You can store subclasses of an object, inside that object. ie B can be stored in A, but A is a superclass of type B. Basically, any class below class X in the inheritance chain can be referred to as type X, but any class above class X in the inheritance chain can NOT be referred to as type X.
Your example
A[] tab = new A[10]; // Perfectly fine. Equal objects.
tab[0] = new B(); // B is a subclass of A, so this is allowed.
A[] tab = new B[10]; // Same principle as above, B is a subclass of A
tab[0] = new A(); // A is a SUPERCLASS of B, so it can not be referred to as A
In short, X can only be referred to as Y, if X is a subclass of Y. (Or a subclass of a subclass. It's a recursive definition).
Let's use some English terms
Instead of A and B, let's have Item and Book.
public class Item {}
public class Book extends Item {}
Now, it makes sense to say:
Item b = new Book(); // All Books are items.
It does not make sense to say:
Book b = new Item(); // All items are definitely not books.