將映射列表轉換爲使用帶有Java 8流的ImmutablePair的映射

Question

如何重寫這種舊式代碼以使用Java 8流？我知道這可以通過一行代碼來完成。

Map<String, ImmutablePair<Double, Double>> dataCache = new LinkedHashMap<>() 
List<Map<String, Object>> data = new ArrayList<>(); 
for(Map<String, Object> rec : data) { 
    String code = (String) rec.get("code"); 
    Double x0 = (Double) rec.get("x0"); 
    Double x1 = (Double) rec.get("x1"); 
    dataCache.put(code, new ImmutablePair<>(x0, x1)); 
} 

本質上，這個想法是將記錄的通用列表重新映射到更加結構化的哈希查找中。

Answer 1

既然你想要的流碼單線：

Map<String, ImmutablePair<Double, Double>> dataCache = data.stream().collect(Collectors.toMap(rec -> (String) rec.get("code"), rec -> new ImmutablePair<>((Double) rec.get("x0"), (Double) rec.get("x1")), (a, b) -> b, LinkedHashMap::new)); 

Answer 2

Map<String, ImmutablePair<Double, Double>> dataCache = data.stream() 
    .collect(
     Collectors.toMap(
      rec -> (String) rec.get("code"), 
      rec -> new ImmutablePair<>(
       (Double) rec.get("x0"), (Double) rec.get("x1")))); 

如果你關心的地圖實現，你可能想

Map<String, ImmutablePair<Double, Double>> dataCache = data.stream() 
    .collect(
     Collectors.toMap(
      rec -> (String) rec.get("code"), 
      rec -> new ImmutablePair<>(
       (Double) rec.get("x0"), (Double) rec.get("x1")), 
      (p1, p2) -> { throw new IllegalArgumentException(); }, 
      LinkedHashMap::new));