Extending Classes - The Java Programming Language - page 136

The types higher up the type hierarchy are said to be wider, or less
specific, than the types lower down the hierarchy. The lower types are
said to be narrower, or more specific, than their supertypes. When you
are expecting a supertype and receive a subtype, a widening conversion
takes place. Such a conversion causes the subtype object to be treated
as an instance of the supertype and can be checked at compile time.
No action is needed by the programmer in a widening conversion. Going
the other waytaking a reference to a supertype and converting it to a
reference to a subtypeis known as a narrowing conversion. Narrowing
conversions must be explicitly requested using the cast operator.
A variable, or an expression, of primitive type can be automatically con-
verted to a reference type using an instance of the wrapper class corres-
ponding to that primitive typesuch as an int value becoming an Integer
object. Conversely, a reference to a wrapper object can be converted
to the primitive value that is being wrapped. These primitivetowrapper
conversions (termed boxing conversions) are discussed in Chapter 8 .
The existence of the boxing conversions means that a variable of type
Object can be assigned the value of any expression that you can form in
the Java programming language.
The type compatibility of an expression in a given context (assignment,
argument-passing, operand, etc.) is affected by different type conver-
sions that can be applied manually or automatically, depending on that
context. Widening and boxing conversions are examples of type conver-
sions that will be automatically applied in some contexts. The different
type conversions that exist and the contexts in which they are applied
are discussed in " Type Conversions " on page 216 . If an expression re-
quires multiple conversions, or if there is no applicable automatic con-
version (as in the case of a narrowing conversion), then you must tell
the compiler what conversion to apply by using the cast operator.
3.4.2. Explicit Type Casting
A cast is used to tell the compiler that an expression should be treated
as having the type specified by the castand when applied to primitive
types can also affect the value of the expression. A cast consists of a
type name within parentheses, applied to an expression. In the previous