328: Highlights

• Programs should obey the explicit-representation principle, with classes included to reflect natural categories.
• Programs should obey the no-duplication principle, with instance methods situated among class definitions to facilitate sharing.
• Programs should obey the look-it-up principle, with class definitions including instance variables for stable, frequently requested information.
• Programs should obey the need-to-know principle, with public interfaces designed to restrict instance-variable and instance-method access, thus facilitating the improvement and maintenance of nonpublic program elements.
• If you find yourself using the phrase an X is a Y when describing the relation between two classes, then the X class is a subclass of the Y class.
• If you find yourself using X has a Y when describing the relation between two classes, then instances of the Y class appear as parts of instances of the X class.