IsSortedByMatcher.java

package gmjonker.matchers;

import com.google.common.collect.Ordering;
import org.hamcrest.Description;
import org.hamcrest.Factory;
import org.hamcrest.TypeSafeMatcher;

import java.util.Comparator;
import java.util.List;
import java.util.function.Function;

public class IsSortedByMatcher<T, U extends Comparable<U>> extends TypeSafeMatcher<List<T>>
{
    private final Function<T, U> mapper;
    private final boolean reverse;

    public IsSortedByMatcher(Function<T, U> mapper, boolean reverse)
    {
        this.mapper = mapper;
        this.reverse = reverse;
    }

    @Override
    public boolean matchesSafely(List<T> list)
    {
        if (reverse) {
            Comparator<T> comparator = (t1, t2) -> mapper.apply(t1).compareTo(mapper.apply(t2));
            return Ordering.from(comparator).reverse().isOrdered(list);
        } else {
            Comparator<T> comparator = (t1, t2) -> mapper.apply(t1).compareTo(mapper.apply(t2));
            return Ordering.from(comparator).isOrdered(list);
        }
    }

    public void describeTo(Description description)
    {
        if (reverse)
            description.appendText("a reverse-sorted list after mapping" );
        else
            description.appendText("a sorted list after mapping");
    }

    @Factory
    public static <S, U extends Comparable<U>> IsSortedByMatcher<S,U> isSortedOn(Function<S,U> mapper)
    {
        return new IsSortedByMatcher<>(mapper, false);
    }

    @Factory
    public static <S, U extends Comparable<U>> IsSortedByMatcher<S,U> isSortedReverselyOn(Function<S,U> mapper)
    {
        return new IsSortedByMatcher<>(mapper, true);
    }
}