当名义 class 需要访问修改时无法使用交集类型

Question

接口：

interface PublicCloneable {
    Object clone();
}

interface HasPosition {
    // doesn't matter
}

尝试使用交集类型：

@SuppressWarnings("unchecked")
<E extends PublicCloneable & HasPosition> E cloneAndIncrementPosition(E elem) {
    final E clone = (E)elem.clone();
    // rest omitted
}

尝试用javac 1.8.0_60编译：

$ javac xx.java
xx.java:13: error: clone() in Object cannot implement clone() in PublicCloneable
    <E extends PublicCloneable & HasPosition> E cloneAndIncrementPosition(E elem) {
     ^
  attempting to assign weaker access privileges; was public
xx.java:14: error: clone() has protected access in Object
        final E clone = (E)elem.clone();
                               ^
2 errors

为什么这个交集类型对 javac 无效？

Answer 1

正如第一个错误试图告诉您的那样，您的代码无法工作，因为您限制了 clone() 方法具有两个不兼容签名的类型。

Answer 2

这看起来像一个 javac 错误。

http://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.4

The members of a type variable X with bound T & I1 & ... & In are the members of the intersection type (§4.9) T & I1 & ... & In

http://docs.oracle.com/javase/specs/jls/se8/html/jls-4.html#jls-4.9

Every intersection type T1 & ... & Tn induces a notional class or interface for the purpose of identifying the members of the intersection type ...

If Ck is Object, a notional interface is induced ... has direct superinterfaces T1', ..., Tn'

因此，PublicCloneable & HasPosition引入了一个概念接口，扩展了两者，应该可以。

当名义 class 需要访问修改时无法使用交集类型

unable to use an intersection type when notional class requires access modification

java

javac