由网友(说一句不爱我便走)分享简介：这是我的列表 fooListHere is my List fooListclass Foo {private String name;private int code;private int account;private int time;private String others;... constructo...

这是我的列表 fooList

Here is my List fooList

class Foo {
    private String name;
    private int code;
    private int account;
    private int time;
    private String others;

    ... constructor, getters & setters
}

例如(帐户的所有值都已设置为1)

e.g.(all the value of account has been set to 1)

new Foo(First, 200, 1, 400, other1), 
new Foo(First, 200, 1, 300, other1),
new Foo(First, 201, 1, 10, other1),
new Foo(Second, 400, 1, 20, other2),
new Foo(Second, 400, 1, 40, other2),
new Foo(Third, 100, 1, 200, other3),
new Foo(Third, 101, 1, 900, other3)

我想通过将名称"分组来转换这些值.和代码"，计算数字，并对时间"求和，例如

I want to transform these values by grouping "name" and "code", accounting for the number, and summing the "time", e.g.

new Foo(First, 200, 2, 700, other1), 
new Foo(First, 201, 1, 10, other1),
new Foo(Second, 400, 2, 60, other2),
new Foo(Third, 100, 1, 200, other3),
new Foo(Third, 101, 1, 900, other3)

我知道我应该使用这样的流:

I know that I should use a stream like this:

Map<String, List<Foo>> map = fooList.stream().collect(groupingBy(Foo::getName()));

但我如何按代码对它们进行分组，然后进行会计和汇总工作?

but how can I group them by code then do the accounting and summing job?

另外，如果我想计算平均时间怎么办?例如

Also, what if I want to calculate the average time? e.g.

new Foo(First, 200, 2, 350, other1), 
new Foo(First, 201, 1, 10, other1),
new Foo(Second, 400, 2, 30, other2),
new Foo(Third, 100, 1, 200, other3),
new Foo(Third, 101, 1, 900, other3)

我可以同时使用 summingInt(Foo::getAccount) 和 averagingInt(Foo::getTime) 吗?

Can I use both of summingInt(Foo::getAccount) and averagingInt(Foo::getTime) instead?

推荐答案

一种解决方法是处理 grouping 键为 List 并在映射回对象时进行转换输入.

A workaround could be to deal with grouping with key as List and casting while mapping back to object type.

List<Foo> result = fooList.stream()
        .collect(Collectors.groupingBy(foo ->
                        Arrays.asList(foo.getName(), foo.getCode(), foo.getAccount()),
                Collectors.summingInt(Foo::getTime)))
        .entrySet().stream()
        .map(entry -> new Foo((String) entry.getKey().get(0),
                (Integer) entry.getKey().get(1),
                entry.getValue(),
                (Integer) entry.getKey().get(2)))
        .collect(Collectors.toList());

更简洁的方法是公开用于合并功能的 API 并执行 toMap.

Cleaner way would be to expose APIs for merge function and performing a toMap.

编辑:toMap 的简化如下所示

List<Foo> result = new ArrayList<>(fooList.stream()
        .collect(Collectors.toMap(foo -> Arrays.asList(foo.getName(), foo.getCode()),
                Function.identity(), Foo::aggregateTime))
        .values());

其中 aggregateTime 是 Foo 中的静态方法，例如:

where the aggregateTime is a static method within Foo such as this :

static Foo aggregateTime(Foo initial, Foo incoming) {
    return new Foo(incoming.getName(), incoming.getCode(),
            incoming.getAccount(), initial.getTime() + incoming.getTime());
}