为什么Java泛型类型推断在链式方法调用中中断?

原学程将引见为何Java泛型典型揣摸在链式办法挪用中中止?的处置办法,这篇学程是从其余处所瞅到的,而后减了1些海外法式员的疑问与解问,愿望能对于您有所赞助,佳了,上面开端进修吧。

为什么Java泛型类型推断在链式方法调用中中断? 教程 第1张

成绩描写

检查以下示例中泛型典型的典型揣摸,我说没有出为何methodAutoTypeInference任务患上很佳,但是methodNotCompilable(简直雷同)没法编译,为了治理它,编译器须要其余技能,如methodWorkaroundTypeHint或者methodWorkaroundTypeCast

methodNotCompilable招致编译器没有肯定表白式典型以及办法成果典型能否兼容的成绩是甚么?

Stream<CharSequence> methodAutoTypeInference() {
  return Stream.of("a");
}

Stream<CharSequence> methodNotCompilable() {
  return Stream.of("a").distinct(); 
  // incompatible types:  java.util.stream.Stream<java.lang.String>
  // cannot be converted to  java.util.stream.Stream<java.lang.CharSequence>
}

Stream<CharSequence> methodWorkaroundTypeHint() {
  return Stream.<CharSequence>of("a").distinct();
}

Stream<CharSequence> methodWorkaroundTypeCast() {
  return Stream.of((CharSequence) "a").distinct();
}

推举谜底

This answer from JDK Developers themselves简直笼罩了雷同的地区。只需留意Stuart Marks说:该编译器能够会在将来的刊行版中获得加强以涵盖这类情形。虽然有lambdas的情形,但是这与您的情形出有太年夜分歧。这便是(今朝)编译器的任务方法。我们对于此疑神疑鬼。

您不妨经由过程以下方法检查编译器对于return Stream.of("a").distinct();的意见并决议应用哪类典型:

javac --debug=verboseResolution=all

这是1个未记载在案的标记。假如您应用该标记停止编译,叨教瞅到1些很年夜的输入:

  with actuals: no arguments
  with type-args: no arguments
  candidates:
#0 applicable method found: Object()
DeleteMe.java:六0: Note: resolving method of in type Stream to candidate 一
  return Stream.of("a").distinct();
^
  phase: BASIC
  with actuals: String
  with type-args: no arguments
  candidates:
#0 not applicable method found: <T#一>of(T#一...)
  (cannot infer type-variable(s) T#一
 (argument mismatch; String cannot be converted to T#一[]))
#一 applicable method found: <T#二>of(T#二)
  (partially instantiated to: (String)Stream<String>)
  where T#一,T#二 are type-variables:
 T#一 extends Object declared in method <T#一>of(T#一...)
 T#二 extends Object declared in method <T#二>of(T#二)
DeleteMe.java:六0: Note: Deferred instantiation of method <T>of(T)
  return Stream.of("a").distinct();
^
  instantiated signature: (String)Stream<String>
  target-type: <none>
  where T is a type-variable:
 T extends Object declared in method <T>of(T)
DeleteMe.java:六0: Note: resolving method distinct in type Stream to candidate 0
  return Stream.of("a").distinct();
  ^
  phase: BASIC
  with actuals: no arguments
  with type-args: no arguments
  candidates:
#0 applicable method found: distinct()
  where T is a type-variable:
 T extends Object declared in interface Stream
DeleteMe.java:六0: error: incompatible types: Stream<String> cannot be converted to Stream<CharSequence>
  return Stream.of("a").distinct();
  ^
一 error

我想最主要的部门是(partially instantiated to: (String)Stream<String>)

您不妨瞅到,T典型的剖析是鉴于办法挪用完成的;而没有是全部挪用链。趁便说1句,假如是如许的话,这会使编译器的任务变患上异常庞杂。关于如许1个简略的链条去说,工作能够瞅起去眇乎小哉,但是当有许多工作时,它便会变患上异常、异常辣手以及庞杂。特别是当您发明non-denotable types的时刻,这会让工作变患上加倍庞杂。

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