Make SinglyLinkedList Iterable (#4334)

src/main/java/com/thealgorithms/datastructures/lists/SinglyLinkedList.java

@@ -1,11 +1,13 @@
 package com.thealgorithms.datastructures.lists;
 
+import java.util.Iterator;
+import java.util.NoSuchElementException;
 import java.util.StringJoiner;
 
 /**
  * https://en.wikipedia.org/wiki/Linked_list
  */
-public class SinglyLinkedList extends Node {
+public class SinglyLinkedList implements Iterable<Integer> {
 
     /**
      * Head refer to the front of the list
@@ -213,10 +215,8 @@ public class SinglyLinkedList extends Node {
      */
     public int count() {
         int count = 0;
-        Node cur = head;
-        while (cur != null) {
-            cur = cur.next;
-            count++;
+        for (final var element : this) {
+            ++count;
         }
         return count;
     }
@@ -228,13 +228,11 @@ public class SinglyLinkedList extends Node {
      * @return {@code true} if key is present in the list, otherwise
      * {@code false}.
      */
-    public boolean search(int key) {
-        Node cur = head;
-        while (cur != null) {
-            if (cur.value == key) {
+    public boolean search(final int key) {
+        for (final var element : this) {
+            if (element == key) {
                 return true;
             }
-            cur = cur.next;
         }
         return false;
     }
@@ -242,10 +240,8 @@ public class SinglyLinkedList extends Node {
     @Override
     public String toString() {
         StringJoiner joiner = new StringJoiner("->");
-        Node cur = head;
-        while (cur != null) {
-            joiner.add(cur.value + "");
-            cur = cur.next;
+        for (final var element : this) {
+            joiner.add(element + "");
         }
         return joiner.toString();
     }
@@ -452,6 +448,34 @@ public class SinglyLinkedList extends Node {
         instance.deleteDuplicates();
         instance.print();
     }
+
+    @Override
+    public Iterator<Integer> iterator() {
+        return new SinglyLinkedListIterator();
+    }
+
+    private class SinglyLinkedListIterator implements Iterator<Integer> {
+        private Node current;
+
+        SinglyLinkedListIterator() {
+            current = head;
+        }
+
+        @Override
+        public boolean hasNext() {
+            return current != null;
+        }
+
+        @Override
+        public Integer next() {
+            if (!hasNext()) {
+                throw new NoSuchElementException();
+            }
+            final var value = current.value;
+            current = current.next;
+            return value;
+        }
+    }
 }
 
 /**

src/test/java/com/thealgorithms/datastructures/lists/SinglyLinkedListTest.java

@@ -3,6 +3,7 @@ package com.thealgorithms.datastructures.lists;
 import static org.junit.jupiter.api.Assertions.*;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import org.junit.jupiter.api.Test;
 
@@ -207,4 +208,50 @@ public class SinglyLinkedListTest {
             i--;
         }
     }
+
+    @Test
+    void readWithEnhancedForLoopTest() {
+        final var expeced = new ArrayList<Integer>(Arrays.asList(10, 20, 30));
+
+        SinglyLinkedList list = new SinglyLinkedList();
+        for (final var x : expeced) {
+            list.insert(x);
+        }
+
+        var readElements = new ArrayList<Integer>();
+        for (final var x : list) {
+            readElements.add(x);
+        }
+
+        assertEquals(readElements, expeced);
+    }
+
+    @Test
+    void toStringTest() {
+        SinglyLinkedList list = new SinglyLinkedList();
+        list.insert(1);
+        list.insert(2);
+        list.insert(3);
+        assertEquals("1->2->3", list.toString());
+    }
+
+    @Test
+    void toStringForEmptyListTest() {
+        SinglyLinkedList list = new SinglyLinkedList();
+        assertEquals("", list.toString());
+    }
+
+    @Test
+    void countTest() {
+        SinglyLinkedList list = new SinglyLinkedList();
+        list.insert(10);
+        list.insert(20);
+        assertEquals(2, list.count());
+    }
+
+    @Test
+    void countForEmptyListTest() {
+        SinglyLinkedList list = new SinglyLinkedList();
+        assertEquals(0, list.count());
+    }
 }