Chapeter 4 Classes and Interfaces

Item 13 Minimize the accessibility of classes and members

• 1.A fundamental software design tenet: information hiding or encapsulation
  • it decouples the modules that comprise a system;
    • (1) allow modules to be [1]developed, [2]tested, [3]optimized, [4]used, [5]understood, and [6]modified in isolation
    • (2) can be developed in parallel
    • (3) It eases the burden of maintenance
    • (4) modules can be optimized without affecting the correctness of other modules
    • (5) information hiding decreases the risk in building large systems
• 2.facilities to aid in information hiding(access control)
  • The access control mechanism specifies the accessibility of classes, interfaces, and members.
    • access modifiers: private , protected , and public
  • For top-level (non-nested) classes and interfaces
    • TWO possible access levels: package-private and public.
  • For package-private top-level class (or interface) is used by only one class
    • consider making the top-level class a private nested class of the sole class that uses it (Item 22)
  • For members: (fields, methods, nested classes, and nested interfaces)
    • four possible access levels: private, package-private, protected, public
  • For instances:
    • Instance fields should never be public, classes with public mutable fields are not thread-safe, even if a field is final and refers to an immutable object
  • For static final fields:
    • he same advice applies to static fields. expose constants via public static final fields. such field can contain either primitive values or references to a immutable objects. If a final field containing a reference to a mutable object has all the disadvantages of a nonfinal field.
  • The rule of thumb is simple: make each class or member as inaccessible as possible
  • one rule that restricts your ability to reduce the accessibility of methods.
    • If a method overrides a superclass method, it is not permitted to have a lower access level in the subclass than it does in the superclass
    • [A special case: implicitly public] if a class implements an interface, all of the class methods that are also present in the interface must be declared public.
• 3.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.

          // it is wrong for a class to have a public static final array field, or an accessor that returns such a field
    
          ===============security hole======================
          // Potential security hole!
          public static final Thing[] VALUES = { ... };
          ==============Alternative 1=======================
          private static final Thing[] PRIVATE_VALUES = { ... };
          public static final List<Thing> VALUES =
              Collections.unmodifiableList(Arrays.asList(PRIVATE_VALUES));
          ==============Alternative 2=======================
          private static final Thing[] PRIVATE_VALUES = { ... };
          public static final Thing[] values() {
              return PRIVATE_VALUES.clone();
          }
    

Prevent any stray classes, interfaces, or members from becoming a part of the API