I have this method:

public static <T,U,R> Stream<R> cartesian(
    List<? extends T> list1,
    List<? extends U> l,java,generics,java-8,java-stream"/>
	














    

		

        

            
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首页  > 解决方案 > 溪流 instead of Stream being produced when invoked on list

I have this method:


public static <T,U,R> Stream<R> cartesian(
    List<? extends T> list1,
    List<? extends U> l












java - 溪流 instead of Stream being produced when invoked on list

I have this method:


public static <T,U,R> Stream<R> cartesian(
    List<? extends T> list1,
    List<? extends U> l





问题描述



I have this method:


public static <T,U,R> Stream<R> cartesian(
    List<? extends T> list1,
    List<? extends U> list2,
    BiFunction<? super T, ? super U, R> func) 
{
    return list1.stream() // list1.stream() produces Stream<Object> not Stream<? extends T>!
        .flatMap(x -> {
            return list2.stream()
                .map(appendable ->x + "" +appendable);
        });
}



Because of the following error, my whole pipeline now return a Stream<Object> instead of the intended Stream<R>.


Any reasons why this error occur which I arrowed occurs?









The "expected type: ... actual type: ..." message is not about list1.stream(), it's about the value you return in the lambda in flatMap. Observe where the red squiggles are:


enter image description here


It's saying that the code in the x -> ... lambda is returning a Stream<Object>, rather than the expected Stream<R>.


Note that map(spendable -> x + "" + appendable) can totally return a Stream<Object> with the right type arguments ( if it were .<Object>map(appendable ->x + "" +appendable) for example).


And in fact, the "expected type: ... actual type: ..." message is only produced by IntelliJ, so it's probably just IntelliJ being a bit silly. If you run javac, you get a much better error message:


incompatible types: inference variable R#1 has incompatible bounds
        .flatMap(x -> {
                ^
    equality constraints: R#2
    lower bounds: R#3,String
  where R#1,T#1,R#2,T#2,U,R#3 are type-variables:
    R#1 extends Object declared in method <R#1>flatMap(Function<? super T#1,? extends Stream<? extends R#1>>)
    T#1 extends Object declared in interface Stream
    R#2 extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    T#2 extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    U extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    R#3 extends Object declared in method <R#3>map(Function<? super T#1,? extends R#3>)



That says that the type parameter R in flatMap cannot be inferred, because its bounds cannot be satisfied simultaneously. R in flatMap has to be the same as the R in cartesian, but its lower bounds are R in map, and String.


Anyway, it seems like what you wanted to do was:


return list1.stream()
    .flatMap(x -> list2.stream().map(y -> func.apply(x, y)));



标签： javagenericsjava-8java-stream


解决方案









“预期类型：...实际类型：...”消息不是关于list1.stream()，而是关于您在 lambda 中返回的值flatMap。观察红色曲线的位置：


在此处输入图像描述


这是说x -> ...lambda 中的代码返回 a Stream<Object>，而不是预期的Stream<R>。


请注意，map(spendable -> x + "" + appendable)可以完全返回Stream<Object>带有正确类型参数的 a （例如，如果是.<Object>map(appendable ->x + "" +appendable)的话）。


实际上，“预期类型：...实际类型：...”消息仅由 IntelliJ 生成，因此可能只是 IntelliJ 有点傻。如果你运行javac，你会得到一个更好的错误信息：


incompatible types: inference variable R#1 has incompatible bounds
        .flatMap(x -> {
                ^
    equality constraints: R#2
    lower bounds: R#3,String
  where R#1,T#1,R#2,T#2,U,R#3 are type-variables:
    R#1 extends Object declared in method <R#1>flatMap(Function<? super T#1,? extends Stream<? extends R#1>>)
    T#1 extends Object declared in interface Stream
    R#2 extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    T#2 extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    U extends Object declared in method <T#2,U,R#2>cartesian(List<? extends T#2>,List<? extends U>,BiFunction<? super T#2,? super U,R#2>)
    R#3 extends Object declared in method <R#3>map(Function<? super T#1,? extends R#3>)



这表示无法推断Rin的类型参数，因为它的边界不能同时满足。in必须与in相同，但其下限为in和。flatMapRflatMapRcartesianRmapString


无论如何，看起来你想做的是：


return list1.stream()
    .flatMap(x -> list2.stream().map(y -> func.apply(x, y)));











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