Effective Java, Third Edition

To recap, there are four different kinds of nested classes, and each has its place. If a nested class needs to be visible outside of a single method or is too long to fit comfortably inside a method, use a member class. If each instance of a member class needs a reference to its enclosing instance, make it nonstatic; otherwise, make it static. Assuming the class belongs inside a method, if you need to create instances from only one location and there is a preexisting type that characterizes the class, make it an anonymous class; otherwise, make it a local class.

In summary, interfaces should be used only to define types. They should not be used merely to export constants.

Default methods are, however, extremely useful for providing standard method implementations when an interface is created, to ease the task of implementing the interface (Item 20).

eliminate the class’s self-use of overridable methods entirely. In doing so, you’ll create a class that is reasonably safe to subclass. Overriding a method will never affect the behavior of any other method.

The toString method should return a concise, useful description of the object

providing a good toString implementation makes your class much more pleasant to use and makes systems using the class easier to debug.

when is it appropriate to override equals? It is when a class has a notion of logical equality that differs from mere object identity and a superclass has not already overridden equals.

The Boolean.valueOf(boolean) method illustrates this technique: it never creates an object. This technique is similar to the Flyweight pattern [Gamma95]. It can greatly improve performance if equivalent objects are requested often, especially if they are expensive to create.

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.

a method overrides a superclass method, it cannot have a more restrictive access level in the subclass than in the superclass