我有一個List<BatchDTO>與以下類public class BatchDTO { private String batchNumber; private Double quantity;..//Getters and setters}如果 batchNumber 重復，我要做的是總結總數。我使用 LinkedHashMap 來實現它，并進行了迭代。但我想要的是一種更優化的方式。我可以使用流以優化的方式執行此操作嗎？private static List<BatchDTO > getBatchDTO (Map<String, BatchDTO > batchmap) { return batchmap.values().stream().collect(Collectors.toList());}private static Map<String, BatchDTO > getBatchMap(List<BatchDTO > batchList, Map<String, BatchDTO > batchMap) { for (BatchDTO batchDTO : batchList) { batchMap = getBatchMap(batchMap, batchDTO ); } return batchMap;}private static Map<String, BatchDTO > getBatchMap(Map<String, BatchDTO > batchMap, BatchDTO batchObject) { String batchCode = batchObject.getBatchNumber(); if(!batchMap.containsKey(batchCode)) { batchMap.put(batchCode, batchObject); } else { batchObject.setQuantity(getTotalQuantity(batchMap,batchObject)); batchMap.put(batchCode, batchObject); } return batchMap;}private static Double getTotalQuantity(Map<String, BatchDTO > batchmap, BatchDTO batchObject) { return batchmap.get(batchObject.getBatchNumber()).getQuantity() + batchObject.getQuantity();}
2 回答
九州編程
TA貢獻1785條經驗 獲得超4個贊
假設BatchDTO擁有所有 args 構造函數，您可以從返回Map到List<BatchDTO>
List<BatchDTO>?collect?=?list.stream() ????????.collect(groupingBy(BatchDTO::getBatchNumber,?summingDouble(BatchDTO::getQuantity))) ????????.entrySet().stream() ????????.map(entry?->?new?BatchDTO(entry.getKey(),?entry.getValue())) ????????.collect(Collectors.toList());
JavaDoc:?groupingBy()?,?summingDouble()
飲歌長嘯
TA貢獻1951條經驗 獲得超3個贊
代碼中的注釋可能有點難以理解，但這就是我的全部時間。
// Result will be a Map where the keys are the unique 'batchNumber's, and the
// values are the sum of the 'quantities' for those with that 'batchNumber'.
public Map<String, Double> countBatchQuantities(final List<BatchDTO> batches) {
// Stream over all the batches...
return batches.stream()
// Group them by 'batch number' (gives a Map<String, List<BatchDTO>>)
.collect(Collectors.groupingBy(BatchDTO::getBatchNumber))
// Stream over all the entries in that Map (gives Stream<Map.Entry<String, List<BatchDTO>>>)
.entrySet().stream()
// Build a map from the Stream of entries
// Keys stay the same
.collect(Collectors.toMap(Entry::getKey,
// Values are now the result of streaming the List<BatchDTO> and summing 'getQuantity'
entry -> entry.getValue().stream().mapToDouble(BatchDTO::getQuantity).sum()));
}
注意：我不保證這比您現有的方法更優化......但它可以使用 Streams 完成工作。quantity注意：如果是null針對您的任何一個，這將引發異常BatchDTO...
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