如何使用Java 8 lambda和Steam API获取元素之和[duplicate]

Question

如何将以下内容转换为流API和lambda表达式。

double journalSize = 0;
List<PersistenceAdapter> allKahadb = ((MultiKahaDBPersistenceAdapter)persistenceAdapter).getAdapters();



for (PersistenceAdapter persistenceAdapter : allKahadb) {
    Journal journal = ((KahaDBPersistenceAdapter) persistenceAdapter).getStore().getJournal();
    int noOfJournels = journal.getFiles().size();
    long journalMaxSize = journal.getMaxFileLength();
    journalSize += (journalMaxSize * noOfJournels)/ Memory.ONE_MB;
}

我试着用“地图”和“溪流之和”来做，但什么都没用？基本上我需要日志大小的总和。

Answer 1

将对象映射到日志实例，然后将它们映射到要求和的双值：

double journalSize =
    allKahadb.stream()
             .map(pa -> ((KahaDBPersistenceAdapter)pa).getStore().getJournal())
             .mapToDouble(j -> ((double) j.getMaxFileLength() * j.getFiles().size()) / Memory.ONE_MB)
             .sum();