1장. Introduction중에서
"A program should read like a book."
The goal of this book is to help you communicate your intentions through your code.
p4 그림 1.1

2장. Patterns중에서
Most programs follow a small set of laws:
- Programs are read more often than they are written.
- There is no such thing as "done". Much more investment will be spent modifying programs than developing them initially.
- They are structured using a basic set of state and control flow concepts.
- Readers need to understand programs in detail and in concept. Sometimes they move from detail to concept, sometimes from concept to detail.

3장. A Theory of Programming중에서
Each pattern carries with it a little bit of theory. there are larger and more pervasive forces at work in programming than are covered in individual patterns, however. They are divided here into two types: values and principles.
The patterns describe what to do. The values provide motivation. The principles help translate motive into action.

Values
Three values that consistent with excellence in programming are communication, simplicity, and flexibility.
Code communicates well when a reader can understand it, modify it, or use it.
Eliminating excess complexity enables those reading, using, and modifying programs to understand them more quickly.
Just as good prose is written with an audience in mind, so good programs are written with an audience in mind. Challenging your audience a little is fine, but too much complexity will lose them.

Programs should be flexible, but only in ways they change.

Principles
Principles can provide an explanation for the motivation behind a pattern, one connected to general rather than specific ideas.

Local Consequences
Structure the code so changes have local consequences.

Minimize Repetition, Logic and Data Together

Symmetry
Once readers understand one half of the symmetry, they can quickly understand the other half.
Symmetry in code is where the same idea is expressed the same way everywhere it  appears in the code.

Declarative Expression
Express as much of my intention as possible declaratively.

Rate of Change
Put logic or data that changes at the same rate together and separate logic or data that chnages at different rates. These rates of change are a form of temporal symmetry.

4장. Motivation중에서
The premature attempts to make the code general enough to meet future needs often interfere with the unanticipated changes that turn out to be necessary.
My strategy for reducing overall costs is to ask all programmers to address the cost of understanding code during the maintenace phase by focusing on communicating, programmer-to-programmer.
Conversations about programming are also more interesting when they cover both theory and practice.

Use class names to tell the story of your code.

Interface
One way of saying "Here's what I want to accomplish and beyound that are details that shouldn't concern me".

Abstract Class
The super class is abstract in the sense that it can be replaced at runtime with any subclass.
Interfaces and class hierarchies are not mutually exclusive. You can provide an interface which says, "Here's how to access this kind of functionality," and a superclass which says, "Here's one way to implement this functionality." In this case, variables should be declared with the interface as teir type so future maintainers are free to substitute new implementations as necessary.

Versioned Interface
역할을 인터페이스로 구현하였을 경우, 그 역할을 사용하는 client와 구현은 다수일 수 있다. 이 경우 인터페이스를 변경하는 것은 큰 영향을 미칠 수 있다. 이런 경우 versioned interface를 사용한다.
Users who want the new functionality use the extended interface while existing users remain oblivious to the existence of the new interface. Anywhere you want to access the new operation you must explicitly check the type of your object and downcast to the new type.

Value Object
While objects-with-changing-state is one valuable way to think about computation, it is not the only way to think. Mathematics has developed over millennia as a way to think about situations that can be reduced to an abstract wourld of absolute truth and certainty, where statements can be reduced to an abstract world of absolute truth and certainty, where statements can be made about eternal verities.
This functional style of computing never changes any state, it only creates new values. When you have a (perhaps momentarily) static situation about which you'd like to make satements or about which you'd like to ask questions, then the functional style is appropriate.
The beauty of mathematical representations is that sequence seldom matters. You are creating a world in which you can make absolute, timeless statements. Create micro-worlds of mathemetics wherever possible. Manage them from an object with changing state.

The beauty of mathematical representations is that sequence seldom matters.

To implement value-style object, first draw the boundary between the world of state and the world of value.

Having gotten this far, I sense that there is much more to be written about programming with the three main styles - objects, functions, and procedures - and how to blend them effectively.

direct access inside of a class - indirect access for clients
Rectangle void setWidth(int width)
{
  this.width = width;
  area = width * height;
}

Local Variable
지역변수의 공통 역할들
- collector, count, explaining, reuse, iterated element
복잡한 식을 갖을 때 식의 부분들이 지역변수에 할당 될 수 있다. 지역변수는 한 단계 더 나아가 helper 메소드들이 될 수 있다.

iterated element에는 each를, 개수를 세는 것에는 count를 변수이름으로 사용한다. 같은 이름이 주어져야 한다면 각 이름을 해당 컨텍스트에 맞게 구체화한다. 예를 들어, exchX, rowCount

Lazy initialization says, "Performance is important here."