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Chapter 4
Converting and Casting
Unary Numeric Promotion
Contexts:
·
Operand of the unary arithmetic operators + and –
·
Operand of the unary integer bit-wise complement operator ~
·
During array creation, for example new int[x], where the dimension
expression x must evaluate to an int value.
·
Indexing array elements, for example table[‘a’], where the
index expression must evaluate to an int value.
·
Individual operands of the shift operators.
Binary numeric promotion
Contexts:
·
Operands of arithmetic operators *, / , %, + and –
·
Operands of relational operators <, <= , > and >=
·
Numeric Operands of equality operators == and !=
·
Integer Operands of bit-wise operators &, ^ and |
Conversion of Primitives
1.
3 types of conversion – assignment conversion, method call conversion
and arithmetic promotion
2.
boolean may not be converted to/from any non-boolean type.
3.
Widening conversions accepted. Narrowing conversions rejected.
4.
byte, short can’t be converted to char and vice versa.
5.
Arithmetic promotion
5.1
Unary operators
·
if the operand is byte, short or char
{
convert it to int;
}
else {
do nothing; no
conversion needed;
}
5.2
Binary operators
·
if one operand is double {
all double; convert the other
operand to double;
}
else
if one operand is float {
all float; convert the other
operand to float;
}
else if one
operand is long {
all long; convert the other
operand to long;
}
else {
all int; convert all to int;
}
6.
When assigning a literal value to a variable, the range of the
variable’s data type is checked against the value of the literal and
assignment is allowed or compiler will produce an error.
char c = 3;
// this will compile, even though a numeric literal is by default an int since
the range of char will accept the value
int a = 3;
char d = a;
// this won’t compile, since we’re assigning an int to char
char e = -1;
// this also won’t compile, since the value is not in the range of char
float f = 1.3;
// this won’t compile, even though the value is within float range. Here range
is not important, but precision is.
1.3 is by default a double, so a specific cast or f = 1.3f will work.
float f = 1/3; // this will
compile, since RHS evaluates to an int.
Float f = 1.0 / 3.0; // this
won’t compile, since RHS evaluates to a double.
7.
Also when assigning a final variable to a variable, even if the final
variable’s data type is wider than the variable, if the value is within the
range of the variable an implicit conversion is done.
byte b;
final int a = 10;
b = a; // Legal, since value of
‘a’ is determinable and within range of b
final int x = a;
b = x; // Legal, since value of
‘x’ is determinable and within range of b
int y;
final int z = y;
b = z; // Illegal, since value of
‘z’ is not determinable
8.
Method call conversions always look for the exact data type or a wider
one in the method signatures. They will not do narrowing conversions to resolve
methods, instead we will get a compile error.
Here is the figure of allowable primitive conversion.
byte à
short à
int à
long à
float à
double
char
Casting of Primitives
9.
Needed with narrowing conversions. Use with care – radical information
loss. Also can be used with widening conversions, to improve the clarity of the
code.
10.
Can cast any non-boolean type to another non-boolean type.
11.
Cannot cast a boolean or to a boolean type.
Conversion of Object references
12.
Three types of reference variables to denote objects - class, interface
or array type.
13.
Two kinds of objects can be created – class or array.
14.
Two types of conversion – assignment and method call.
15.
Permitted if the direction of the conversion is ‘up’ the inheritance
hierarchy. Means that types can be assigned/substituted to only super-types –
super-classes or interfaces. Not the other way around, explicit casting is
needed for that.
16.
Interfaces can be used as types when declaring variables, so they
participate in the object reference conversion. But we cannot instantiate an
interface, since it is abstract and doesn’t provide any implementation. These
variables can be used to hold objects of classes that implement the interface.
The reason for having interfaces as types may be, I think, several unrelated
classes may implement the same interface and if there’s a need to deal with
them collectively one way of treating them may be an array of the interface type
that they implement.
17.
Primitive arrays can be converted to only the arrays of the same
primitive type. They cannot be converted to another type of primitive array.
Only object reference arrays can be converted / cast.
18.
Primitive arrays can be converted to an Object reference, but not to an
Object[] reference. This is because all arrays (primitive arrays and Object[])
are extended from Object.
Casting of Object references
19.
Allows super-types to be assigned to subtypes. Extensive checks done both
at compile and runtime. At compile time, class of the object may not be known,
so at runtime if checks fail, a ClassCastException is thrown.
20.
Cast operator, instanceof operator and the == operator behave the same
way in allowing references to be the operands of them. You cannot cast or apply
instanceof or compare unrelated references,
sibling references or any incompatible
references.
Compile-time Rules
·
When old and new types are classes, one class must be the
sub-class of the other.
·
When old and new types are arrays, both must contain reference
types and it must be legal to cast between those types (primitive arrays cannot
be cast, conversion possible only between same type of primitive arrays).
·
We can always cast between an interface and a non-final object.
Run-time rules
·
If new type is a class, the class of the expression being
converted must be new type or extend new type.
·
If new type is an interface, the class of the expression being
converted must implement the interface.
An Object reference can be converted to: (java.lang.Object)
·
an Object reference
·
a Cloneable interface reference, with casting, with runtime check
·
any class reference, with casting, with runtime check
·
any array referenece, with casting, with runtime check
·
any interface reference, with casting, with runtime check
A Class type reference can be converted to:
·
any super-class type reference, (including Object)
·
any sub-class type reference, with casting, with runtime check
·
an interface reference, if the class implements that interface
·
any interface reference, with casting, with runtime check (except
if the class is final and doesn’t implement the interface)
An Interface reference can be converted to:
·
an Object reference
·
a super-interface reference
·
any interface/class reference with casting, with runtime check
(except if the class is final and doesn’t implement the interface)
A Primitive Array reference can be converted to:
·
an Object reference
·
a Cloneable interface reference
·
a primitive array reference of the same type
An Object Array reference can be converted to:
·
an Object reference
·
a Cloneable interface
reference
·
a super-class Array reference, including an Object Array reference
·
any sub-class Array reference with casting, with runtime check
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