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();
}
Conclusion
- 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 |