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Thinking in Java, 3rd ed. Revision 2.0

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5: Hiding the Implementation

A primary consideration in object-oriented design is “separating the things that change from the things that stay the same.”

This is particularly important for libraries. The user (client programmer) of that library must be able to rely on the part they use, and know that they won’t need to rewrite code if a new version of the library comes out. On the flip side, the library creator must have the freedom to make modifications and improvements with the certainty that the client programmer’s code won’t be affected by those changes. Comment

This can be achieved through convention. For example, the library programmer must agree to not remove existing methods when modifying a class in the library, since that would break the client programmer’s code. The reverse situation is thornier, however. In the case of a data member, how can the library creator know which data members have been accessed by client programmers? This is also true with methods that are only part of the implementation of a class, and not meant to be used directly by the client programmer. But what if the library creator wants to rip out an old implementation and put in a new one? Changing any of those members might break a client programmer’s code. Thus the library creator is in a strait jacket and can’t change anything. Comment

To solve this problem, Java provides access specifiers to allow the library creator to say what is available to the client programmer and what is not. The levels of access control from “most access” to “least access” are public, protected, “friendly” (which has no keyword), and private. From the previous paragraph you might think that, as a library designer, you’ll want to keep everything as “private” as possible, and expose only the methods that you want the client programmer to use. This is exactly right, even though it’s often counterintuitive for people who program in other languages (especially C) and are used to accessing everything without restriction. By the end of this chapter you should be convinced of the value of access control in Java. Comment

The concept of a library of components and the control over who can access the components of that library is not complete, however. There’s still the question of how the components are bundled together into a cohesive library unit. This is controlled with the package keyword in Java, and the access specifiers are affected by whether a class is in the same package or in a separate package. So to begin this chapter, you’ll learn how library components are placed into packages. Then you’ll be able to understand the complete meaning of the access specifiers. Comment

package: the library unit

A package is what you get when you use the import keyword to bring in an entire library, such as

import java.util.*;

This brings in the entire utility library that’s part of the standard Java distribution. Since, for example, the class ArrayList is in java.util, you can now either specify the full name java.util.ArrayList (which you can do without the import statement), or you can simply say ArrayList (because of the import). Comment

If you want to bring in a single class, you can name that class in the import statement

import java.util.ArrayList;

Now you can use ArrayList with no qualification. However, none of the other classes in java.util are available. Comment

The reason for all this importing is to provide a mechanism to manage “name spaces.” The names of all your class members are insulated from each other. A method f( ) inside a class A will not clash with an f( ) that has the same signature (argument list) in class B. But what about the class names? Suppose you create a stack class that is installed on a machine that already has a stack class that’s written by someone else? With Java on the Internet, this can happen without the user knowing it, since classes can get downloaded automatically in the process of running a Java program. Comment

This potential clashing of names is why it’s important to have complete control over the name spaces in Java, and to be able to create a completely unique name regardless of the constraints of the Internet. Comment

So far, most of the examples in this book have existed in a single file and have been designed for local use, and haven’t bothered with package names. (In this case the class name is placed in the “default package.”) This is certainly an option, and for simplicity’s sake this approach will be used whenever possible throughout the rest of this book. However, if you’re planning to create libraries or programs that are friendly to other Java programs on the same machine, you must think about preventing class name clashes. Comment

When you create a source-code file for Java, it’s commonly called a compilation unit (sometimes a translation unit). Each compilation unit must have a name ending in .java, and inside the compilation unit there can be a public class that must have the same name as the file (including capitalization, but excluding the .java filename extension). There can be only one public class in each compilation unit, otherwise the compiler will complain. The rest of the classes in that compilation unit, if there are any, are hidden from the world outside that package because they’re not public, and they comprise “support” classes for the main public class. Comment

When you compile a .java file you get an output file with exactly the same name but an extension of .class for each class in the .java file. Thus you can end up with quite a few .class files from a small number of .java files. If you’ve programmed with a compiled language, you might be used to the compiler spitting out an intermediate form (usually an “obj” file) that is then packaged together with others of its kind using a linker (to create an executable file) or a librarian (to create a library). That’s not how Java works. A working program is a bunch of .class files, which can be packaged and compressed into a JAR file (using Java’s jar archiver). The Java interpreter is responsible for finding, loading, and interpreting these files[33]. Comment

A library is also a bunch of these class files. Each file has one class that is public (you’re not forced to have a public class, but it’s typical), so there’s one component for each file. If you want to say that all these components (that are in their own separate .java and .class files) belong together, that’s where the package keyword comes in. Comment

When you say:

package mypackage;

at the beginning of a file (if you use a package statement, it must appear as the first noncomment in the file), you’re stating that this compilation unit is part of a library named mypackage. Or, put another way, you’re saying that the public class name within this compilation unit is under the umbrella of the name mypackage, and if anyone wants to use the name they must either fully specify the name or use the import keyword in combination with mypackage (using the choices given previously). Note that the convention for Java package names is to use all lowercase letters, even for intermediate words. Comment

For example, suppose the name of the file is MyClass.java. This means there can be one and only one public class in that file, and the name of that class must be MyClass (including the capitalization):

package mypackage;
public class MyClass {
  // . . .

Now, if someone wants to use MyClass or, for that matter, any of the other public classes in mypackage, they must use the import keyword to make the name or names in mypackage available. The alternative is to give the fully qualified name:

mypackage.MyClass m = new mypackage.MyClass();

The import keyword can make this much cleaner:

import mypackage.*;
// . . . 
MyClass m = new MyClass();

It’s worth keeping in mind that what the package and import keywords allow you to do, as a library designer, is to divide up the single global name space so you won’t have clashing names, no matter how many people get on the Internet and start writing classes in Java. Comment

Creating unique package names

You might observe that, since a package never really gets “packaged” into a single file, a package could be made up of many .class files, and things could get a bit cluttered. To prevent this, a logical thing to do is to place all the .class files for a particular package into a single directory; that is, use the hierarchical file structure of the operating system to your advantage. This is one way that Java references the problem of clutter; you’ll see the other way later when the jar utility is introduced. Comment

Collecting the package files into a single subdirectory solves two other problems: creating unique package names, and finding those classes that might be buried in a directory structure someplace. This is accomplished, as was introduced in Chapter 2, by encoding the path of the location of the .class file into the name of the package. The compiler enforces this, but by convention, the first part of the package name is the Internet domain name of the creator of the class, reversed. Since Internet domain names are guaranteed to be unique, if you follow this convention it’s guaranteed that your package name will be unique and thus you’ll never have a name clash. (That is, until you lose the domain name to someone else who starts writing Java code with the same path names as you did.) Of course, if you don’t have your own domain name then you must fabricate an unlikely combination (such as your first and last name) to create unique package names. If you’ve decided to start publishing Java code it’s worth the relatively small effort to get a domain name. Comment

The second part of this trick is resolving the package name into a directory on your machine, so when the Java program runs and it needs to load the .class file (which it does dynamically, at the point in the program where it needs to create an object of that particular class, or the first time you access a static member of the class), it can locate the directory where the .class file resides. Comment

The Java interpreter proceeds as follows. First, it finds the environment variable CLASSPATH (set via the operating system, sometimes by the installation program that installs Java or a Java-based tool on your machine). CLASSPATH contains one or more directories that are used as roots for a search for .class files. Starting at that root, the interpreter will take the package name and replace each dot with a slash to generate a path name from the CLASSPATH root (so package foo.bar.baz becomes foo\bar\baz or foo/bar/baz or possibly something else, depending on your operating system). This is then concatenated to the various entries in the CLASSPATH. That’s where it looks for the .class file with the name corresponding to the class you’re trying to create. (It also searches some standard directories relative to where the Java interpreter resides). Comment

To understand this, consider my domain name, which is bruceeckel.com. By reversing this, com.bruceeckel establishes my unique global name for my classes. (The com, edu, org, etc., extension was formerly capitalized in Java packages, but this was changed in Java 2 so the entire package name is lowercase.) I can further subdivide this by deciding that I want to create a library named simple, so I’ll end up with a package name:

package com.bruceeckel.simple;

Now this package name can be used as an umbrella name space for the following two files: Comment

//: com:bruceeckel:simple:Vector.java
// Creating a package.
package com.bruceeckel.simple;

public class Vector {
  public Vector() {
} ///:~

When you create your own packages, you’ll discover that the package statement must be the first noncomment code in the file. The second file looks much the same: Comment

//: com:bruceeckel:simple:List.java
// Creating a package.
package com.bruceeckel.simple;

public class List {
  public List() {
} ///:~

Both of these files are placed in the subdirectory on my system: Comment


If you walk back through this, you can see the package name com.bruceeckel.simple, but what about the first portion of the path? That’s taken care of in the CLASSPATH environment variable, which is, on my machine: Comment


You can see that the CLASSPATH can contain a number of alternative search paths. Comment

There’s a variation when using JAR files, however. You must put the name of the JAR file in the classpath, not just the path where it’s located. So for a JAR named grape.jar your classpath would include:


Once the classpath is set up properly, the following file can be placed in any directory:

//: c05:LibTest.java
// Uses the library.
import com.bruceeckel.simpletest.*;
import com.bruceeckel.simple.*;

public class LibTest {
  static Test monitor = new Test();
  public static void main(String[] args) {
    Vector v = new Vector();
    List l = new List();
    monitor.expect(new String[] {
} ///:~

When the compiler encounters the import statement, it begins searching at the directories specified by CLASSPATH, looking for subdirectory com\bruceeckel\simple, then seeking the compiled files of the appropriate names (Vector.class for Vector and List.class for List). Note that both the classes and the desired methods in Vector and List must be public. Comment

Setting the CLASSPATH has been such a trial for beginning Java users (it was for me, when I started) that Sun made the JDK in Java 2 a bit smarter. You’ll find that, when you install it, even if you don’t set a CLASSPATH you’ll be able to compile and run basic Java programs. To compile and run the source-code package for this book (available on the CD ROM packaged with this book, or at www.BruceEckel.com), however, you will need to make some modifications to your CLASSPATH (these are explained in the source-code package). Comment


What happens if two libraries are imported via * and they include the same names? For example, suppose a program does this:

import com.bruceeckel.simple.*;
import java.util.*;

Since java.util.* also contains a Vector class, this causes a potential collision. However, as long as you don’t write the code that actually causes the collision, everything is OK—this is good because otherwise you might end up doing a lot of typing to prevent collisions that would never happen. Comment

The collision does occur if you now try to make a Vector:

Vector v = new Vector();

Which Vector class does this refer to? The compiler can’t know, and the reader can’t know either. So the compiler complains and forces you to be explicit. If I want the standard Java Vector, for example, I must say:

java.util.Vector v = new java.util.Vector();

Since this (along with the CLASSPATH) completely specifies the location of that Vector, there’s no need for the import java.util.* statement unless I’m using something else from java.util. Comment

A custom tool library

With this knowledge, you can now create your own libraries of tools to reduce or eliminate duplicate code. Consider, for example, creating an alias for System.out.println( ) to reduce typing. This can be part of a package called tools:

//: com:bruceeckel:tools:P.java
// The P.rint & P.rintln shorthand.
package com.bruceeckel.tools;

public class P {
  public static void rint(String s) {
  public static void rintln(String s) {
} ///:~

You can use this shorthand to print a String either with a newline (P.rintln( )) or without a newline (P.rint( )). Comment

You can guess that the location of this file must be in a directory that starts at one of the CLASSPATH locations, then continues com/bruceeckel/tools. After compiling, the P.class file can be used anywhere on your system with an import statement:

//: c05:ToolTest.java
// Uses the tools library.
import com.bruceeckel.simpletest.*;
import com.bruceeckel.tools.*;

public class ToolTest {
  static Test monitor = new Test();
  public static void main(String[] args) {
    P.rintln("Available from now on!");
    P.rintln("" + 100); // Force it to be a String
    P.rintln("" + 100L);
    P.rintln("" + 3.14159);
    monitor.expect(new String[] {
      "Available from now on!",
} ///:~

Notice that all objects can easily be forced into String representations by putting them in a String expression; in the above case, starting the expression with an empty String does the trick. But this brings up an interesting observation. If you call System.out.println(100), it works without casting it to a String. With some extra overloading, you can get the P class to do this as well (this is an exercise at the end of this chapter). Comment

So from now on, whenever you come up with a useful new utility, you can add it to the tools directory. (Or to your own personal util or tools directory.) Comment

Using imports to change behavior

A feature that is missing from Java is C’s conditional compilation, which allows you to change a switch and get different behavior without changing any other code. The reason such a feature was left out of Java is probably because it is most often used in C to solve cross-platform issues: different portions of the code are compiled depending on the platform that the code is being compiled for. Since Java is intended to be automatically cross-platform, such a feature should not be necessary. Comment

However, there are other valuable uses for conditional compilation. A very common use is for debugging code. The debugging features are enabled during development, and disabled in the shipping product. Allen Holub (www.holub.com) came up with the idea of using packages to mimic conditional compilation. He used this to create a Java version of C’s very useful assertion mechanism, whereby you can say “this should be true” or “this should be false” and if the statement doesn’t agree with your assertion you’ll find out about it. Such a tool is quite helpful during debugging. Comment

Here is the class that you’ll use for debugging:

//: com:bruceeckel:tools:debug:Assert.java
// Assertion tool for debugging.
package com.bruceeckel.tools.debug;

public class Assert {
  private static void perr(String msg) {
  public final static void is_true(boolean exp) {
    if(!exp) perr("Assertion failed");
  public final static void is_false(boolean exp){
    if(exp) perr("Assertion failed");
  public final static void 
  is_true(boolean exp, String msg) {
    if(!exp) perr("Assertion failed: " + msg);
  public final static void 
  is_false(boolean exp, String msg) {
    if(exp) perr("Assertion failed: " + msg);
} ///:~

This class simply encapsulates Boolean tests, which print error messages if they fail. In Chapter 10, you’ll learn about a more sophisticated tool for dealing with errors called exception handling, but the perr( ) method will work fine in the meantime. Comment

The output is printed to the console standard error stream by writing to System.err. Comment

When you want to use this class, you add a line in your program:

import com.bruceeckel.tools.debug.*;

To remove the assertions so you can ship the code, a second Assert class is created, but in a different package: Comment

//: com:bruceeckel:tools:Assert.java
// Turning off the assertion output 
// so you can ship the program.
package com.bruceeckel.tools;

public class Assert {
  public final static void is_true(boolean exp){}
  public final static void is_false(boolean exp){}
  public final static void 
  is_true(boolean exp, String msg) {}
  public final static void 
  is_false(boolean exp, String msg) {}
} ///:~

Now if you change the previous import statement to:

import com.bruceeckel.tools.*;

The program will no longer print assertions. Here’s an example: Comment

//: c05:TestAssert.java
// Demonstrating the assertion tool.
// Comment the following, and uncomment the
// subsequent line to change assertion behavior:
import com.bruceeckel.simpletest.*;
import com.bruceeckel.tools.debug.*;
// import com.bruceeckel.tools.*;

public class TestAssert {
  static Test monitor = new Test();
  public static void main(String[] args) {
    Assert.is_true((2 + 2) == 5);
    Assert.is_false((1 + 1) == 2);
    Assert.is_true((2 + 2) == 5, "2 + 2 == 5");
    Assert.is_false((1 + 1) == 2, "1 +1 != 2");
    monitor.expect(new String[] {
      "Assertion failed",
      "Assertion failed",
      "Assertion failed: 2 + 2 == 5",
      "Assertion failed: 1 +1 != 2"
} ///:~

By changing the package that’s imported, you change your code from the debug version to the production version. This technique can be used for any kind of conditional code. Comment

Package caveat

It’s worth remembering that anytime you create a package, you implicitly specify a directory structure when you give the package a name. The package must live in the directory indicated by its name, which must be a directory that is searchable starting from the CLASSPATH. Experimenting with the package keyword can be a bit frustrating at first, because unless you adhere to the package-name to directory-path rule, you’ll get a lot of mysterious run-time messages about not being able to find a particular class, even if that class is sitting there in the same directory. If you get a message like this, try commenting out the package statement, and if it runs you’ll know where the problem lies. Comment

Java access specifiers

When used, the Java access specifiers public, protected, and private are placed in front of each definition for each member in your class, whether it’s a field or a method. Each access specifier controls the access for only that particular definition. This is a distinct contrast to C++, in which the access specifier controls all the definitions following it until another access specifier comes along. Comment

One way or another, everything has some kind of access specified for it. In the following sections, you’ll learn all about the various types of access, starting with the default access. Comment


What if you give no access specifier at all, as in all the examples before this chapter? The default access has no keyword, but it is commonly referred to as “friendly.” It means that all the other classes in the current package have access to the friendly member, but to all the classes outside of this package the member appears to be private. Since a compilation unit—a file—can belong only to a single package, all the classes within a single compilation unit are automatically friendly with each other. Thus, friendly elements are also said to have package access. Comment

Friendly access allows you to group related classes together in a package so that they can easily interact with each other. When you put classes together in a package (thus granting mutual access to their friendly members; i.e., making them “friends”) you “own” the code in that package. It makes sense that only code you own should have friendly access to other code you own. You could say that friendly access gives a meaning or a reason for grouping classes together in a package. In many languages the way you organize your definitions in files can be willy-nilly, but in Java you’re compelled to organize them in a sensible fashion. In addition, you’ll probably want to exclude classes that shouldn’t have access to the classes being defined in the current package. Comment

The class controls which code has access to its members. There’s no magic way to “break in.” Code from another package can’t show up and say, “Hi, I’m a friend of Bob’s!” and expect to see the protected, friendly, and private members of Bob. The only way to grant access to a member is to: Comment

  1. Make the member public. Then everybody, everywhere, can access it. Comment
  2. Make the member friendly by leaving off any access specifier, and put the other classes in the same package. Then the other classes can access the member. Comment
  3. As you’ll see in Chapter 6, when inheritance is introduced, an inherited class can access a protected member as well as a public member (but not private members). It can access friendly members only if the two classes are in the same package. But don’t worry about that now. Comment
  4. Provide “accessor/mutator” methods (also known as “get/set” methods) that read and change the value. This is the most civilized approach in terms of OOP, and it is fundamental to JavaBeans, as you’ll see in Chapter 13. Comment

public: interface access

When you use the public keyword, it means that the member declaration that immediately follows public is available to everyone, in particular to the client programmer who uses the library. Suppose you define a package dessert containing the following compilation unit: Comment

//: c05:dessert:Cookie.java
// Creates a library.
package c05.dessert;

public class Cookie {
  public Cookie() { 
   System.out.println("Cookie constructor"); 
  void bite() { System.out.println("bite"); }
} ///:~

Remember, Cookie.java must reside in a subdirectory called dessert, in a directory under c05 (indicating Chapter 5 of this book) that must be under one of the CLASSPATH directories. Don’t make the mistake of thinking that Java will always look at the current directory as one of the starting points for searching. If you don’t have a ‘.’ as one of the paths in your CLASSPATH, Java won’t look there. Comment

Now if you create a program that uses Cookie:

//: c05:Dinner.java
// Uses the library.
import com.bruceeckel.simpletest.*;
import c05.dessert.*;

public class Dinner {
  static Test monitor = new Test();
  public Dinner() {
   System.out.println("Dinner constructor");
  public static void main(String[] args) {
    Cookie x = new Cookie();
    //! x.bite(); // Can't access
    monitor.expect(new String[] {
      "Cookie constructor"
} ///:~

you can create a Cookie object, since its constructor is public and the class is public. (We’ll look more at the concept of a public class later.) However, the bite( ) member is inaccessible inside Dinner.java since bite( ) is friendly only within package dessert. Comment

The default package

You might be surprised to discover that the following code compiles, even though it would appear that it breaks the rules:

//: c05:Cake.java
// Accesses a class in a 
// separate compilation unit.
import com.bruceeckel.simpletest.*;

class Cake {
  static Test monitor = new Test();
  public static void main(String[] args) {
    Pie x = new Pie();
    monitor.expect(new String[] {
} ///:~

In a second file, in the same directory:

//: c05:Pie.java
// The other class.

class Pie {
  void f() { System.out.println("Pie.f()"); }
} ///:~

You might initially view these as completely foreign files, and yet Cake is able to create a Pie object and call its f( ) method! (Note that you must have ‘.’ in your CLASSPATH in order for the files to compile.) You’d typically think that Pie and f( ) are friendly and therefore not available to Cake. They are friendly—that part is correct. The reason that they are available in Cake.java is because they are in the same directory and have no explicit package name. Java treats files like this as implicitly part of the “default package” for that directory, and therefore friendly to all the other files in that directory. Comment

private: you can’t touch that!

The private keyword means that no one can access that member except that particular class, 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. Comment

The default “friendly” package access often provides an adequate amount of hiding; remember, a “friendly” member is inaccessible to the user of the package. 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 14.) Comment

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 example above, you cannot create a Sundae object via its constructor; instead you must call the makeASundae( ) method to do it for you[34]. Comment

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. Comment

The same is true for a private field inside a class. Unless you must expose the underlying implementation (which is a much rarer situation 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.) Comment

protected: “sort of friendly”

The protected access specifier requires a jump ahead to understand. First, you should be aware that you don’t need to understand this section to continue through this book up through inheritance (Chapter 6). But for completeness, here is a brief description and example using protected. Comment

The protected keyword deals with a concept called inheritance, which takes an existing class and adds new members to that class without touching the existing class, which we refer to as the base class. You can also change the behavior of existing members of the class. To inherit from an existing class, you say that your new class extends an existing class, like this:

class Foo extends Bar {

The rest of the class definition looks the same. Comment

If you create a new package and you inherit from a class in another package, the only members you have access to are the public members of the original package. (Of course, if you perform the inheritance in the same package, you have the normal package access to all the “friendly” members.) Sometimes the creator of the base class would like to take a particular member and grant access to derived classes but not the world in general. That’s what protected does. If you refer back to the file Cookie.java, the following class cannot access the “friendly” member:

//: c05:ChocolateChip.java
// Can't access friendly member
// in another class.
import com.bruceeckel.simpletest.*;
import c05.dessert.*;

public class ChocolateChip extends Cookie {
  static Test monitor = new Test();
  public ChocolateChip() {
     "ChocolateChip constructor");
  public static void main(String[] args) {
    ChocolateChip x = new ChocolateChip();
    //! x.bite(); // Can't access bite
    monitor.expect(new String[] {
      "Cookie constructor",
      "ChocolateChip constructor"
} ///:~

One of the interesting things about inheritance is that if a method bite( ) exists in class Cookie, then it also exists in any class inherited from Cookie. But since bite( ) is “friendly” in a foreign package, it’s unavailable to us in this one. Of course, you could make it public, but then everyone would have access and maybe that’s not what you want. If we change the class Cookie as follows:

public class Cookie {
  public Cookie() { 
    System.out.println("Cookie constructor");
  protected void bite() {

then bite( ) still has “friendly” access within package dessert, but it is also accessible to anyone inheriting from Cookie. However, it is not public.

protected also gives package access – that is, other classes in the same package may access protected elements. Comment

Interface and implementation

Access control is often referred to as implementation hiding. Wrapping data and methods within classes in combination with implementation hiding is often called encapsulation[35]. The result is a data type with characteristics and behaviors. Comment

Access control puts boundaries within a data type for two important reasons. The first is to establish what the client programmers can and can’t use. You can build your internal mechanisms into the structure without worrying that the client programmers will accidentally treat the internals as part of the interface that they should be using. Comment

This feeds directly into the second reason, which is to separate the interface from the implementation. If the structure is used in a set of programs, but client programmers can’t do anything but send messages to the public interface, then you can change anything that’s not public (e.g., “friendly,” protected, or private) without requiring modifications to client code. Comment

We’re now in the world of object-oriented programming, where a class is actually describing “a class of objects,” as you would describe a class of fishes or a class of birds. Any object belonging to this class will share these characteristics and behaviors. The class is a description of the way all objects of this type will look and act. Comment

In the original OOP language, Simula-67, the keyword class was used to describe a new data type. The same keyword has been used for most object-oriented languages. This is the focal point of the whole language: the creation of new data types that are more than just boxes containing data and methods. Comment

The class is the fundamental OOP concept in Java. It is one of the keywords that will not be set in bold in this book—it becomes annoying with a word repeated as often as “class.” Comment

For clarity, you might prefer a style of creating classes that puts the public members at the beginning, followed by the protected, friendly, and private members. The advantage is that the user of the class can then read down from the top and see first what’s important to them (the public members, because they can be accessed outside the file), and stop reading when they encounter the non-public members, which are part of the internal implementation:

public class X {
  public void pub1( ) { /* . . . */ }
  public void pub2( ) { /* . . . */ }
  public void pub3( ) { /* . . . */ }
  private void priv1( ) { /* . . . */ }
  private void priv2( ) { /* . . . */ }
  private void priv3( ) { /* . . . */ }
  private int i;
  // . . .

This will make it only partially easier to read because the interface and implementation are still mixed together. That is, you still see the source code—the implementation—because it’s right there in the class. In addition, the comment documentation supported by javadoc (described in Chapter 2) lessens the importance of code readability by the client programmer. Displaying the interface to the consumer of a class is really the job of the class browser, a tool whose job is to look at all the available classes and show you what you can do with them (i.e., what members are available) in a useful fashion. By the time you read this, browsers should be an expected part of any good Java development tool. Comment

Class access

In Java, the access specifiers can also be used to determine which classes within a library will be available to the users of that library. If you want a class to be available to a client programmer, you place the public keyword somewhere before the opening brace of the class body. This controls whether the client programmer can even create an object of the class. Comment

To control the access of a class, the specifier must appear before the keyword class. Thus you can say:

public class Widget {

Now if the name of your library is mylib any client programmer can access Widget by saying

import mylib.Widget;


import mylib.*;

However, there’s an extra set of constraints: Comment

  1. There can be only one public class per compilation unit (file). The idea is that each compilation unit has a single public interface represented by that public class. It can have as many supporting “friendly” classes as you want. If you have more than one public class inside a compilation unit, the compiler will give you an error message. Comment
  2. The name of the public class must exactly match the name of the file containing the compilation unit, including capitalization. So for Widget, the name of the file must be Widget.java, not widget.java or WIDGET.java. Again, you’ll get a compile-time error if they don’t agree. Comment
  3. It is possible, though not typical, to have a compilation unit with no public class at all. In this case, you can name the file whatever you like. Comment

What if you’ve got a class inside mylib that you’re just using to accomplish the tasks performed by Widget or some other public class in mylib? You don’t want to go to the bother of creating documentation for the client programmer, and you think that sometime later you might want to completely change things and rip out your class altogether, substituting a different one. To give you this flexibility, you need to ensure that no client programmers become dependent on your particular implementation details hidden inside mylib. To accomplish this, you just leave the public keyword off the class, in which case it becomes friendly. (That class can be used only within that package.) Comment

Note that a class cannot be private (that would make it accessible to no one but the class), or protected[36]. So you have only two choices for class access: “friendly” or public. If you don’t want anyone else to have access to that class, you can make all the constructors private, thereby preventing anyone but you, inside a static member of the class, from creating an object of that class[37]. Here’s an example: Comment

//: c05:Lunch.java
// Demonstrates class access specifiers.
// Make a class effectively private
// with private constructors:

class Soup {
  private Soup() {}
  // (1) Allow creation via static method:
  public static Soup makeSoup() {
    return new Soup();
  // (2) Create a static object and
  // return a reference upon request.
  // (The "Singleton" pattern):
  private static Soup ps1 = new Soup();
  public static Soup access() {
    return ps1;
  public void f() {}

class Sandwich { // Uses Lunch
  void f() { new Lunch(); }

// Only one public class allowed per file:
public class Lunch {
  void test() {
    // Can't do this! Private constructor:
    //! Soup priv1 = new Soup();
    Soup priv2 = Soup.makeSoup();
    Sandwich f1 = new Sandwich();
} ///:~

Up to now, most of the methods have been returning either void or a primitive type, so the definition:

  public static Soup access() {
    return ps1;

might look a little confusing at first. The word before the method name (access) tells what the method returns. So far this has most often been void, which means it returns nothing. But you can also return a reference to an object, which is what happens here. This method returns a reference to an object of class Soup. Comment

The class Soup shows how to prevent direct creation of a class by making all the constructors private. Remember that if you don’t explicitly create at least one constructor, the default constructor (a constructor with no arguments) will be created for you. By writing the default constructor, it won’t be created automatically. By making it private, no one can create an object of that class. But now how does anyone use this class? The above example shows two options. First, a static method is created that creates a new Soup and returns a reference to it. This could be useful if you want to do some extra operations on the Soup before returning it, or if you want to keep count of how many Soup objects to create (perhaps to restrict their population). Comment

The second option uses what’s called a design pattern, which is covered in Thinking in Patterns with Java, downloadable at www.BruceEckel.com. This particular pattern is called a “singleton” because it allows only a single object to ever be created. The object of class Soup is created as a static private member of Soup, so there’s one and only one, and you can’t get at it except through the public method access( ). Comment

As previously mentioned, if you don’t put an access specifier for class access it defaults to “friendly.” This means that an object of that class can be created by any other class in the package, but not outside the package. (Remember, all the files within the same directory that don’t have explicit package declarations are implicitly part of the default package for that directory.) However, if a static member of that class is public, the client programmer can still access that static member even though they cannot create an object of that class. Comment


In any relationship it’s important to have boundaries that are respected by all parties involved. When you create a library, you establish a relationship with the user of that library—the client programmer—who is another programmer, but one putting together an application or using your library to build a bigger library. Comment

Without rules, client programmers can do anything they want with all the members of a class, even if you might prefer they don’t directly manipulate some of the members. Everything’s naked to the world. Comment

This chapter looked at how classes are built to form libraries; first, the way a group of classes is packaged within a library, and second, the way the class controls access to its members. Comment

It is estimated that a C programming project begins to break down somewhere between 50K and 100K lines of code because C has a single “name space” so names begin to collide, causing an extra management overhead. In Java, the package keyword, the package naming scheme, and the import keyword give you complete control over names, so the issue of name collision is easily avoided. Comment

There are two reasons for controlling access to members. The first is to keep users’ hands off tools that they shouldn’t touch; tools that are necessary for the internal machinations of the data type, but not part of the interface that users need to solve their particular problems. So making methods and fields private is a service to users because they can easily see what’s important to them and what they can ignore. It simplifies their understanding of the class. Comment

The second and most important 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. You might build a class one way at first, and then discover that restructuring your code will provide much greater speed. If the interface and implementation are clearly separated and protected, you can accomplish this without forcing the user to rewrite their code. Comment

Access specifiers in Java give valuable control to the creator of a class. The users of the class can clearly see exactly what they can use and what to ignore. More important, though, is the ability to ensure that no user becomes dependent on any part of the underlying implementation of a class. If you know this as the creator of the class, you can change the underlying implementation with the knowledge that no client programmer will be affected by the changes because they can’t access that part of the class. Comment

When you have the ability to change the underlying implementation, you can not only improve your design later, but you also have the freedom to make mistakes. No matter how carefully you plan and design you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn faster, and you’ll finish your project sooner. Comment

The public interface to a class is what the user does see, so that is the most important part of the class to get “right” during analysis and design. Even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more methods, as long as you don’t remove any that client programmers have already used in their code. Comment


Solutions to selected exercises can be found in the electronic document The Thinking in Java Annotated Solution Guide, available for a small fee from www.BruceEckel.com.

  1. Write a program that creates an ArrayList object without explicitly importing java.util.*. Comment
  2. In the section labeled “package: the library unit,” turn the code fragments concerning mypackage into a compiling and running set of Java files. Comment
  3. In the section labeled “Collisions,” take the code fragments and turn them into a program, and verify that collisions do in fact occur. Comment
  4. Generalize the class P defined in this chapter by adding all the overloaded versions of rint( ) and rintln( ) necessary to handle all the different basic Java types. Comment
  5. Change the import statement in TestAssert.java to enable and disable the assertion mechanism. Comment
  6. Create a class with public, private, protected, and “friendly” data members and method members. Create an object of this class and see what kind of compiler messages you get when you try to access all the class members. Be aware that classes in the same directory are part of the “default” package. Comment
  7. Create a class with protected data. Create a second class in the same file with a method that manipulates the protected data in the first class. Comment
  8. Change the class Cookie as specified in the section labeled “protected: ‘sort of friendly.’” Verify that bite( ) is not public. Comment
  9. In the section titled “Class access” you’ll find code fragments describing mylib and Widget. Create this library, then create a Widget in a class that is not part of the mylib package. Comment
  10. Create a new directory and edit your CLASSPATH to include that new directory. Copy the P.class file (produced by compiling com.bruceeckel.tools.P.java) to your new directory and then change the names of the file, the P class inside and the method names. (You might also want to add additional output to watch how it works.) Create another program in a different directory that uses your new class. Comment
  11. Following the form of the example Lunch.java, create a class called ConnectionManager that manages a fixed array of Connection objects. The client programmer must not be able to explicitly create Connection objects, but can only get them via a static method in ConnectionManager. When the ConnectionManager runs out of objects, it returns a null reference. Test the classes in main( ). Comment
  12. Create the following file in the c05/local directory (presumably in your CLASSPATH):
// c05:local:PackagedClass.java
package c05.local;
class PackagedClass {
  public PackagedClass() {
      "Creating a packaged class");

Then create the following file in a directory other than c05:

// c05:foreign:Foreign.java
package c05.foreign;
import c05.local.*;
public class Foreign {
   public static void main (String[] args) {
      PackagedClass pc = new PackagedClass();

Explain why the compiler generates an error. Would making the Foreign class part of the c05.local package change anything? Comment

[33] There’s nothing in Java that forces the use of an interpreter. There exist native-code Java compilers that generate a single executable file.

[34] There’s another effect in this case: Since the default constructor is the only one defined, and it’s private, it will prevent inheritance of this class. (A subject that will be introduced in Chapter 6.)

[35] However, people often refer to implementation hiding alone as encapsulation.

[36] Actually, an inner class can be private or protected, but that’s a special case. These will be introduced in Chapter 7.

[37] You can also do it by inheriting (Chapter 6) from that class.

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