Java流按多个字段进行分组并求和

Question

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

    ... constructor, getters & setters
}

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)

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)

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

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？

Answer 1

解决办法可以是处理分组，将键作为list并在映射回对象类型时进行强制转换。

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());

更干净的方法是为合并函数公开API，并执行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());

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