The hidden implementation
It is helpful to break up the playing field into class creators (those who create new data types) and client programmers (the class consumers who use the data types in their applications). The goal of the client programmer is to collect a toolbox full of classes to use for rapid application development. The goal of the class creator is to build a class that exposes only whats necessary to the client programmer and keeps everything else hidden. Why? Because if its hidden, the client programmer cant access it, which means that the class creator can change the hidden portion at will without worrying about the impact on anyone else. The hidden portion usually represents the tender insides of an object that could easily be corrupted by a careless or uninformed client programmer, so hiding the implementation reduces program bugs.
The concept of implementation hiding cannot be overemphasized. In any relationship its important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the client programmer, who is also a programmer, but one who is putting together an application by using your library, possibly to build a bigger library. If all the members of a class are available to everyone, then the client programmer can do anything with that class and theres no way to enforce rules. Even though you might really prefer that the client programmer not directly manipulate some of the members of your class, without access control theres no way to prevent it. Everythings naked to the world.
So the first reason for access control is to keep client programmers hands off portions they shouldnt touchparts that are necessary for the internal operation of the data type but not part of the interface that users need in order to solve their particular problems. This is actually a service to users because they can easily see whats important to them and what they can ignore.
The second reason for access control is to allow the library designer to change the internal workings of the class without worrying about how it will affect the client programmer. For example, you might implement a particular class in a simple fashion to ease development, and then later discover that you need to rewrite it in order to make it run faster. If the interface and implementation are clearly separated and protected, you can accomplish this easily.
Java uses three explicit keywords to set the boundaries in a class: public, private, and protected. Their use and meaning are quite straightforward. These access specifiers determine who can use the definitions that follow. public means the following element is available to everyone. The private keyword, on the other hand, means that no one can access that element except you, the creator of the type, inside methods of that type. private is a brick wall between you and the client programmer. Someone who tries to access a private member will get a compile-time error. The protected keyword acts like private, with the exception that an inheriting class has access to protected members, but not private members. Inheritance will be introduced shortly.
Java also has a default access, which comes into play if you dont use one of the aforementioned specifiers. This is usually called package access because classes can access the members of other classes in the same package, but outside of the package those same members appear to be private.