[New] add CollectionsHelper.singletonCollector(Supplier)

li.strolch.utils/src/main/java/li/strolch/utils/collections/CollectionsHelper.java

@@ -3,11 +3,30 @@ package li.strolch.utils.collections;
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
+import java.util.function.Supplier;
 import java.util.stream.Collector;
 import java.util.stream.Collectors;
 
+/**
+ * A helper class to handle collections
+ */
 public class CollectionsHelper {
 
+	/**
+	 * Returns true if the elements in the two given lists are equal according to the given comparator regardless of the
+	 * sequence
+	 *
+	 * @param one
+	 * 		the first list
+	 * @param two
+	 * 		the second list
+	 * @param comparator
+	 * 		the comparator
+	 * @param <T>
+	 * 		the type of elements being compared
+	 *
+	 * @return true if the lists have the same elements, regardless of their order in the given lists
+	 */
 	public static <T> boolean equalsUnordered(List<T> one, List<T> two, Comparator<T> comparator) {
 		if (one == null && two == null)
 			return true;
@@ -24,17 +43,63 @@ public class CollectionsHelper {
 		return one.equals(two);
 	}
 
-	public static <T> Collector<T, List<T>, T> singletonCollector() {
-		return singletonCollector(null);
+	/**
+	 * Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
+	 * element is in the stream
+	 *
+	 * @param <T>
+	 * 		the type of element to return
+	 *
+	 * @return the singleton collector
+	 *
+	 * @throws IllegalStateException
+	 * 		if not 1 and only 1 element is in the stream
+	 */
+	public static <T> Collector<T, List<T>, T> singletonCollector() throws IllegalStateException {
+		return singletonCollector(() -> null);
 	}
 
-	public static <T> Collector<T, List<T>, T> singletonCollector(String errorMsg) {
+	/**
+	 * Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
+	 * element is in the stream
+	 *
+	 * @param errorMsg
+	 * 		the error message to use if not 1 and only 1 element is in the collection
+	 * @param <T>
+	 * 		the type of element to return
+	 *
+	 * @return the singleton collector
+	 *
+	 * @throws IllegalStateException
+	 * 		if not 1 and only 1 element is in the stream
+	 */
+	public static <T> Collector<T, List<T>, T> singletonCollector(String errorMsg) throws IllegalStateException {
+		return singletonCollector(() -> errorMsg);
+	}
+
+	/**
+	 * Returns a collector which returns exactly one element from a stream, and throws an exception if not exactly one
+	 * element is in the stream
+	 *
+	 * @param errorMsgSupplier
+	 * 		the supplier for an error message to use if not 1 and only 1 element is in the collection
+	 * @param <T>
+	 * 		the type of element to return
+	 *
+	 * @return the singleton collector
+	 *
+	 * @throws IllegalStateException
+	 * 		if not 1 and only 1 element is in the stream
+	 */
+	public static <T> Collector<T, List<T>, T> singletonCollector(Supplier<String> errorMsgSupplier)
+			throws IllegalStateException {
 		return Collector.of(ArrayList::new, List::add, (left, right) -> {
 			left.addAll(right);
 			return left;
 		}, list -> {
 
 			if (list.isEmpty()) {
+				String errorMsg = errorMsgSupplier.get();
 				if (errorMsg == null) {
 					throw new IllegalStateException("Expect one element, but received no elements");
 				} else
@@ -43,6 +108,7 @@ public class CollectionsHelper {
 
 			if (list.size() != 1) {
 				String listS = list.stream().map(Object::toString).collect(Collectors.joining(", "));
+				String errorMsg = errorMsgSupplier.get();
 				if (errorMsg == null) {
 					throw new IllegalStateException(
 							"Expect one element, but received " + list.size() + " in list " + listS);