Difference between Iterable and Iterator (#12079)

* Difference between Iterable and Iterator

Add examples of the Iterable and the Iterator usage.

* Minor changes

Add UnitTests for Iterable examples

* Fix iterable example

Add Custom collection for the Iterable implementation example.

* Fix iterable example

Change the example for the Iterable interface implementation

* Revert "Fix iterable example"

This reverts commit 4015cbc038b1b657c54fa666193c0c7019a474ad.

* Minor changes

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterable/CustomIterableClient.java

@@ -0,0 +1,17 @@
+package com.baeldung.collections.iterable;
+
+class CustomIterableClient {
+
+    public static void main(String[] args) {
+
+        ShoppingCart<Product> shoppingCart = new ShoppingCart<>();
+        shoppingCart.add(new Product("Tuna", 42));
+        shoppingCart.add(new Product("Eggplant", 65));
+        shoppingCart.add(new Product("Salad", 45));
+        shoppingCart.add(new Product("Banana", 29));
+
+        for (Product product : shoppingCart) {
+            System.out.println(product.getName());
+        }
+    }
+}

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterable/IterableExample.java

@@ -0,0 +1,32 @@
+package com.baeldung.collections.iterable;
+
+import java.util.Iterator;
+import java.util.List;
+
+public class IterableExample {
+
+    public void iterateUsingIterator(List<Integer> numbers) {
+        Iterator<Integer> iterator = numbers.iterator();
+        while (iterator.hasNext()) {
+            System.out.println(iterator.next());
+        }
+    }
+
+    public void iterateUsingEnhancedForLoop(List<Integer> numbers) {
+        for (Integer number : numbers) {
+            System.out.println(number);
+        }
+    }
+
+    public void iterateUsingForEachLoop(List<Integer> numbers) {
+        numbers.forEach(System.out::println);
+    }
+
+    public void removeElementsUsingIterator(List<Integer> numbers) {
+        Iterator<Integer> iterator = numbers.iterator();
+        while (iterator.hasNext()) {
+            iterator.next();
+            iterator.remove();
+        }
+    }
+}

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterable/Product.java

@@ -0,0 +1,28 @@
+package com.baeldung.collections.iterable;
+
+class Product {
+
+    private String name;
+    private double price;
+
+    public Product(String code, double price) {
+        this.name = code;
+        this.price = price;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public double getPrice() {
+        return price;
+    }
+
+    public void setPrice(double price) {
+        this.price = price;
+    }
+}

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterable/ShoppingCart.java

@@ -0,0 +1,72 @@
+package com.baeldung.collections.iterable;
+
+import java.util.Arrays;
+import java.util.ConcurrentModificationException;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+public class ShoppingCart<E> implements Iterable<E> {
+
+    private E[] elementData;
+    private int size;
+
+    public ShoppingCart() {
+        this.elementData = (E[]) new Object[]{};
+    }
+
+    public void add(E element) {
+        ensureCapacity(size + 1);
+        elementData[size++] = element;
+    }
+
+    private void ensureCapacity(int minCapacity) {
+        int oldCapacity = elementData.length;
+        int newCapacity = oldCapacity + (oldCapacity >> 1);
+        if (newCapacity - minCapacity < 0) {
+            newCapacity = minCapacity;
+        }
+        elementData = Arrays.copyOf(elementData, newCapacity);
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return new ShoppingCartIterator();
+    }
+
+    public class ShoppingCartIterator implements Iterator<E> {
+        int cursor;
+        int lastReturned = -1;
+
+        public boolean hasNext() {
+            return cursor != size;
+        }
+
+        public E next() {
+            return getNextElement();
+        }
+
+        private E getNextElement() {
+            int current = cursor;
+            exist(current);
+
+            E[] elements = ShoppingCart.this.elementData;
+            validate(elements, current);
+
+            cursor = current + 1;
+            lastReturned = current;
+            return elements[lastReturned];
+        }
+
+        private void exist(int current) {
+            if (current >= size) {
+                throw new NoSuchElementException();
+            }
+        }
+
+        private void validate(E[] elements, int current) {
+            if (current >= elements.length) {
+                throw new ConcurrentModificationException();
+            }
+        }
+    }
+}

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterator/CustomIteratorClient.java

@@ -0,0 +1,14 @@
+package com.baeldung.collections.iterator;
+
+import java.util.Iterator;
+
+class CustomIteratorClient {
+
+    public static void main(String[] args) {
+        Iterator<Integer> iterator = Numbers.iterator();
+
+        while (iterator.hasNext()) {
+            System.out.println(iterator.next());
+        }
+    }
+}

core-java-modules/core-java-collections-2/src/main/java/com/baeldung/collections/iterator/Numbers.java

@@ -0,0 +1,60 @@
+package com.baeldung.collections.iterator;
+
+import java.util.Arrays;
+import java.util.Iterator;
+import java.util.List;
+import java.util.NoSuchElementException;
+
+class Numbers {
+
+    private static final List<Integer> NUMBER_LIST = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
+
+    private Numbers() {
+    }
+
+    public static Iterator<Integer> iterator() {
+        return new PrimeIterator();
+    }
+
+    private static class PrimeIterator implements Iterator<Integer> {
+
+        private int cursor;
+
+        @Override
+        public Integer next() {
+            exist(cursor);
+            return NUMBER_LIST.get(cursor++);
+        }
+
+        private void exist(int current) {
+            if (current >= NUMBER_LIST.size()) {
+                throw new NoSuchElementException();
+            }
+        }
+
+        @Override
+        public boolean hasNext() {
+            if (cursor > NUMBER_LIST.size()) {
+                return false;
+            }
+
+            for (int i = cursor; i < NUMBER_LIST.size(); i++) {
+                if (isPrime(NUMBER_LIST.get(i))) {
+                    cursor = i;
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private boolean isPrime(int number) {
+            for (int i = 2; i <= number / 2; ++i) {
+                if (number % i == 0) {
+                    return false;
+                }
+            }
+            return true;
+        }
+    }
+}

core-java-modules/core-java-collections-2/src/test/java/com/baeldung/collections/iterable/IterableUnitTest.java

@@ -0,0 +1,54 @@
+package com.baeldung.collections.iterable;
+
+import com.baeldung.collections.iterable.IterableExample;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class IterableUnitTest {
+
+    private static List<Integer> getNumbers() {
+        List<Integer> numbers = new ArrayList<>();
+        numbers.add(10);
+        numbers.add(20);
+        numbers.add(30);
+        numbers.add(40);
+        return numbers;
+    }
+
+    @Test
+    void givenNumbers_whenUsingIterator_thenCorrectSize() {
+        List<Integer> numbers = getNumbers();
+        IterableExample iterableExample = new IterableExample();
+        iterableExample.iterateUsingIterator(numbers);
+        assertEquals(4, numbers.size());
+    }
+
+    @Test
+    void givenNumbers_whenRemoveElements_thenEmptyList() {
+        List<Integer> numbers = getNumbers();
+        IterableExample iterableExample = new IterableExample();
+        iterableExample.removeElementsUsingIterator(numbers);
+        assertEquals(0, numbers.size());
+    }
+
+    @Test
+    void givenNumbers_whenIterateUsingEnhancedForLoop_thenCorrectSize() {
+        List<Integer> numbers = getNumbers();
+        IterableExample iterableExample = new IterableExample();
+        iterableExample.iterateUsingEnhancedForLoop(numbers);
+        assertEquals(4, numbers.size());
+    }
+
+    @Test
+    void givenNumbers_whenIterateUsingForEachLoop_thenCorrectSize() {
+        List<Integer> numbers = getNumbers();
+        IterableExample iterableExample = new IterableExample();
+        iterableExample.iterateUsingForEachLoop(numbers);
+        assertEquals(4, numbers.size());
+    }
+}