Single item from stream

import java.util.List;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;


public class StreamUtils {


    public static <T> Collector<T, ?, T> singletonCollector() {
        return getCollectorFunction(getListTSingleFunction());
    }


    public static <T> Collector<T, ?, T> firstInListCollector() {
        return getCollectorFunction(getListTFirstFunction());
    }


    public static <T> Collector<T, ?, T> lastInListCollector() {
        return getCollectorFunction(getLastTFirstFunction());
    }


    private static <T> Collector<T, ?, T> getCollectorFunction(Function<List<T>, T> listTFunction) {
        return Collectors.collectingAndThen(Collectors.toList(), listTFunction);
    }


    private static <T> Function<List<T>, T> getListTSingleFunction() {
        return list -> {
            if (list.size() != 1) {
                throw new IllegalStateException();
            }
            return list.get(0);
        };
    }


    private static <T> Function<List<T>, T> getListTFirstFunction() {
        return list -> {
            if (list.size() == 0) {
                throw new IllegalStateException();
            }
            return list.get(0);
        };
    }


    private static <T> Function<List<T>, T> getLastTFirstFunction() {
        return list -> {
            if (list.size() == 0) {
                throw new IllegalStateException();
            }
            return list.get(list.size()-1);
        };
    }

}