Item 13 - Minimize the accessibility of classes and members

From Effective Java 2/e by Joshua Bloch

Information Hiding or Encapsulation

Principle of segregation of the design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed. The protection involves providing a stable interface which protects the remainder of the program from the implementation (the details that are most likely to change).

  • Decouples the modules that comprise a system, allowing them to be developed, tested, optimized, used, understood, and modified in isolation
  • Speeds up system development because modules can be developed in parallel
  • Eases the burden of maintenance because modules can be understood more quickly and debugged with little fear of harming other modules
  • Enables effective performance tuning: once a system is complete and profiling has determined which modules are causing performance problems

Public Class

  • Instance fields should never be public:
    • Impossible to take any action when the field is modified
    • Mostly private
    • Sometimes package-private
    • Protected fields is part of the class’s exported API and must be supported forever
    • Classes with public mutable fields are not thread-safe

Public static final fields

// Exception
public static final String CLASS_NAME = "Point";

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

// Alternative #1: Private array and add a public immutable list
private static final Thing[] PRIVATE_VALUES = { ... };
public static final List<Thing> VALUES = Collections.unmodifiableList(Arrays.asList(PRIVATE_VALUES));

// Alternative #2: Private array 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();


  • Always reduce accessibility as much as possible
  • Prevent any stray classes, interfaces, or members from becoming a part of the API
  • Except immutable public static final fields, public classes should have no public fields

Access Levels

Modifier Class Package Subclass World
public Y Y Y Y
protected Y Y Y N
no modifier Y Y N N
private Y N N N

Posted by The Finest Artist