Test and avoid bugs around empty and singleton iterators.

src/main/java/io/github/jervenbolleman/handlegraph4j/iterators/AutoClosedIterator.java

@@ -58,22 +58,25 @@ public interface AutoClosedIterator<T> extends AutoCloseable, Iterator<T> {
 	public static <T> AutoClosedIterator<T> of(T stat) {
 
 		return new AutoClosedIterator<>() {
-			T t = stat;
-
+			private final T t = stat;
+			boolean consumed = false;
 			@Override
 			public void close() {
 			}
 
 			@Override
 			public boolean hasNext() {
-				return t != null;
+				return !consumed;
 			}
 
 			@Override
 			public T next() {
-				T temp = t;
-				t = null;
-				return temp;
+				if (consumed) {
+					throw new NoSuchElementException();
+				} else {
+					consumed = true;
+					return t;
+				}
 			}
 		};
 	}
@@ -100,7 +103,7 @@ public boolean hasNext() {
 
 			@Override
 			public T next() {
-				if (useFirst) {
+				if (useFirst && first.hasNext()) {
 					T next = first.next();
 					useFirst = first.hasNext();
 					return next;

src/test/java/io/github/jervenbolleman/handlegraph4j/iterators/AutoClosedIteratorTest.java

@@ -0,0 +1,44 @@
+package io.github.jervenbolleman.handlegraph4j.iterators;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.NoSuchElementException;
+
+import org.junit.jupiter.api.Test;
+
+public class AutoClosedIteratorTest {
+
+	@Test
+	public void empty() {
+		try (AutoClosedIterator<Object> empty = AutoClosedIterator.empty()) {
+			assertFalse(empty.hasNext());
+			assertThrows(NoSuchElementException.class, () -> empty.next());
+		}
+	}
+
+	@Test
+	public void concatTwoEmptyIterators() {
+		try (AutoClosedIterator<Object> empty1 = AutoClosedIterator.empty();
+				AutoClosedIterator<Object> empty2 = AutoClosedIterator.empty();
+				AutoClosedIterator<Object> both = AutoClosedIterator.concat(empty1, empty2)) {
+			assertFalse(both.hasNext());
+			assertThrows(NoSuchElementException.class, () -> both.next());
+		}
+	}
+
+	@Test
+	public void concatEmptyThenFullIterators() {
+		try (AutoClosedIterator<Object> empty1 = AutoClosedIterator.empty();
+				AutoClosedIterator<Object> notEmpty= AutoClosedIterator.of("String");
+				AutoClosedIterator<Object> both = AutoClosedIterator.concat(empty1, notEmpty)) {
+			assertTrue(both.hasNext());
+			assertNotNull(both.next());
+			assertFalse(both.hasNext());
+			assertThrows(NoSuchElementException.class, () -> both.next());
+		}
+	}
+}
+