modifying ListOfThree class

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/accessmodifiers/publicmodifier/ListOfThree.java

@@ -1,31 +1,31 @@
 package com.baeldung.accessmodifiers.publicmodifier;
 
 import java.util.AbstractList;
+import java.util.Arrays;
 
 public class ListOfThree<E> extends AbstractList<E> {
 
-    private static final int MAX_LENGTH = 3;
-    private int size;
-    private Object[] elements = new Object[MAX_LENGTH];
+    private static final int LENGTH = 3;
+    private Object[] elements;
+    
+    public ListOfThree(E[] data) {
+        if(data == null 
+            || data.length != LENGTH)
+            throw new IllegalArgumentException();
+        
+        this.elements = Arrays.copyOf(data, data.length); //shallow copy
+        
+    }
     
     @Override
     @SuppressWarnings("unchecked")
-    public E get(int index) {        
+    public E get(int index) {      
         return (E)elements[index];
     }
-
-    @Override
-    public boolean add(E e) {
-        
-        elements[size] = e;
-        size++;
-        
-        return true;
-    }
     
     @Override
     public int size() {
-        return size;
+        return LENGTH;
     }
 
 }

core-java-modules/core-java-lang-oop-2/src/test/java/com/baeldung/accessmodifiers/PublicAccessModifierUnitTest.java

@@ -48,31 +48,21 @@ public class PublicAccessModifierUnitTest {
     @Test
     public void whenCreatingCustomList_concreteAndInheritedMethodsWork() {
 
-        List<String> list1 = new ListOfThree<String>();
-        list1.add("zero"); //inherited implementation
-        list1.add("one");
-        list1.add("two");
+        String[] dataSet1 = new String[] {"zero", "one", "two"};
+        
+        List<String> list1 = new ListOfThree<String>(dataSet1);
         
         //our implemented methods
-        assertEquals("zero", list1.get(0));
-      
-        assertThrows(ArrayIndexOutOfBoundsException.class, () -> {
-            list1.get(4);
-        });
-        
-        assertThrows(ArrayIndexOutOfBoundsException.class, () -> {
-            list1.add("three");
-        });
-        
+        assertEquals("one", list1.get(1));
         assertEquals(3, list1.size());
 
-        list1.indexOf("one"); //inherited implementation
+        //inherited implementations
+        assertEquals(1, list1.indexOf("one"));
         
-        List<String> list2 = new ListOfThree<String>();
-        list2.add("zero");
-        list2.add("one");
+        String[] dataSet2 = new String[] {"two", "zero", "one"};
+        List<String> list2 = new ListOfThree<String>(dataSet2);
         
-        assertTrue(list1.containsAll(list2));  //inherited implementation
+        assertTrue(list1.containsAll(list2));
     }
     
 }