A Closer Look at Methods and Classes - Introducing Access Control
(Page 7 of 12 )
As you know, encapsulation links data with the code that manipulates it. However, encapsulation provides another important attribute: access control. Through encapsulation, you can control what parts of a program can access the members of a class. By controlling access, you can prevent misuse. For example, allowing access to data only through a well-defined set of methods, you can prevent the misuse of that data. Thus, when correctly implemented, a class creates a “black box” which may be used, but the inner workings of which are not open to tampering. However, the classes that were presented earlier do not completely meet this goal. For example, consider the Stack class shown at the end of Chapter 6. While it is true that the methods push( ) and pop( ) do provide a controlled interface to the stack, this interface is not enforced. That is, it is possible for another part of the program to bypass these methods and access the stack directly. Of course, in the wrong hands, this could lead to trouble. In this section you will be introduced to the mechanism by which you can precisely control access to the various members of a class.
How a member can be accessed is determined by the access specifier that modifies its declaration. Java supplies a rich set of access specifiers. Some aspects of access control are related mostly to inheritance or packages. (A package is, essentially, a grouping of classes.) These parts of Java’s access control mechanism will be discussed later. Here, let’s begin by examining access control as it applies to a single class. Once you understand the fundamentals of access control, the rest will be easy.
Java’s access specifiers are public, private, and protected. Java also defines a default access level. protected applies only when inheritance is involved. The other access specifiers are described next.
Let’s begin by defining public and private. When a member of a class is modified by the public specifier, then that member can be accessed by any other code. When a member of a class is specified as private, then that member can only be accessed by other members of its class. Now you can understand why main( ) has always been preceded by the public specifier. It is called by code that is outside the program—that is, by the Java run-time system. When no access specifier is used, then by default the member of a class is public within its own package, but cannot be accessed outside of its package. (Packages are discussed in the following chapter.)
In the classes developed so far, all members of a class have used the default access mode, which is essentially public. However, this is not what you will typically want to be the case. Usually, you will want to restrict access to the data members of a class—allowing access only through methods. Also, there will be times when you will want to define methods which are private to a class.
An access specifier precedes the rest of a member’s type specification. That is, it must begin a member’s declaration statement. Here is an example:
public int i;
private double j;
private int myMethod(int a, char b) { // ...
To understand the effects of public and private access, consider the following program:
/* This program demonstrates the difference between
public and private.
*/
class Test {
int a; // default access
public int b; // public access
private int c; // private access
// methods to access c
void setc(int i) { // set c's value
c = i;
}
int getc() { // get c's value
return c;
}
}
class AccessTest {
public static void main(String args[]) {
Test ob = new Test();
// These are OK, a and b may be accessed directly
ob.a = 10;
ob.b = 20;
// This is not OK and will cause an error
// ob.c = 100; // Error!
// You must access c through its methods
ob.setc(100); // OK
System.out.println("a, b, and c: " + ob.a + " " +
ob.b + " " + ob.getc());
}
}
As you can see, inside the Test class, a uses default access, which for this example is the same as specifying public. b is explicitly specified as public. Member c is given private access. This means that it cannot be accessed by code outside of its class. So, inside the AccessTest class, c cannot be used directly. It must be accessed through its public methods: setc( ) and getc( ). If you were to remove the comment symbol from the beginning of the following line,
// ob.c = 100; // Error!
then you would not be able to compile this program because of the access violation.
To see how access control can be applied to a more practical example, consider the following improved version of the Stack class shown at the end of Chapter 6.
// This class defines an integer stack that can hold 10 values.
class Stack {
/* Now, both stck and tos are private. This means
that they cannot be accidentally or maliciously
altered in a way that would be harmful to the stack.
*/
private int stck[] = new int[10];
private int tos;
// Initialize top-of-stack
Stack() {
tos = -1;
}
// Push an item onto the stack
void push(int item) {
if(tos==9)
System.out.println("Stack is full.");
else
stck[++tos] = item;
}
// Pop an item from the stack
int pop() {
if(tos < 0) {
System.out.println("Stack underflow.");
return 0;
}
else
return stck[tos--];
}
}
As you can see, now both stck, which holds the stack, and tos, which is the index of the top of the stack, are specified as private. This means that they cannot be accessed or altered except through push( ) and pop( ). Making tos private, for example, prevents other parts of your program from inadvertently setting it to a value that is beyond the end of the stck array.
The following program demonstrates the improved Stack class. Try removing the commented-out lines to prove to yourself that the stck and tos members are, indeed, inaccessible.
class TestStack {
public static void main(String args[]) {
Stack mystack1 = new Stack();
Stack mystack2 = new Stack();
// push some numbers onto the stack
for(int i=0; i<10;; i++) mystack1.push(i);
for(int i=10; i<20;; i++) mystack2.push(i);
// pop those numbers off the stack
System.out.println("Stack in mystack1:");
for(int i=0; i<10;; i++)
System.out.println(mystack1.pop());
System.out.println("Stack in mystack2:");
for(int i=0; i<10;; i++)
System.out.println(mystack2.pop());
// these statements are not legal
// mystack1.tos = -2;
// mystack2.stck[3] = 100;
}
}
Although methods will usually provide access to the data defined by a class, this does not always have to be the case. It is perfectly proper to allow an instance variable to be public when there is good reason to do so. For example, most of the simple classes in this book were created with little concern about controlling access to instance variables for the sake of simplicity. However, in most real-world classes, you will need to allow operations on data only through methods. The next chapter will return to the topic of access control. As you will see, it is particularly important when inheritance is involved.
Next: Understanding static >>
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 This article is excerpted from chapter 7 of Java: the Complete Reference, J2SE 5 Edition, written by Herbert Schildt (McGraw-Hill/Osborne, 2004; ISBN: 0072230738). Check it out at your favorite bookstore. Buy this book now.
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