 | Chapter 4 The Java Language |  |
| Chapter 4 The Java Language | |
The type system of a programming language describes how its data elements (variables and constants) are associated with actual storage. In a statically typed language, such as C or C++, the type of a data element is a simple, unchanging attribute that often corresponds directly to some underlying hardware phenomenon, like a register value or a pointer indirection. In a more dynamic language like Smalltalk or Lisp, variables can be assigned arbitrary elements and can effectively change their type throughout their lifetime. A considerable amount of overhead goes into validating what happens in these languages at run-time. Scripting languages like Perl and Tcl achieve ease of use by providing drastically simplified type systems in which only certain data elements can be stored in variables, and values are unified into a common representation such as strings.
Java combines the best features of both statically and dynamically typed languages. As in a statically typed language, every variable and programming element in Java has a type that is known at compile-time, so the interpreter doesn't normally have to check the type validity of assignments while the code is executing. Unlike C or C++ though, Java also maintains run-time information about objects and uses this to allow safe run-time polymorphism.
Java data types fall into two categories. Primitive types represent simple values that have built-in functionality in the language; they are fixed elements like literal constants and numeric expressions. Reference types (or class types) include objects and arrays; they are called reference types because they are passed "by reference" as I'll explain shortly.
Numbers, characters, and boolean values are fundamental elements in Java. Unlike some other (perhaps more pure) object-oriented languages, they are not objects. For those situations where it's desirable to treat a primitive value as an object, Java provides "wrapper" classes (see Chapter 9). One major advantage of treating primitive values as such is that the Java compiler can more readily optimize their usage.
Another advantage of working with the Java virtual-machine
architecture is that primitive types are precisely defined. For
example, you never have to worry about the size of an
int on a particular platform; it's always a
32-bit, signed, two's complement number. Table 4.2 summarizes Java's primitive types.
| Type | Definition |
|---|---|
boolean |
|
char | 16-bit Unicode character |
byte | 8-bit signed two's complement integer |
short | 16-bit signed two's complement integer |
int | 32-bit signed two's complement integer |
long | 64-bit signed two's complement integer |
float | 32-bit IEEE 754 floating-point value |
double | 64-bit IEEE 754 floating-point value |
If you think the primitive types look like an idealization of C scalar types on a byte-oriented 32-bit machine, you're absolutely right. That's how they're supposed to look. The 16-bit characters were forced by Unicode, and generic pointers were deleted for other reasons, but the syntax and semantics of Java primitive types are meant to fit a C programmer's mental habits. You'll probably find this saves you a lot of mental effort in learning the language.
Variables are declared inside of methods or classes in C style. For example:
int foo; double d1, d2; boolean isFun;
Variables can optionally be initialized with an appropriate expression when they are declared:
int foo = 42; double d1 = 3.14, d2 = 2 * 3.14; boolean isFun = true;
Variables that are declared as instance variables in a class are set to
default values if they are not initialized. In this case, they act much
like static variables in C or C++. Numeric types
default to the appropriate flavor of zero, characters are set to the null
character "\0," and boolean variables
have the value false. Local variables declared
in methods, on the other hand, must be explicitly initialized before they
can be used.
Integer literals can be specified in octal (base 8), decimal (base 10), or hexadecimal (base 16). A decimal integer is specified by a sequence of digits beginning with one of the characters 1-9:
int i = 1230;
Octal numbers are distinguished from decimal by a leading zero:
int i = 01230; // i = 664 decimal
As in C, a hexadecimal number is denoted by the leading characters
0x or 0X (zero "x"), followed by
digits and the characters a-f or A-F, which represent the decimal
values 10-15 respectively:
int i = 0xFFFF; // i = 65535 decimal
Integer literals are of type int unless they are
suffixed with an L, denoting that they are to be
produced as a long value:
long l = 13L; long l = 13; // equivalent--13 is converted from type int
(The lowercase character l ("el") is also
acceptable, but should be avoided because it often looks like the
numeral 1.)
When a numeric type is used in an assignment or an expression involving
a type with a larger range, it can be promoted to the larger type. For
example, in the second line of the above example, the number 13 has the
default type of int, but it's promoted to type
long for assignment to the long
variable. Certain other numeric and comparison operations also cause this
kind of arithmetic promotion. A numeric value can never be assigned to a type
with a smaller range without an explicit (C-style) cast, however:
int i = 13; byte b = i; // Compile time error--explicit cast needed byte b = (byte) i; // Okay
Conversions from floating point to integer types always require an explicit cast because of the potential loss of precision.
Floating-point values can be specified in decimal or scientific
notation. Floating-point literals are of type
double unless they are suffixed with an
f denoting that they are to be
produced as a float value:
double d = 8.31; double e = 3.00e+8; float f = 8.31F; float g = 3.00e+8F;
A literal character value can be specified either as a single-quoted character or as an escaped ASCII or Unicode sequence:
char a = 'a'; char newline = '\n'; char smiley = '\u263a';
In C, you can make a new, complex data type by creating a
structure. In Java (and other object-oriented
languages), you instead create a class that defines
a new type in the language. For instance, if we create a new class
called Foo in Java, we are also implicitly creating
a new type called Foo. The type of an item governs
how it's used and where it's assigned. An item of type
Foo can, in general, be assigned to a variable of
type Foo or passed as an argument to a method that
accepts a Foo value.
In an object-oriented language like Java, a type is not
necessarily just a simple attribute. Reference types are related in
the same way as the classes they represent. Classes exist in a
hierarchy, where a subclass is a specialized kind of its parent
class. The corresponding types have a similar relationship, where the
type of the child class is considered a subtype of the parent
class. Because child classes always extend their parents and have, at
a minimum, the same functionality, an object of the
child's type can be used in place of an object of the
parent's type. For example, if I create a new class,
Bar, that extends Foo, there is
a new type Bar that is considered a subtype of
Foo. Objects of type Bar can
then be used anywhere an object of type Foo
could be used; an object of type Bar is said to be
assignable to a variable of type Foo. This is
called subtype polymorphism and is one of the
primary features of an object-oriented language. We'll look more
closely at classes and objects in Chapter 5.
Primitive types in Java are used and passed "by value." In other words, when a primitive value is assigned or passed as an argument to a method, it's simply copied. Reference types, on the other hand, are always accessed "by reference." A reference is simply a handle or a name for an object. What a variable of a reference type holds is a reference to an object of its type (or of a subtype). A reference is like a pointer in C or C++, except that its type is strictly enforced and the reference value itself is a primitive entity that can't be examined directly. A reference value can't be created or changed other than through assignment to an appropriate object. When references are assigned or passed to methods, they are copied by value. You can think of a reference as a pointer type that is automatically dereferenced whenever it's mentioned.
Let's run through an example. We specify a variable of
type Foo, called myFoo, and
assign it an appropriate object:
Foo myFoo = new Foo(); Foo anotherFoo = myFoo;
myFoo is a reference type variable that holds a
reference to the newly constructed Foo object. For
now, don't worry about the details of creating an object;
we'll cover that in Chapter 5. We designate a
second Foo type variable,
anotherFoo, and assign it to the same object. There are
now two identical references: myFoo and
anotherFoo. If we change things in the state of
the Foo object itself, we will see the same
effect by looking at it with
either reference. The comparable code in C++ would be:
// C++ Foo& myFoo = *(new Foo()); Foo& anotherFoo = myFoo;
We can pass one of the variables to a method, as in:
myMethod( myFoo );
An important, but sometimes confusing, distinction to make at this
point is that the reference itself is passed by value. That is, the
argument passed to the method (a local variable from the method's
point of view) is actually a third copy of the reference. The method
can alter the state of the Foo object itself
through that reference, but it can't change the caller's notion of the
reference to
myFoo. That is, the method can't change the
caller's myFoo to point to a different Foo object.
For the times we want a method to change a reference for us, we have
to pass a reference to the object that contains it.
Reference types always point to objects, and objects are always defined by classes. However, two special kinds of reference types specify the type of object they point to in a slightly different way. Arrays in Java have a special place in the type system. They are a special kind of object automatically created to hold a number of some other type of object, known as the base type. Declaring an array-type reference implicitly creates the new class type, as you'll see in the next section.
Interfaces are a bit sneakier. An interface defines a set of methods and a corresponding type. Any object that implements all methods of the interface can be treated as an object of that type. Variables and method arguments can be declared to be of interface types, just like class types, and any object that implements the interface can be assigned to them. This allows Java to cross the lines of the class hierarchy in a type safe way.
Strings in Java are objects; they are therefore a reference
type. String objects do, however, have some special
help from the Java compiler that makes them look more
primitive. Literal string values in Java source code are turned into
String objects by the compiler. They can be used
directly, passed as arguments to methods, or assigned to
String type variables:
System.out.println( "Hello World..." ); String s = "I am the walrus..."; String t = "John said: \"I am the walrus...\"";
The + symbol in Java is overloaded to provide
string concatenation; this is the only overloaded operator in Java:
String quote = "Four score and " + "seven years ago,"; String more = quote + " our" + " fathers" + " brought...";
Java builds a single String object from the concatenated
strings and provides it as the result of the expression. We will discuss
the String class in Chapter 9.
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| 4.2 Comments |  | 4.4 Statements and Expressions |
