feat(commons): add StreamUtil for common stream utilities

commons/src/main/java/io/gitlab/jfronny/commons/StreamUtil.java

@@ -0,0 +1,23 @@
+package io.gitlab.jfronny.commons;
+
+import java.util.function.BiConsumer;
+import java.util.function.Consumer;
+
+public class StreamUtil {
+    /**
+     * A BiConsumer that keeps only elements of the specified type.
+     * To be used with Stream.mapMulti.
+     *
+     * @param type The type to keep
+     * @param <E> The type of the elements
+     * @param <T> The type to keep
+     * @return A BiConsumer that keeps only elements of the specified type
+     */
+    public static <E, T> BiConsumer<E, Consumer<T>> keepOnly(Class<T> type) {
+        return (element, consumer) -> {
+            if (type.isInstance(element)) {
+                consumer.accept(type.cast(element));
+            }
+        };
+    }
+}