introduced StreamUtils

util/src/main/java/de/muehlencord/shared/util/StreamUtils.java

@@ -0,0 +1,52 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package de.muehlencord.shared.util;
+
+import java.util.List;
+import java.util.function.Function;
+import java.util.stream.Collector;
+import java.util.stream.Collectors;
+
+/**
+ *
+ * @author Joern Muehlencord (joern@muehlencord.de)
+ */
+public class StreamUtils {
+
+    public static <T> Collector<T, ?, T> singletonCollector() {
+        return getCollectorFunction(getListTSingleFunction());
+    }
+
+    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 == null) {
+                throw new IllegalStateException("List must not be null");
+            }
+            if (list.size() != 1) {
+                throw new IllegalStateException("List contains not exactly one element (size=" + list.size() + ")");
+            }
+            return list.get(0);
+        };
+    }
+
+}