Java lambda 表达式中的错误返回类型
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bad return type in lambda expression
提问by Roland Illig
The following code compiles fine in IntelliJ and Eclipse, but the JDK compiler 1.8.0_25 complains. First, the code.
以下代码在 IntelliJ 和 Eclipse 中编译良好，但 JDK 编译器 1.8.0_25 会报错。首先，代码。
import java.util.function.Predicate;
public abstract class MyStream<E> {
static <T> MyStream<T> create() {
return null;
}
abstract MyStream<E> filter(MyPredicate<? super E> predicate);
public interface MyPredicate<T> extends Predicate<T> {
@Override
boolean test(T t);
}
public void demo() {
MyStream.<Boolean> create().filter(b -> b);
MyStream.<String> create().filter(s -> s != null);
}
}
The output from javac 1.8.0_25 is:
javac 1.8.0_25 的输出是：
MyStream.java:18: error: incompatible types: incompatible parameter types in lambda expression
MyStream.<Boolean> create().filter(b -> b);
^
MyStream.java:18: error: incompatible types: bad return type in lambda expression
MyStream.<Boolean> create().filter(b -> b);
^
? super Boolean cannot be converted to boolean
MyStream.java:19: error: bad operand types for binary operator '!='
MyStream.<String> create().filter(s -> s != null);
^
first type: ? super String
second type: <null>
MyStream.java:19: error: incompatible types: incompatible parameter types in lambda expression
MyStream.<String> create().filter(s -> s != null);
^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
4 errors
When I replace ? super Ewith simply E, JDK compiles successfully.
当我用simple替换? super E时E，JDK 编译成功。
When I replace filter(MyPredicatewith filter(Predicate, JDK compiles successfully.
当我用 替换filter(MyPredicate时filter(Predicate，JDK 编译成功。
Since it works with JDK 1.8.0_60, I suspect it is a compiler bug.
由于它适用于 JDK 1.8.0_60，我怀疑这是一个编译器错误。
Any details on what caused this and when it has been fixed?
有关导致此问题的原因以及何时修复的任何详细信息？
采纳答案by ZhongYu
If a lambda expression appears in a target type with wildcards (as in most cases)
如果 lambda 表达式出现在带有通配符的目标类型中（在大多数情况下）
Consumer<? super Boolean> consumer = b->{...}
the question arises - what's the type of the lambda expression; in particular, the type of b.
问题出现了 - lambda 表达式的类型是什么；特别是b.
Of course, there could be many choices due to the wildcards; e.g. we could explicitly choose
当然，由于通配符，可能有很多选择；例如，我们可以明确地选择
Consumer<? super Boolean> consumer = (Object b)->{...}
However, implicitly, bshould be inferred as Boolean. This makes sense since the consumer should only be fed with Booleananyway.
但是，隐含地，b应该推断为Boolean。这是有道理的，因为Boolean无论如何都应该给消费者喂食。
http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.27.3
http://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.27.3
If T is a wildcard-parameterized functional interface type and the lambda expression is implicitly typed, then the ground target type is the non-wildcard parameterization of T
如果 T 是通配符参数化的函数接口类型并且 lambda 表达式是隐式类型的，则基本目标类型是 T 的非通配符参数化
(This probably assumes that wildcards are use properly variance-wise on the target type; we might find some hilarious examples if the assumption doesn't hold)
（这可能假设通配符在目标类型上正确使用方差；如果假设不成立，我们可能会发现一些有趣的例子）
回答by Bohemian
<? super E>includes Object, which isn't a Boolean. ie
<? super E>包括Object，它不是Boolean. IE
MyPredicate<? super E>
Could be a
可能是一个
MyPredicate<Object>
and you can't return an Objectas a boolean.
并且您不能将 anObject作为boolean.
Try changing your lambda to:
尝试将您的 lambda 更改为：
MyStream.<Boolean> create().filter(b -> Boolean.TRUE.equals(b));
which will compile and execute without error not matter the type of the stream, but will return truefor elements that are trueBooleanvalues.
无论流的类型如何，它都会编译和执行而不会出错，但会返回值的true元素trueBoolean。
