使用带连接字符分隔符的收集器

Question

我正在构建一个名为“id”的查询参数，它是一个列表，所以我有类似于localhost:8080/application？id=1

（我知道我的方法不够，我需要写第一个？id=）

方法

public Map<Long, String> getSkus(List<ItemPedido> itensPedido) {
    Map<Long, String> skus = new HashMap<>();
    String ids = itensPedido.stream()
            .filter(i -> i.getProduto() != null)
            .map(i -> i.getProduto().getId())
            .collect(Collectors.joining("&id="));
    //TODO 
    return skus;
}

我的问题是：为什么我收到这个错误在Collectors.joining（"

no suitable method found for collect(java.util.stream.Collector<java.lang.CharSequence,capture#1 of ?,java.lang.String>)
[ERROR]     method java.util.stream.Stream.<R>collect(java.util.function.Supplier<R>,java.util.function.BiConsumer<R,? super java.lang.Long>,java.util.function.BiConsumer<R,R>) is not applicable
[ERROR]       (cannot infer type-variable(s) R
[ERROR]         (actual and formal argument lists differ in length))
[ERROR]     method java.util.stream.Stream.<R,A>collect(java.util.stream.Collector<? super java.lang.Long,A,R>) is not applicable
[ERROR]       (cannot infer type-variable(s) R,A
[ERROR]         (argument mismatch; java.util.stream.Collector<java.lang.CharSequence,capture#1 of ?,java.lang.String> cannot be converted to java.util.stream.Collector<? super java.lang.Long,A,R>))

我正在使用OpenJDK

$ java -version
openjdk version "1.8.0_151"
OpenJDK Runtime Environment (build 1.8.0_151-b12)
OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)

Answer 1

将流映射到字符串流：

.map(i -> i.getProduto().getId().toString())