diff --git a/core-java-modules/core-java-collections-array-list/src/main/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacity.java b/core-java-modules/core-java-collections-array-list/src/main/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacity.java new file mode 100644 index 0000000000..34f202a4dd --- /dev/null +++ b/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; + } + +} diff --git a/core-java-modules/core-java-collections-array-list/src/test/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacityUnitTest.java b/core-java-modules/core-java-collections-array-list/src/test/java/com/baeldung/defaultarraylistcapacity/DefaultArrayListCapacityUnitTest.java new file mode 100644 index 0000000000..0690e93879 --- /dev/null +++ b/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 myList = new ArrayList<>(); + int defaultCapacity = DefaultArrayListCapacity.getDefaultCapacity(myList); + + assertEquals(0, defaultCapacity); + + } + + @Test + void givenEmptyArrayList_whenAddItemAndGetDefaultCapacity_thenReturn10() throws Exception { + + ArrayList myList = new ArrayList<>(); + myList.add("ITEM 1"); + + int defaultCapacity = DefaultArrayListCapacity.getDefaultCapacity(myList); + + assertEquals(10, defaultCapacity); + + } + +}