Expressions - The Java Language Specification - page 528

pression completes abruptly for the same reason and the right-hand operand is not
evaluated and no assignment occurs.
• Otherwise, the right-hand operand is evaluated. If this evaluation completes ab-
ruptly, then the assignment expression completes abruptly for the same reason and
no assignment occurs.
• Otherwise, if the value of the array reference subexpression is null , then no assign-
ment occurs and a NullPointerException is thrown.
• Otherwise, the value of the array reference subexpression indeed refers to an array.
If the value of the index subexpression is less than zero, or greater than or equal to
the length of the array, then no assignment occurs and an ArrayIndexOutOfBoundsEx-
ception is thrown.
• Otherwise, the value of the index subexpression is used to select a component of
the array referred to by the value of the array reference subexpression.
This component is a variable; call its type SC . Also, let TC be the type of the left-
hand operand of the assignment operator as determined at compile time. Then
there are two possibilities:
♦ If TC is a primitive type, then SC is necessarily the same as TC .
The value of the right-hand operand is converted to the type of the selected ar-
ray component, is subjected to value set conversion (§ 5.1.13 ) to the appropriate
standard value set (not an extended-exponent value set), and the result of the
conversion is stored into the array component.
♦ If TC is a reference type, then SC may not be the same as TC , but rather a type
that extends or implements TC .
Let RC be the class of the object referred to by the value of the right-hand oper-
and at run time.
A Java compiler may be able to prove at compile time that the array component
will be of type TC exactly (for example, TC might be final ). But if a Java com-
piler cannot prove at compile time that the array component will be of type TC
exactly, then a check must be performed at run time to ensure that the class RC
is assignment compatible (§ 5.2 ) with the actual type SC of the array compon-
ent.
This check is similar to a narrowing cast (§ 5.5 , § 15.16 ), except that if the
check fails, an ArrayStoreException is thrown rather than a ClassCastException .
If class RC is not assignable to type SC , then no assignment occurs and an Ar-
rayStoreException is thrown.