How to assign Stream method to a Function in java 8?

Question

I have a case as below:

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

I've two cases where I need to find either min or max depending on a case? how can i replace the call max at third line by a Function so that it direcly uses .apply() method and performs the operation?

I'v successfully assigned Collection.max or Collection min as below:

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

and i use it as below:

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

Holger · Accepted Answer · 2017-08-09 15:54:03Z

The choice to use min instead of max is not different to still using the max operation, but with a reversed order, so you could use

Initialization:

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

Actual operation

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

Reversing the order has no performance impact. It just implies calling r2.getId().compareTo(r1.getId()) instead of r1.getId().compareTo(r2.getId())…

An alternative would be

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();

This anticipates what min and max would do…

You can use either variant to create your BiFunction, e.g.

Variant 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();

Variant 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();

Of course, you could also live with a code duplication

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();

In either case, the condition expComponents[0] == 'M' is checked only once when the BiFunction is created and never evaluated when mrOrlrOperation.apply(…) is invoked.

assylias · Accepted Answer · 2017-08-09 15:52:02Z

It would probably be simpler to just use a condition:

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();

You can store the min/max operation in a separate Function if you want:

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);

Yeah, looks like the option, but i want to avoid the case lookup at runtime
i do not want to perform any if else operations. i've left it as a last option
@Ajeetkumar I'm not sure I understand: how can you check the value of expComponents without a condition?
check my second example of Collections::max or ::min. So that function whenever runs it will directly use .apply, it won't check for min or max because it is already told to it that min or max is to be run. getting the idea?

Jean-Baptiste Yunès · Accepted Answer · 2017-08-10 14:03:24Z

max is a reducing operation, then you can use the generic 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); ...

Hi, i find your answer interesting but have difficulty understanding it fully. BiFunction parameters are Integer, Integer, Integer ? in case of Integer::max, have you missed myFunction.apply call?

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