How to match stream elements but return false if non exists?

Question

I have a stream and would like to check if all match a filter. If all match, return true.

But, if the stream is empty, I\'d like to return false

Answer 1

I already added two answers I now removed but the problem kept nagging on me so I gave it a third try and this time it works including keeping allMatch. Here is the code for method validate.

public static boolean validate(Stream stream) {
    final boolean[] streamNotEmpty = new boolean[1];
    return stream.filter(elem -> {
        streamNotEmpty[0] = true;
        return true;
    }).allMatch(Whatever::someCheck) && streamNotEmpty[0];
}

A shorter version would be

public static boolean validate(Stream stream) {
    final boolean[] streamNotEmpty = new boolean[1];
    return stream.allMatch(elem -> {
        streamNotEmpty[0] = true;
        return elem.someCheck();
    }) && streamNotEmpty[0];
}

The idea behind this is that we want to know if there was at least one element in the stream, so I created a final boolean[] where the value is changed within the filter-call. So at the end of the stream's processing we can check the stream for being empty or not and act accordingly when we return the result.

Here is a complete class including testcases to prove that - this time finally - I provided a valid solution.

import java.util.ArrayList;
import java.util.stream.Stream;

public class StreamTest {

    public static boolean validate(Stream stream) {
        final boolean[] streamNotEmpty = new boolean[1];
        return stream.allMatch(elem -> {
            streamNotEmpty[0] = true;
            return elem.someCheck();
        }) && streamNotEmpty[0];
    }

    static class Whatever {
        private boolean checkResult;

        public Whatever() {
            this(false);
        }

        public Whatever(boolean b) {
            this.checkResult = b;
        }

        public boolean someCheck() {
            return checkResult;
        }
    }

    public final static void main(String[] args) {
        ArrayList list = new ArrayList<>();
        System.out.println("empty list: " + validate(list.stream()));
        System.out.println("empty list parallel: " + validate(list.parallelStream()));

        for (int i = 0; i < 10000; i++) {
            list.add(new Whatever(true));
        }

        System.out.println("non empty true list: " + validate(list.stream()));
        System.out.println("non empty true list parallel: " + validate(list.parallelStream()));

        for (int i = 0; i < 10000; i += 1000) {
            list.add(new Whatever(false));
        }

        System.out.println("non empty false list: " + validate(list.stream()));
        System.out.println("non empty false list parallel: " + validate(list.parallelStream()));
    }
}

The output when executing it is:

empty list: false empty list parallel: false non empty true list: true non empty true list parallel: true non empty false list: false non empty false list parallel: false