Abstraction function——抽象函式
抽象函式是表示值到其對應的抽象值的對映——AF: R->A。
對於抽象函式來說，僅僅寬泛的說抽象域表示了什麼並不夠。抽象函式的作用是規定合法的表示值會如何被解釋到抽象域。作為一個函式，我們應該清晰的知道從一個輸入到一個輸入是怎麼對應的。
Rep invariant——表示不變數
註明抽象值的合法區域。
說明合法/不合法的原因。
程式碼示例

// Immutable type representing a tweet.
public class Tweet {

private final String author;
private final String text;
 
private final Date timestamp;

// Rep invariant:
// author is a Twitter username (a nonempty string of letters, digits, underscores)
// text.length <= 140
// Abstraction function:
// AF(author, text, timestamp) = a tweet posted by author, with content text, 
// at time timestamp 
// Safety from rep exposure:
// All fields are private;
 
// author and text are Strings, so are guaranteed immutable;
// timestamp is a mutable Date, so Tweet() constructor and getTimestamp() 
// make defensive copies to avoid sharing the rep's Date object with clients.

// Operations (specs and method bodies omitted to save space)
public Tweet(String author, String text, Date timestamp) { ... }
 
public String getAuthor() { ... }
public String getText() { ... }
public Date getTimestamp() { ... }
}