java - JDK 8 的类型推断如何与泛型一起工作？

Question

我的代码无法使用 JDK 7 进行编译，但可以使用 JDK 8 进行编译。

抽象实际代码:

interface A {
...
}

class B implements A {
...
}

public void AAA(List<A> list) {...}

AAA(Collections.singletonList(new B()));

Collections.singletonList 定义为

public static <T> List<T> singletonList(T o) {
    return new SingletonList<>(o);
}

据我所知，基于泛型，T 将被推断为 B，因此 Collections.singletonList(new B()) 将是无法分配给 List 的列表，因为 Java 泛型是不变的。

但是对于 JDK 8，T 被推断为 A 并且编译成功。

我想知道如何将 T 推断为 A，因为这里有两个类型 T 的变量:A 和 B。

有先后顺序吗？还是编译器找到共同的祖先类？

附上官方文档更胜一筹!

提前致谢!

ps1。 JDK 7版本为Oracle 1.7.0_79，JDK 8版本为Oracle 1.8.0_66。

PS2。以下是实际代码的链接:

https://github.com/apache/storm/blob/85a31e2fdec1ffef83e1ff438cd765a821fb06e4/examples/storm-opentsdb-examples/src/main/java/org/apache/storm/opentsdb/SampleOpenTsdbBoltTopology.java#L48

https://github.com/apache/storm/blob/85a31e2fdec1ffef83e1ff438cd765a821fb06e4/external/storm-opentsdb/src/main/java/org/apache/storm/opentsdb/bolt/OpenTsdbBolt.java#L77

https://github.com/apache/storm/blob/85a31e2fdec1ffef83e1ff438cd765a821fb06e4/external/storm-opentsdb/src/main/java/org/apache/storm/opentsdb/bolt/TupleOpenTsdbDatapointMapper.java#L37

Answer 1

嗯，有一个全新的章节，§18. Type Inference , 在语言规范中，但它并不容易阅读。即使是第一部分的摘要(准确地解决您的问题)也很难:

In comparison to the Java SE 7 Edition of The Java® Language Specification, important changes to inference include:

• Adding support for lambda expressions and method references as method invocation arguments.
• Generalizing to define inference in terms of poly expressions, which may not have well-defined types until after inference is complete. This has the notable effect of improving inference for nested generic method and diamond constructor invocations.
• Describing how inference is used to handle wildcard-parameterized functional interface target types and most specific method analysis.
• Clarifying the distinction between invocation applicability testing (which involves only the invocation arguments) and invocation type inference (which incorporates a target type).
• Delaying resolution of all inference variables, even those with lower bounds, until invocation type inference, in order to get better results.
• Improving inference behavior for interdependent (or self-dependent) variables.
• Eliminating bugs and potential sources of confusion. This revision more carefully and precisely handles the distinction between specific conversion contexts and subtyping, and describes reduction by paralleling the corresponding non-inference relations. Where there are intentional departures from the non-inference relations, these are explicitly identified as such.
• Laying a foundation for future evolution: enhancements to or new applications of inference will be easier to integrate into the specification.

第二个项目符号对您的代码示例影响最大。你有一个泛型方法的嵌套方法调用，没有指定显式类型参数，这使它成为一个所谓的聚合表达式，其实际类型可以从目标类型推断出来，这是AAA的参数类型在你的情况下。

所以这是一个相当简单的星座 AAA不是通用的，关于它的参数类型没有歧义。总是List<A> .这里没有搜索“公共(public)祖先类”，所有需要检查的是参数表达式的类型 ( B ) 是否与推断类型 ( A ) 兼容。