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 Tcl and awk 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.
As I described in Chapter 1, Yet Another Language?, 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 7, Basic Utility Classes). 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.
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 we'll touch on later, but in general the syntax and semantics of Java primitive types are meant to fit a C programmer's mental habits. If you're like most of this book's readers, 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
(An interesting, but meaningless, observation is that this would make the number 0 an octal value in the eyes of the compiler.)
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 octalff = \u00ff;
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, Objects in Java.
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, Objects in Java. 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 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, as shown in Chapter 5, Objects in Java.
Reference types always point to objects, and objects are always defined by classes. However, there are two special kinds of reference types that 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, as you'll see in Chapter 5, Objects in Java.
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 7, Basic Utility Classes.