使用 Java 8 Stream API 减少 Map
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Reducing Map by using Java 8 Stream API
提问by Fab
I have a Map in the following form:
我有以下形式的地图：
Map<Integer, Map<String,Double>> START
Let INNER be the inner map, i.e.
让 INNER 成为内部映射，即
Map<String,Double>
For example, I'd like to reduce START map in a new one
例如，我想在新的地图中减少 START 地图
Map<Integer, Double> END
which have the same keys, but different values. In particular, for each key, I want the new Double value be the SUM of values in the INNER map for the corresponding key.
它们具有相同的键，但具有不同的值。特别是，对于每个键，我希望新的 Double 值是对应键的 INNER 映射中的值的总和。
How could I achieve this by using JAVA 8's STREAM API?
我如何通过使用 JAVA 8 的 STREAM API 来实现这一点？
Thanks everyone.
感谢大家。
EDIT: A sample map is
编辑：示例地图是
------------------------------
| | 2016-10-02 3.45 |
| ID1 | 2016-10-03 1.23 |
| | 2016-10-04 0.98 |
------------------------------
| | 2016-10-02 1.00 |
| ID2 | 2016-10-03 2.00 |
| | 2016-10-04 3.00 |
------------------------------
e I'd like a new map like the following one:
e 我想要一张像下面这样的新地图：
--------------------------------
| | |
| ID1 | SUM(3.45,1.23,0.98) |
| | |
--------------------------------
| | |
| ID2 | SUM(1.00,2.00,3.00) |
| | |
--------------------------------
回答by cringineer
It will work for you
它会为你工作
Map<Integer, Double> collect = START.entrySet()
.stream()
.collect(
Collectors.toMap(
Map.Entry::getKey,
e -> e.getValue()
.values()
.stream()
.reduce(0d, (a, b) -> a + b)
)
);
回答by GROX13
This should be a good example:
这应该是一个很好的例子：
public class Main {
public static void main(final String[] args) {
final Map<Integer, Map<String, Double>> tmp = new HashMap<>();
tmp.put(1, new HashMap<String, Double>() {{
put("1", 3.45);
put("2", 1.23);
put("3", 0.98);
}});
tmp.put(2, new HashMap<String, Double>() {{
put("1", 1.00);
put("2", 2.00);
put("3", 3.00);
}});
System.out.println(tmp.entrySet().stream()
.collect(
toMap(Map.Entry::getKey,
data ->
data.getValue()
.values().stream()
.mapToDouble(Number::doubleValue).sum())));
}
}
output will be {1=5.66, 2=6.0}and all this does is takes entry set of map, gets a stream of it and collects to new map sums of inner map values.
输出将是{1=5.66, 2=6.0}，所有这些都是获取映射的条目集，获取它的流并收集内部映射值的新映射总和。
