Local Variable Declarations

A feature that Java has borrowed from C++ is the ability to declare and initialize local variables anywhere in a method body or other block of code. Declarations and their initializers no longer have to be the first statements in any block--you can declare them where it is convenient and fits well with the structure of your code.

Don't let this freedom make you sloppy, however! For someone reading your program, it is nice to have variable declarations grouped together in one place. As a rule of thumb, put your declarations at the top of the block, unless you have some good organizational reason for putting them elsewhere.

Forward References

For compiler efficiency, C requires that variables and functions must be defined, or at least declared, before they can be used or called. That is, forward references are not allowed in C. Java does not make this restriction, and by lifting it, it also does away with the whole concept of a variable or function declaration that is separate from the definition.

Java allows very flexible forward references. A method may refer to a variable or another method of its class, regardless of where in the current class the variable or method is defined. Similarly, it may refer to any class, regardless of where in the current file (or outside of the file) that class is defined. The only place that forward references are not allowed is in variable initialization. A variable initializer (for local variables, class variables, or instance variables) may not refer to other variables that have not yet been declared and initialized.

Method Overloading

A technique that Java borrows from C++ is called method overloading. Overloaded methods are methods that have the same name, but have different signatures. In other words, they take different types of arguments, a different number of arguments, or the same type of arguments in different positions in the argument list. You cannot overload a method by changing only its return type. Two methods with the same name may have different return types, but only if the method arguments also differ. Similarly, two overloaded methods may throw different exceptions, but only if their arguments differ as well.

Method overloading is commonly used in Java to define a number of related functions with the same name, but different arguments. Overloaded methods usually perform the same basic operation, but allow the programmer to specify arguments in different ways depending on what is convenient in a given situation. Method overloading is discussed in more detail in the next chapter.

The void Keyword

The void keyword is used in Java, as in C, to indicate that a function returns no value. (As we will see in the next section, constructor methods are an exception to this rule.)

Java differs from C (and is similar to C++) in that methods that take no arguments are declared with empty parentheses, not with the void keyword. Java does not have any void * type, nor does it use a (void) cast in order to ignore the result returned by a call to a non-void method.

Modifiers

Java defines a number of modifier keywords that may be applied to variable and/or method declarations to provide additional information or place restrictions on the variable or method:

final

The final keyword is a modifier that may be applied to classes, methods, and variables. It has a similar, but not identical, meaning in each case. A final class may never be subclassed. A final method may never be overridden. A final variable may never have its value set. In Java 1.1, this modifier may also be applied to local variables and method parameters. This modifier is discussed in more detail in the next chapter.

native

native is a modifier that may be applied to method declarations. It indicates that the method is implemented elsewhere in C, or in some other platform-dependent fashion. A native method should have a semicolon in place of its body.

synchronized

We saw the synchronized keyword in a previous section where it was a statement that marked a critical section of code. The same keyword can also be used as a modifier for class or instance methods. It indicates that the method modifies the internal state of the class or the internal state of an instance of the class in a way that is not thread-safe. Before running a synchronized class method, Java obtains a lock on the class, to ensure that no other threads can modif the class concurrently. Before running a synchronized instance method, Java obtains a lock on the instance that invoked the method, ensuring that no other thread can modify the object at the same time.

transient

The transient keyword is a modifier that may be applied to instance fields in a class. This modifier is legal but unused in Java 1.0. In Java 1.1, it indicates a field that is not part of an object's persistent state and thus does not need to be serialized with the object.

volatile

The volatile keyword is a modifier that may be applied to fields. It specifies that the field is used by synchronized threads and that the compiler should not attempt to perform optimizations with it. For example, it should read the variable's value from memory every time and not attempt to save a copy of it on the stack.

No Structures or Unions

Java does not support C struct or union types. Note, however, that a class is essentially the same thing as a struct, but with more features. And you can simulate the important features of a union by subclassing.

No Enumerated Types

Java does not support the C enum keyword for defining types that consist of one of a specified number of named values. This is somewhat surprising for a strongly-typed language like Java. Enumerated types can be partially simulated with the use of static final constant values.

No Method Types

C allows you to store the address of a function in a variable and to pass function addresses to other functions. You cannot do this in Java: methods are not data, and cannot be manipulated by Java programs. Note, however, that objects are data, and that objects can define methods. [9] So, when you need to pass a method to another method, you declare a class that defines the desired method and pass an instance of that class. See, for example, the FilenameFilter interface in the java.io package.

[9] An interesting way to think about objects in Java is as a kind of method that defines multiple entry points.

No Bitfields

Java does not support the C ability to define variables that occupy particular bits within struct and union types. This feature of C is usually only used to interface directly to hardware devices, which is never necessary with Java's platform-independent programming model.

No typedef

Java does not support the C typedef keyword to define aliases for type names. Java has a much simpler type naming scheme than C does, however, and so there is no need for something like typedef.

No Variable-Length Argument Lists

Java does not allow you to define methods that take a variable number of arguments, as C does. This is because Java is a strongly typed language and there is no way to do appropriate type checking for a method with variable arguments. Method overloading allows you to simulate C "varargs" functions for simple cases, but there is no general replacement for this C feature.