Learning Java, 4th Edition

Safety of Implementation

It’s one thing to create a language that prevents you from shooting yourself in the foot; it’s quite another to create one that prevents others from shooting you in the foot.

Encapsulation is the concept of hiding data and behavior within a class; it’s an important part of object-oriented design. It helps you write clean, modular software. In most languages, however, the visibility of data items is simply part of the relationship between the programmer and the compiler. It’s a matter of semantics, not an assertion about the actual security of the data in the context of the running program’s environment.

When Bjarne Stroustrup chose the keyword private to designate hidden members of classes in C++, he was probably thinking about shielding a developer from the messy details of another developer’s code, not the issues of shielding that developer’s classes and objects from attack by someone else’s viruses and Trojan horses. Arbitrary casting and pointer arithmetic in C or C++ make it trivial to violate access permissions on classes without breaking the rules of the language. Consider the following code:

    // C++ code
    class Finances {
        private:
            char creditCardNumber[16];
            ...
    };

    main() {
        Finances finances;

        // Forge a pointer to peek inside the class
        char *cardno = (char *)&finances;
        printf("Card Number = %.16s\n", cardno);
    }

In this little C++ drama, we have written some code that violates the encapsulation of the Finances class and pulls out some secret information. ...