CODEWITHSUNDEEP
javalambdajava-stream
gaurs
gaurs

Reputation: 605

java8 reducing a stream using accumulator and combiner

I am trying to write a simple reduce function that will check if all the elements of srcList are present in targetList:

List<String> srcList = Arrays.asList("a", "b");
        List<String> targetList = Collections.singletonList("a");

        Boolean result = srcList.stream().reduce(true
                , (prevResult, srcElement) -> targetList.contains(srcElement)
                , (prevResult, currentResult) -> prevResult && currentResult);

System.out.println(result); // prints false as expected

The above mentioned lambda does not seem to be working as expected. For instance if we change the element in the target list the result is printed as true:

List<String> srcList = Arrays.asList("a", "b");
        List<String> targetList = Collections.singletonList("b");

        Boolean result = srcList.stream().reduce(true
                , (prevResult, srcElement) -> targetList.contains(srcElement)
                , (prevResult, currentResult) -> prevResult && currentResult);

System.out.println(result); // prints true

It looks like that the expression is returning the result of last accumulator evaluation only (i.e. the contains operation in this case) and the combiner is not performing the && on all the results.

Can someone please explain?

Upvotes: 0

Views: 112

Answers (1)

Andy Turner
Andy Turner

Reputation: 140309

The issue is that you ignore prevResult in the accumulator:

(prevResult, srcElement) -> targetList.contains(srcElement)

should be, for example

(prevResult, srcElement) -> prevResult && targetList.contains(srcElement)

But honestly, it would be easier to use containsAll:

targetList.containsAll(srcList)

Upvotes: 2

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