我一直想着如何使用不可变的case类在Scala中实现双链接树或列表。对于大多数“更新”操作，我一直在使用复制和更新方法。例如，在设置父母的孩子时，我说
parent = parent.copy(child=child)
或者在设置孩子的父母时，我说
child = child.copy(parent=parent)
我意识到如果我将父级设置为包含子级，然后创建并更新新子级以包含父级，则父级将包含对旧子级的引用。同样，如果我试图以相反的方式进行，那么孩子将包含对旧父母的引用。
我希望我的树能够双重连接，所以我可以双向爬行：从根到他的孩子，或者从叶子向上到父母。是否有可能以这种方式“同时”链接父节点和子节点，为我提供循环引用，然后我可以双向抓取？
我可以使用可变数据轻松完成此操作，但在我的情况下，双重链接树将在创建后存在很长时间，并且我希望尽可能保持不变。
答案 0 :(得分：23)
让我们一步一步地尝试。
根据经验，在创建不可变对象时，所有构造函数参数都应该在实例化时知道，但是让我们按名称欺骗并传递构造函数参数，然后使用延迟字段来延迟评估，这样我们就可以创建双向链接元素之间：
// p and n are passed by name
// and won't be evaluated until prev and next are accessed
// for the first time
class Element [T] (val value: T, p : => Element[T], n : => Element [T]) {
lazy val prev = p
lazy val next = n
}
val e1:Element[Int] = new Element [Int] (1,null,e2)
val e2:Element[Int] = new Element [Int] (2,e1,e3)
val e3:Element[Int] = new Element [Int] (3,e2,e4)
val e4:Element[Int] = new Element [Int] (4,e3,null)
一旦我们运行代码，我们将收到一个不可变的双向链表：
null←e1（1）↔e2（2）↔e3（3）↔e4（4）→null
并且能够来回穿越它：
println(e1.next.next.next.value)
println(e4.prev.prev.prev.value)
4
1
现在，假设我们要在列表末尾添加第五个元素，以便它看起来像这样：
null←e1（1）↔e2（2）↔e3（3）↔e4（4）↔e5（5）→ null
val e5:Element[Int] = new Element [Int] (5,e4,null)
我们得到了：
null ← e1(1) ↔ e2(2) ↔ e3(3) ↔ e4(4) → null
↖ ↑
e5(5)
等一下，看起来不对劲！ e4 应指向 e5 ，而不是指向 null ，但 e4 是不可变的，我们无法更改元素本身，所以看起来唯一的选择是制作副本，并将其指向 e3 和 e5 。让我们尝试将这种新方法应用于初始列表：
null←e1（1）↔e2（2）↔e3（3）↔e4（4）→null
val e4_b: Element[Int] = new Element [Int] (e4.value, // keeping original value
e3,e5)
val e5 : Element[Int] = new Element [Int] (5,e4_b,null)
那更好，e4_b会导致 e5 导致回到e4_b：
null ← e1(1) ↔ e2(2) ↔ e3(3) ↔ e4(4) → null
↖ ↑
e4_b(4) ↔ e5(5)
但是现在我们有同样的原始问题，只有 e3 仍然指向 e4 。你能看到一种趋势出现吗？如果我们不断复制元素来解决问题，我们最终会：
null ← e1(1) ↔ e2(2) ↔ e3(3) ↔ e4(4) → null
↑ ↑
e1_b(1) ↔ e2_b(2) ↔ e3_b(3) ↔ e4_b(4) ↔ e5(5)
原始列表没有改变一点（因为它变成了我们没有把它称为“不可变”），而是我们最终得到一个全新的列表，尽管保持相同的值。因此，每当我们尝试对不可变的双向链接数据结构进行更改时，我们需要从头开始重建整个事物，保留值。
让我们来看看Scala标准单链接的不可变列表：
e1（1）→e2（2）→e3（3）→e4（4）→无
我们会注意到，我们能够更轻松地派生新列表，而无需从头开始重建整个数据结构，例如删除第二个元素，我们只需要复制第一个和第一个元素它到了第三个：
e1(1) → e2(2) → e3(3) → e4(4) → Nil
↗
e1_b(1)
当然，因为原始列表是不可变的，所以它并没有真正改变。
答案 1 :(得分：8)
你可以用懒惰来做，例如：
trait Link[A] {
def value: A
def get: Link[A]
}
class Circular[A](val value: A, getter: => Link[A]) extends Link[A] {
lazy val get = getter
}
object circles {
def create[A](as: (A, A)): Link[A] = {
lazy val b: Link[A] = new Circular(as._1, new Circular(as._2, b))
b
}
}
话虽这么说，你可能想长时间地问自己为什么要这样的事情。
答案 2 :(得分：2)
我创建了一篇博文，介绍了一个可能解决问题的方法。 http://akikhtenko.github.io/blog/2013/12/15/immutable-double-linked-tree-construction-in-scala/ 它以树为例，但将这个想法应用于其他数据类型应该不是问题。
答案 3 :(得分：0)
scala中的不变性意味着在我们完成构造对象之后它不应该改变。在对象构造期间，它实际上是可变的。解决方案是将一段代码传递给。的构造函数 在字段变为不可变之前计算所需值的对象。
Error starting ApplicationContext. To display the auto-configuration report re-run your application with 'debug' enabled.
2017-01-27 10:20:47.885 ERROR 12097 --- [ main] o.s.boot.SpringApplication : Application startup failed
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'cepController': Unsatisfied dependency expressed through field 'cityService'; nested exception is org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'cityService' defined in file [/home/myUser/rapha2/gcp-ws/target/classes/com/myproj/address/service/CityService.class]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'ICityRepository': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Validation failed for query for method public abstract java.lang.Double com.myproj.address.domain.repository.ICityRepository.findCustom()!
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:588) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:366) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1225) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:552) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:759) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866) ~[spring-context-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542) ~[spring-context-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122) ~[spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:371) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:315) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1186) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1175) [spring-boot-1.4.3.RELEASE.jar:1.4.3.RELEASE]
at com.myproj.GcpApplication.main(GcpApplication.java:12) [classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_111]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_111]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_111]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_111]
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147) [idea_rt.jar:na]
Caused by: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'cityService' defined in file [/home/myUser/rapha2/gcp-ws/target/classes/com/myproj/address/service/CityService.class]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'ICityRepository': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Validation failed for query for method public abstract java.lang.Double com.myproj.address.domain.repository.ICityRepository.findCustom()!
at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:749) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:189) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1154) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1056) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:513) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:207) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1136) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1064) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:585) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
... 24 common frames omitted
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'ICityRepository': Invocation of init method failed; nested exception is java.lang.IllegalArgumentException: Validation failed for query for method public abstract java.lang.Double com.myproj.address.domain.repository.ICityRepository.findCustom()!
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1589) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:554) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:483) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:202) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.config.DependencyDescriptor.resolveCandidate(DependencyDescriptor.java:207) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1136) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1064) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:835) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:741) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
... 37 common frames omitted
Caused by: java.lang.IllegalArgumentException: Validation failed for query for method public abstract java.lang.Double com.myproj.address.domain.repository.ICityRepository.findCustom()!
at org.springframework.data.jpa.repository.query.SimpleJpaQuery.validateQuery(SimpleJpaQuery.java:92) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.SimpleJpaQuery.<init>(SimpleJpaQuery.java:62) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.JpaQueryFactory.fromMethodWithQueryString(JpaQueryFactory.java:72) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.JpaQueryFactory.fromQueryAnnotation(JpaQueryFactory.java:53) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.JpaQueryLookupStrategy$DeclaredQueryLookupStrategy.resolveQuery(JpaQueryLookupStrategy.java:144) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.JpaQueryLookupStrategy$CreateIfNotFoundQueryLookupStrategy.resolveQuery(JpaQueryLookupStrategy.java:212) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.query.JpaQueryLookupStrategy$AbstractQueryLookupStrategy.resolveQuery(JpaQueryLookupStrategy.java:77) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactorySupport$QueryExecutorMethodInterceptor.<init>(RepositoryFactorySupport.java:435) ~[spring-data-commons-1.12.6.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactorySupport.getRepository(RepositoryFactorySupport.java:220) ~[spring-data-commons-1.12.6.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.initAndReturn(RepositoryFactoryBeanSupport.java:280) ~[spring-data-commons-1.12.6.RELEASE.jar:na]
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.afterPropertiesSet(RepositoryFactoryBeanSupport.java:266) ~[spring-data-commons-1.12.6.RELEASE.jar:na]
at org.springframework.data.jpa.repository.support.JpaRepositoryFactoryBean.afterPropertiesSet(JpaRepositoryFactoryBean.java:92) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1648) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1585) ~[spring-beans-4.3.5.RELEASE.jar:4.3.5.RELEASE]
... 48 common frames omitted
Caused by: java.lang.IllegalArgumentException: org.hibernate.hql.internal.ast.QuerySyntaxException: city is not mapped [select count(*)*2.1 from city]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1679) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1602) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1608) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:294) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_111]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_111]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_111]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_111]
at org.springframework.orm.jpa.ExtendedEntityManagerCreator$ExtendedEntityManagerInvocationHandler.invoke(ExtendedEntityManagerCreator.java:347) ~[spring-orm-4.3.5.RELEASE.jar:4.3.5.RELEASE]
at com.sun.proxy.$Proxy125.createQuery(Unknown Source) ~[na:na]
at org.springframework.data.jpa.repository.query.SimpleJpaQuery.validateQuery(SimpleJpaQuery.java:86) ~[spring-data-jpa-1.10.6.RELEASE.jar:na]
... 61 common frames omitted
Caused by: org.hibernate.hql.internal.ast.QuerySyntaxException: city is not mapped [select count(*)*2.1 from city]
at org.hibernate.hql.internal.ast.QuerySyntaxException.generateQueryException(QuerySyntaxException.java:79) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.QueryException.wrapWithQueryString(QueryException.java:103) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:218) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:142) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:115) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.engine.query.spi.HQLQueryPlan.<init>(HQLQueryPlan.java:76) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.engine.query.spi.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:150) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.internal.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:302) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.internal.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:240) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.internal.SessionImpl.createQuery(SessionImpl.java:1894) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.jpa.spi.AbstractEntityManagerImpl.createQuery(AbstractEntityManagerImpl.java:291) ~[hibernate-entitymanager-5.0.11.Final.jar:5.0.11.Final]
... 68 common frames omitted
Caused by: org.hibernate.hql.internal.ast.QuerySyntaxException: city is not mapped
at org.hibernate.hql.internal.ast.util.SessionFactoryHelper.requireClassPersister(SessionFactoryHelper.java:171) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.tree.FromElementFactory.addFromElement(FromElementFactory.java:91) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.tree.FromClause.addFromElement(FromClause.java:76) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.HqlSqlWalker.createFromElement(HqlSqlWalker.java:321) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3687) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:3576) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:716) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:572) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:309) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:257) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:262) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at org.hibernate.hql.internal.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:190) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
... 76 common frames omitted
Process finished with exit code 1
由于问题涉及树木，我还将使用相同的技术生成基本树的生成。
{
// Create a with the creation of b as a parameter.
val a=new A( (uncomplete:A)=>new B(uncomplete) )
}
class A( bFactory:A=>B){
//Call the bFactory then assign the result to b.
val b=bFactory(this);
}
class B(val a:A){
}
执行这样的构造会留下纯粹的不可变结构，而不会增加访问延迟值的开销。
答案 4 :(得分：-1)
class A(val b: B)
abstract class B {
val a: A
}
new B {
val a = new A(this)
}