面向 Java 开发人员的 db4o 指南: 结构化对象和集合
2010-04-01 00:00:00 来源:WEB开发网db4o 中的继承(以及多态)其实就是这么简单。没有用于查询语言的复杂的 IS 扩展,就不会引入不同于 Java 类型系统中现有概念的 “类型” 概念。我所指的只是期望作为查询的一部分的类型,而且这些是构成查询的主要成分。这跟在 SQL 查询中加入表格很相似,方法就是选择其数据应为查询结果一部分的表格。额外的好处是,“父类型” 也是作为查询的一部分隐式地 “加入” 的。清单 6 显示了 清单 3 中 InheritanceTest 的输出:
清单 6. 多态发挥作用
.Found [Employee: Ted Neward (President and CEO)]
Found [Person: firstName = Charlotte lastName = Neward gender = FEMALE age = 35
mood = HAPPY spouse = Ted ]
Found [Person: firstName = Michael lastName = Neward gender = MALE age = 14 mood
= CRANKY ]
Found [Person: firstName = Matthew lastName = Neward gender = MALE age = 8 mood
= CRANKY ]
Found [Employee: Bruce Tate (Chief Technical Officer)]
Found [Person: firstName = Maggie lastName = Tate gender = FEMALE age = 29 mood
= HAPPY spouse = Bruce ]
Found [Person: firstName = Kayla lastName = Tate gender = FEMALE age = 0 mood =
CRANKY ]
Found [Person: firstName = Julia lastName = Tate gender = FEMALE age = 0 mood =
CRANKY ]
Found [Employee: Neal Ford (Meme Wrangler)]
Found [Person: firstName = Candi lastName = Ford gender = FEMALE age = 29 mood =
HAPPY spouse = Neal ]
Found [Employee: Brian Sletten (Bosatsu Master)]
Found [Person: firstName = Kristen lastName = Sletten gender = FEMALE age = 29 m
ood = HAPPY spouse = Brian ]
Found [Employee: Ben Galbraith (Chief UI Director)]
Found [Person: firstName = Jessica lastName = Galbraith gender = FEMALE age = 29
mood = HAPPY spouse = Ben ]
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