Saturday, 7 July 2012

Item 13: Minimize the accessibility of classes and members

A well-designed module hides all of its implementation details, cleanly separating its API from its implementation. Modules then communicate only through their APIs and are oblivious to each others’ inner workings. This concept, known as information hiding or encapsulation, is one of the fundamental tenets of software design.

Information hiding is important for many reasons, most of which stem from the fact that it decouples the modules that comprise a system, allowing them to be developed, tested, optimized, used, understood, and modified in isolation. This speeds up system development because modules can be developed in parallel. It eases the burden of maintenance because modules can be understood more quickly and debugged with little fear of harming other modules.

Finally, information hiding decreases the risk in building large systems, because individual modules may prove successful even if the system does not.

Java has many facilities to aid in information hiding. The access control mechanism [JLS, 6.6] specifies the accessibility of classes, interfaces, and members.

The rule of thumb is simple: make each class or member as inaccessible as possible. In other words, use the lowest possible access level consistent with the proper functioning of the software that you are writing.

For top-level (non-nested) classes and interfaces, there are only two possible access levels: package-private and public. If you declare a top-level class or interface with the public modifier, it will be public; otherwise, it will be package-private. If a top-level class or interface can be made package-private, it should be. By making it package-private, you make it part of the implementation rather than the exported API, and you can modify it, replace it, or eliminate it in a subsequent release without fear of harming existing clients. If you make it public, you are obligated to support it forever to maintain compatibility.

For members (fields, methods, nested classes, and nested interfaces), there are four possible access levels, listed here in order of increasing accessibility:

private—The member is accessible only from the top-level class where it is declared.

package-private—The member is accessible from any class in the package where it is declared. Technically known as default access, this is the access level you get if no access modifier is specified.

protected—The member is accessible from subclasses of the class where it is declared and from any class in the package where it is declared.

public—The member is accessible from anywhere.

After carefully designing your class’s public API, your reflex should be to make all other members private. Only if another class in the same package really needs to access a member should you remove the private modifier, making the member package-private.

Instance fields should never be public (Item 14). If an instance field is nonfinal, or is a final reference to a mutable object, then by making the field public, you give up the ability to limit the values that can be stored in the field.

Classes with public mutable fields are not thread-safe. Even if a field is final and refers to an immutable object, by making the field public you give up the flexibility to switch to a new internal data representation in which the field does not exist.

Note that a nonzero-length array is always mutable, so it is wrong for a class to have a public static final array field, or an accessor that returns such a field. If a class has such a field or accessor, clients will be able to modify the contents of the array. This is a frequent source of security holes:

// Potential security hole!
public static final Thing[] VALUES = { ... };

Beware of the fact that many IDEs generate accessors that return references to private array fields, resulting in exactly this problem. There are two ways to fix the problem. You can make the public array private and add a public immutable list:

private static final Thing[] PRIVATE_VALUES = { ... };
public static final List<Thing> VALUES =

Alternatively, you can make the array private and add a public method that returns a copy of a private array:

private static final Thing[] PRIVATE_VALUES = { ... };
public static final Thing[] values() {
return PRIVATE_VALUES.clone();

To choose between these alternatives, think about what the client is likely to do with the result. Which return type will be more convenient? Which will give better performance?

To summarize, you should always reduce accessibility as much as possible. After carefully designing a minimal public API, you should prevent any stray classes, interfaces, or members from becoming a part of the API. With the exception of public static final fields, public classes should have no public fields. Ensure that objects referenced by public static final fields are immutable.

Reference: Effective Java 2nd Edition by Joshua Bloch