如何將Stream方法分配給java 8中的函數？

Question

我有一個情況如下：

RowDataSet rdsHavingMaxNumericValue = dataList.stream() 
        .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
        .max((r1, r2) -> r1.getId().compareTo(r2.getId())) 
        .get(); 

我有兩種情況下，我需要找到依據的情況下，無論是最小或最大？我怎樣才能取代函數的第三行調用，以便它直接使用.apply（）方法並執行操作？

心中已經成功地分配Collection.max或集合分鐘如下：

BiFunction<List<RowDataSet>, Comparator<RowDataSet>, RowDataSet> mrOrlrOperation = expComponents[0] == 'M' ? Collections::max : Collections::min; 

我用它如下：

RowDataSet rds = mrOrlrOperation.apply(dataList, <some comparator>); 

Answer 1

它可能會更簡單，只需使用一個條件：

Stream<?> stream = dataList.stream() 
          .filter(r -> r.getLong(NUMERIC_VALUE) != null); 
Comparator<RowDataSet> c = Comparator.comparingLong(RowDataSet::getId); 
RowDataSet rdsHavingMaxNumericValue = 
       expComponents[0] == 'M' ? stream.max(c).get() : stream.min(c).get(); 

可以存儲在一個單獨的Function最小值/最大值操作，如果你想：

BiFunction<Stream<RowDataSet>, Comparator<RowDataSet>, RowDataSet>> operation = 
      expComponents[0] == 'M' ? (s, c) -> s.max(c).get() : (s, c) -> s.min(c).get(); 

RowDataSet rdsHavingMaxNumericValue = operation.apply(stream, c); 

Answer 2

選擇使用的min代替max是不是不同的仍然使用max操作，但相反的次序，所以你可以使用

初始化：

Comparator<RowDataSet> c = Comparator.comparingLong(RowDataSet::getId); 
if(expComponents[0] != 'M') c = c.reversed(); 

實際操作

RowDataSet rdsHavingMaxNumericValue = 
    dataList.stream() 
      .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
      .max(c) 
      .get(); 

Rever唱歌的順序沒有性能影響。它只是意味着調用r2.getId().compareTo(r1.getId())代替r1.getId().compareTo(r2.getId()) ...

另一種方法是

Comparator<RowDataSet> c = Comparator.comparingLong(RowDataSet::getId); 
BinaryOperator<RowDataSet> op = expComponents[0] == 'M'? 
           BinaryOperator.maxBy(c): BinaryOperator.minBy(c); 

RowDataSet rdsHavingMaxNumericValue = 
    dataList.stream() 
      .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
      .reduce(op) 
      .get(); 

此預期what min and max would do ...

---

您可以使用變種創建您BiFunction，例如

變體1：

Function<Comparator<RowDataSet>, Comparator<RowDataSet>> 
    maxOrMin = expComponents[0] == 'M'? Function.identity(): Comparator::reversed; 

BiFunction<List<RowDataSet>, Comparator<RowDataSet>, RowDataSet> mrOrlrOperation 
    = (dataList, comparator) -> dataList.stream() 
             .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
             .max(maxOrMin.apply(comparator)) 
             .get(); 

變2：

Function<Comparator<RowDataSet>, BinaryOperator<RowDataSet>> 
    maxOrMin = expComponents[0] == 'M'? BinaryOperator::maxBy: BinaryOperator::minBy; 

BiFunction<List<RowDataSet>, Comparator<RowDataSet>, RowDataSet> mrOrlrOperation 
    = (dataList, comparator) -> dataList.stream() 
             .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
             .reduce(maxOrMin.apply(comparator)) 
             .get(); 

當然，你也可以生活在一個代碼重複

BiFunction<List<RowDataSet>, Comparator<RowDataSet>, RowDataSet> mrOrlrOperation 
    = expComponents[0] == 'M'? 
     (dataList, comparator) -> dataList.stream() 
              .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
              .max(comparator) 
              .get(): 
     (dataList, comparator) -> dataList.stream() 
              .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
              .min(comparator) 
              .get(); 

在這兩種情況下，條件expComponents[0] == 'M'是BiFunction創建時只檢查一次，時從未評估被調用。

Answer 3

max是還原操作，那麼您可以使用通用的reduce：

public void reducing(BiOperator<...> myFunction) { 
    RowDataSet rdsHavingMaxNumericValue = dataList.stream() 
       .filter(r -> r.getLong(NUMERIC_VALUE) != null) 
       .reduce((r1, r2) -> myFunction.apply(r1,r2)) 
       .get(); 
} 
... 
reducing(Integer::max); 
...