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Thinking in Java
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private: you can’t touch that!

The private keyword means that no one can access that member except the class that contains that member, inside methods of that class. Other classes in the same package cannot access private members, so it’s as if you’re even insulating the class against yourself. On the other hand, it’s not unlikely that a package might be created by several people collaborating together, so private allows you to freely change that member without concern that it will affect another class in the same package.

The default package access often provides an adequate amount of hiding; remember, a package-access member is inaccessible to the client programmer using the class. This is nice, since the default access is the one that you normally use (and the one that you’ll get if you forget to add any access control). Thus, you’ll typically think about access for the members that you explicitly want to make public for the client programmer, and as a result, you might not initially think you’ll use the private keyword often since it’s tolerable to get away without it. (This is a distinct contrast with C++.) However, it turns out that the consistent use of private is very important, especially where multithreading is concerned. (As you’ll see in Chapter 13.)

Here’s an example of the use of private:

//: c05:IceCream.java
// Demonstrates "private" keyword.

class Sundae {
  private Sundae() {}
  static Sundae makeASundae() {
    return new Sundae();
  }
}

public class IceCream {
  public static void main(String[] args) {
    //! Sundae x = new Sundae();
    Sundae x = Sundae.makeASundae();
  }
} ///:~


This shows an example in which private comes in handy: you might want to control how an object is created and prevent someone from directly accessing a particular constructor (or all of them). In the preceding example, you cannot create a Sundae object via its constructor; instead, you must call the makeASundae( ) method to do it for you.[28]

Any method that you’re certain is only a “helper” method for that class can be made private, to ensure that you don’t accidentally use it elsewhere in the package and thus prohibit yourself from changing or removing the method. Making a method private guarantees that you retain this option.

The same is true for a private field inside a class. Unless you must expose the underlying implementation (which is less likely than you might think), you should make all fields private. However, just because a reference to an object is private inside a class doesn't mean that some other object can't have a public reference to the same object. (See Appendix A for issues about aliasing.)
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire