推断类型不符合不相关变量的等式约束错误
Inferred type does not conform to equality constraint error for unrelated variables
我有如下一段代码
public class TeeingCollector {
public static void main(String[] args) {
// var strs = List.of("abc");
var dividedStrings = Stream.of("foo", "hello", "bar", "world")
.collect(Collectors.teeing(
Collectors.filtering((String s) -> s.length() <= 3, Collectors.toList()),
Collectors.filtering((String s) -> s.length() > 3, Collectors.toList()),
List::of
));
System.out.println(dividedStrings);
}
private static class Employee {
boolean isActive;
public Employee(boolean isActive) {
this.isActive = isActive;
}
public boolean isActive() {
return isActive;
}
@Override
public String toString() {
return "Employee{" +
"isActive=" + isActive +
'}';
}
}
private static class MaxMin {
int max;
int min;
MaxMin(int max, int min) {
this.max = max;
this.min = min;
}
@Override
public String toString() {
return "MaxMin{" +
"max=" + max +
", min=" + min +
'}';
}
}
}
如果我使用 java src/TeeingCollector.java 从终端执行 class
我收到以下错误：
src/TeeingCollector.java:14: error: incompatible types: inferred type does not conform to equality constraint(s)
.collect(Collectors.teeing(
^
inferred: List<String>
equality constraints(s): List<Object>,R
where R,T,A are type-variables:
R extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
T extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
A extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
1 error
error: compilation failed
如果我取消对行 var strs = List.of("abc"); 的注释，那么代码将毫无问题地执行。
Java 版本（适用于 macOS）：
OpenJDK Runtime Environment (build 12+33)
OpenJDK 64-Bit Server VM (build 12+33, mixed mode, sharing)
运行使用以下版本（旧版本）使用相同的代码不会产生错误
OpenJDK Runtime Environment (build 12-ea+23)
OpenJDK 64-Bit Server VM (build 12-ea+23, mixed mode, sharing)
注意： 如果我编译它 运行，我在 macOS 上的两个构建都没有任何错误，所以似乎只有 java TeeingCollector.java 无法正常工作
TL;DR 这显然是一个错误，因为编译器的行为取决于完全不相关的事物，包括 Java 语言之外的环境方面。
我简化了您的示例并将 Collectors.teeing 和 Predicate.not 的实现集成到示例中，以便能够使用从 JDK 9 到 Java 版本测试您的代码JDK12.
请注意，虽然我最初认为这是 var 的类型推断与右侧泛型构造的某种交互，但更多测试表明问题甚至存在于使用变量的显式类型，允许在测试中包含 JDK 9。
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import java.util.stream.Collector;
public class Temp5 {
public static void main(String[] args) {
// List<Character> strs = List.of("abc");
List<List<Character>> lettersAndNumbers = Stream.of('5', 't', 'o', '9', 'p', '1', 'h')
.collect(teeing(
Collectors.filtering(Character::isLetter, Collectors.toList()),
Collectors.filtering(not(Character::isLetter), Collectors.toList()),
List::of
));
}
public static <T, R1, R2, R>
Collector<T, ?, R> teeing(Collector<? super T, ?, R1> downstream1,
Collector<? super T, ?, R2> downstream2,
BiFunction<? super R1, ? super R2, R> merger) {
return teeing0(downstream1, downstream2, merger);
}
private static <T, A1, A2, R1, R2, R>
Collector<T, ?, R> teeing0(Collector<? super T, A1, R1> downstream1,
Collector<? super T, A2, R2> downstream2,
BiFunction<? super R1, ? super R2, R> merger) {
Objects.requireNonNull(downstream1, "downstream1");
Objects.requireNonNull(downstream2, "downstream2");
Objects.requireNonNull(merger, "merger");
Supplier<A1> c1Supplier = Objects.requireNonNull(downstream1.supplier(), "downstream1 supplier");
Supplier<A2> c2Supplier = Objects.requireNonNull(downstream2.supplier(), "downstream2 supplier");
BiConsumer<A1, ? super T> c1Accumulator =
Objects.requireNonNull(downstream1.accumulator(), "downstream1 accumulator");
BiConsumer<A2, ? super T> c2Accumulator =
Objects.requireNonNull(downstream2.accumulator(), "downstream2 accumulator");
BinaryOperator<A1> c1Combiner = Objects.requireNonNull(downstream1.combiner(), "downstream1 combiner");
BinaryOperator<A2> c2Combiner = Objects.requireNonNull(downstream2.combiner(), "downstream2 combiner");
Function<A1, R1> c1Finisher = Objects.requireNonNull(downstream1.finisher(), "downstream1 finisher");
Function<A2, R2> c2Finisher = Objects.requireNonNull(downstream2.finisher(), "downstream2 finisher");
Collector.Characteristics[] characteristics;
Set<Collector.Characteristics> c1Characteristics = downstream1.characteristics();
Set<Collector.Characteristics> c2Characteristics = downstream2.characteristics();
EnumSet<Collector.Characteristics> c = EnumSet.noneOf(Collector.Characteristics.class);
c.addAll(c1Characteristics);
c.retainAll(c2Characteristics);
c.remove(Collector.Characteristics.IDENTITY_FINISH);
characteristics = c.toArray(new Collector.Characteristics[0]);
class PairBox {
A1 left = c1Supplier.get();
A2 right = c2Supplier.get();
void add(T t) {
c1Accumulator.accept(left, t);
c2Accumulator.accept(right, t);
}
PairBox combine(PairBox other) {
left = c1Combiner.apply(left, other.left);
right = c2Combiner.apply(right, other.right);
return this;
}
R get() {
R1 r1 = c1Finisher.apply(left);
R2 r2 = c2Finisher.apply(right);
return merger.apply(r1, r2);
}
}
return Collector.of(PairBox::new, PairBox::add, PairBox::combine, PairBox::get, characteristics);
}
@SuppressWarnings("unchecked")
static <T> Predicate<T> not(Predicate<? super T> target) {
Objects.requireNonNull(target);
return (Predicate<T>)target.negate();
}
}
结果是灾难性的。每个版本都可以对代码的正确性有不同的看法，即使只是更改次要版本也是如此。好吧，即使使用 OpenJDK 而不是 Oracle 的发行版也会有不同的结果。此外，即使对代码进行最小的更改也会影响它。正如您所注意到的，将注释更改为实际声明可能会改变结果，但即使删除注释也会改变某些 JDK 版本的结果。只需更改 值 ，例如使用 Stream.of('5', 't', 'o', '9', 'p', '1') 而不是 Stream.of('5', 't', 'o', '9', 'p', '1', 'h') 会改变某些版本的结果。
我的结论是编译器实现中的某些东西依赖于特定设置的稳定但实际上不可预测的东西，比如 HashMap 的迭代顺序。 JDK 版本本身似乎是其中的一部分。这也可以解释为什么在使用 MacOS 而不是 Linux 或 Windows 时结果可能会发生变化。即使将此源文件与另一个不相关的源文件一起编译也会改变结果。
我有如下一段代码
public class TeeingCollector {
public static void main(String[] args) {
// var strs = List.of("abc");
var dividedStrings = Stream.of("foo", "hello", "bar", "world")
.collect(Collectors.teeing(
Collectors.filtering((String s) -> s.length() <= 3, Collectors.toList()),
Collectors.filtering((String s) -> s.length() > 3, Collectors.toList()),
List::of
));
System.out.println(dividedStrings);
}
private static class Employee {
boolean isActive;
public Employee(boolean isActive) {
this.isActive = isActive;
}
public boolean isActive() {
return isActive;
}
@Override
public String toString() {
return "Employee{" +
"isActive=" + isActive +
'}';
}
}
private static class MaxMin {
int max;
int min;
MaxMin(int max, int min) {
this.max = max;
this.min = min;
}
@Override
public String toString() {
return "MaxMin{" +
"max=" + max +
", min=" + min +
'}';
}
}
}
如果我使用 java src/TeeingCollector.java 从终端执行 class
我收到以下错误：
src/TeeingCollector.java:14: error: incompatible types: inferred type does not conform to equality constraint(s)
.collect(Collectors.teeing(
^
inferred: List<String>
equality constraints(s): List<Object>,R
where R,T,A are type-variables:
R extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
T extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
A extends Object declared in method <T,A,R>filtering(Predicate<? super T>,Collector<? super T,A,R>)
1 error
error: compilation failed
如果我取消对行 var strs = List.of("abc"); 的注释，那么代码将毫无问题地执行。
Java 版本（适用于 macOS）：
OpenJDK Runtime Environment (build 12+33)
OpenJDK 64-Bit Server VM (build 12+33, mixed mode, sharing)
运行使用以下版本（旧版本）使用相同的代码不会产生错误
OpenJDK Runtime Environment (build 12-ea+23)
OpenJDK 64-Bit Server VM (build 12-ea+23, mixed mode, sharing)
注意： 如果我编译它 运行，我在 macOS 上的两个构建都没有任何错误，所以似乎只有 java TeeingCollector.java 无法正常工作
TL;DR 这显然是一个错误，因为编译器的行为取决于完全不相关的事物，包括 Java 语言之外的环境方面。
我简化了您的示例并将 Collectors.teeing 和 Predicate.not 的实现集成到示例中，以便能够使用从 JDK 9 到 Java 版本测试您的代码JDK12.
请注意，虽然我最初认为这是 var 的类型推断与右侧泛型构造的某种交互，但更多测试表明问题甚至存在于使用变量的显式类型，允许在测试中包含 JDK 9。
import java.util.*;
import java.util.function.*;
import java.util.stream.*;
import java.util.stream.Collector;
public class Temp5 {
public static void main(String[] args) {
// List<Character> strs = List.of("abc");
List<List<Character>> lettersAndNumbers = Stream.of('5', 't', 'o', '9', 'p', '1', 'h')
.collect(teeing(
Collectors.filtering(Character::isLetter, Collectors.toList()),
Collectors.filtering(not(Character::isLetter), Collectors.toList()),
List::of
));
}
public static <T, R1, R2, R>
Collector<T, ?, R> teeing(Collector<? super T, ?, R1> downstream1,
Collector<? super T, ?, R2> downstream2,
BiFunction<? super R1, ? super R2, R> merger) {
return teeing0(downstream1, downstream2, merger);
}
private static <T, A1, A2, R1, R2, R>
Collector<T, ?, R> teeing0(Collector<? super T, A1, R1> downstream1,
Collector<? super T, A2, R2> downstream2,
BiFunction<? super R1, ? super R2, R> merger) {
Objects.requireNonNull(downstream1, "downstream1");
Objects.requireNonNull(downstream2, "downstream2");
Objects.requireNonNull(merger, "merger");
Supplier<A1> c1Supplier = Objects.requireNonNull(downstream1.supplier(), "downstream1 supplier");
Supplier<A2> c2Supplier = Objects.requireNonNull(downstream2.supplier(), "downstream2 supplier");
BiConsumer<A1, ? super T> c1Accumulator =
Objects.requireNonNull(downstream1.accumulator(), "downstream1 accumulator");
BiConsumer<A2, ? super T> c2Accumulator =
Objects.requireNonNull(downstream2.accumulator(), "downstream2 accumulator");
BinaryOperator<A1> c1Combiner = Objects.requireNonNull(downstream1.combiner(), "downstream1 combiner");
BinaryOperator<A2> c2Combiner = Objects.requireNonNull(downstream2.combiner(), "downstream2 combiner");
Function<A1, R1> c1Finisher = Objects.requireNonNull(downstream1.finisher(), "downstream1 finisher");
Function<A2, R2> c2Finisher = Objects.requireNonNull(downstream2.finisher(), "downstream2 finisher");
Collector.Characteristics[] characteristics;
Set<Collector.Characteristics> c1Characteristics = downstream1.characteristics();
Set<Collector.Characteristics> c2Characteristics = downstream2.characteristics();
EnumSet<Collector.Characteristics> c = EnumSet.noneOf(Collector.Characteristics.class);
c.addAll(c1Characteristics);
c.retainAll(c2Characteristics);
c.remove(Collector.Characteristics.IDENTITY_FINISH);
characteristics = c.toArray(new Collector.Characteristics[0]);
class PairBox {
A1 left = c1Supplier.get();
A2 right = c2Supplier.get();
void add(T t) {
c1Accumulator.accept(left, t);
c2Accumulator.accept(right, t);
}
PairBox combine(PairBox other) {
left = c1Combiner.apply(left, other.left);
right = c2Combiner.apply(right, other.right);
return this;
}
R get() {
R1 r1 = c1Finisher.apply(left);
R2 r2 = c2Finisher.apply(right);
return merger.apply(r1, r2);
}
}
return Collector.of(PairBox::new, PairBox::add, PairBox::combine, PairBox::get, characteristics);
}
@SuppressWarnings("unchecked")
static <T> Predicate<T> not(Predicate<? super T> target) {
Objects.requireNonNull(target);
return (Predicate<T>)target.negate();
}
}
结果是灾难性的。每个版本都可以对代码的正确性有不同的看法，即使只是更改次要版本也是如此。好吧，即使使用 OpenJDK 而不是 Oracle 的发行版也会有不同的结果。此外，即使对代码进行最小的更改也会影响它。正如您所注意到的，将注释更改为实际声明可能会改变结果，但即使删除注释也会改变某些 JDK 版本的结果。只需更改 值 ，例如使用 Stream.of('5', 't', 'o', '9', 'p', '1') 而不是 Stream.of('5', 't', 'o', '9', 'p', '1', 'h') 会改变某些版本的结果。
我的结论是编译器实现中的某些东西依赖于特定设置的稳定但实际上不可预测的东西，比如 HashMap 的迭代顺序。 JDK 版本本身似乎是其中的一部分。这也可以解释为什么在使用 MacOS 而不是 Linux 或 Windows 时结果可能会发生变化。即使将此源文件与另一个不相关的源文件一起编译也会改变结果。