Effective Java, Third Edition

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.

classes with public mutable fields are not generally thread-safe

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

The rule of thumb is simple: make each class or member as inaccessible as possible.

Use of the relational operators < and > in compareTo methods is verbose and error-prone and no longer recommended.

the equality test imposed by the compareTo method should generally return the same results as the equals method