GroupingBy with multiple columns_JAVA_编程开发_程序员俱乐部

Below is learnt from my colleague. Next time i wish i can solve it independently.

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class="java" name="code">import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.reducing;
import static java.util.stream.Collectors.toList;

import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;
...

list.stream().collect(groupingBy(item -> item.getType() + item.getLevel(),
                reducing(Item::summarize))) // summarize: static function to aggregate items
        .values().stream().map(Optional::get)
        .collect(toList());

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