BAEL-5931: Default Capacity of ArrayList (#13110)

core-java-modules/core-java-collections-array-list/src/main/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacity.java

@@ -0,0 +1,20 @@
+package com.baeldung.defaultarraylistcapacity;
+
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+
+public class DefaultArrayListCapacity {
+
+    public static int getDefaultCapacity(ArrayList<?> arrayList) throws Exception {
+
+        if (arrayList == null) {
+            return 0;
+        }
+
+        Field field = ArrayList.class.getDeclaredField("elementData");
+        field.setAccessible(true);
+
+        return ((Object[]) field.get(arrayList)).length;
+    }
+
+}

core-java-modules/core-java-collections-array-list/src/test/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacityUnitTest.java

@@ -0,0 +1,33 @@
+package com.baeldung.defaultarraylistcapacity;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.ArrayList;
+
+import org.junit.jupiter.api.Test;
+
+public class DefaultArrayListCapacityUnitTest {
+
+    @Test
+    void givenEmptyArrayList_whenGetDefaultCapacity_thenReturnZero() throws Exception {
+
+        ArrayList<Integer> myList = new ArrayList<>();
+        int defaultCapacity = DefaultArrayListCapacity.getDefaultCapacity(myList);
+
+        assertEquals(0, defaultCapacity);
+
+    }
+
+    @Test
+    void givenEmptyArrayList_whenAddItemAndGetDefaultCapacity_thenReturn10() throws Exception {
+
+        ArrayList<String> myList = new ArrayList<>();
+        myList.add("ITEM 1");
+
+        int defaultCapacity = DefaultArrayListCapacity.getDefaultCapacity(myList);
+
+        assertEquals(10, defaultCapacity);
+
+    }
+
+}