[java/en] Added more info about Collections initialization (#4200)

java.html.markdown

@@ -510,10 +510,12 @@ public class LearnJava {
 
         // toString returns this Object's string representation.
         System.out.println("trek info: " + trek.toString());
+    } // End main method
 
+    private static class TestInitialization {
         // Double Brace Initialization
-        // The Java Language has no syntax for how to create static Collections
+        // Before Java 11, the Java Language had no syntax for how to create
-        // in an easy way. Usually you end up in the following way:
+        // static Collections in an easy way. Usually you end up like this:
         private static final Set<String> COUNTRIES = new HashSet<String>();
         static {
            COUNTRIES.add("DENMARK");
@@ -521,10 +523,10 @@ public class LearnJava {
            COUNTRIES.add("FINLAND");
         }
 
-        // But there's a nifty way to achieve the same thing in an
+        // There's a nifty way to achieve the same thing, 
-        // easier way, by using something that is called Double Brace
+        // by using something that is called Double Brace Initialization.
-        // Initialization.
+        private static final Set<String> COUNTRIES_DOUBLE_BRACE = 
-        private static final Set<String> COUNTRIES = new HashSet<String>() {{
+        new HashSet<String>() {{
             add("DENMARK");
             add("SWEDEN");
             add("FINLAND");
@@ -535,8 +537,45 @@ public class LearnJava {
         // is called when the anonymous inner class is created.
         // This does not only work for Collections, it works for all
         // non-final classes.
+        
 
-    } // End main method
+        // Another option was to initialize the Collection from an array,
+        // using Arrays.asList() method:
+        private static final List<String> COUNTRIES_AS_LIST = 
+                        Arrays.asList("SWEDEN", "DENMARK", "NORWAY");
+        // This has one catch: the list we get is internally backed by the array,
+        // and since arrays can't change their size, the list backed by the array
+        // is not resizeable, which means we can't add new elements to it: 
+        public static void main(String[] args) {
+            COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException!
+            // However, we can replace elements by index, just like in array: 
+            COUNTRIES.set(1, "FINLAND");
+            System.out.println(COUNTRIES); // prints [SWEDEN, FINLAND, NORWAY]
+        }
+        // The resizing problem can be circumvented 
+        // by creating another Collection from the List:
+         private static final Set<String> COUNTRIES_SET = 
+                new HashSet<>(Arrays.asList("SWEDEN", "DENMARK", "NORWAY"));
+        // It's perfectly fine to add anything to the Set of COUNTRIES now. 
+    } // End TestInitialization class
+
+    private static class TestJava11Initialization {
+        // Since Java 11, there is a convenient option to initialize Collections:
+        // Set.of() and List.of() methods. 
+        private static final Set<String> COUNTRIES = 
+                Set.of("SWEDEN", "DENMARK", "NORWAY");
+        // There is a massive catch, though: Lists and Sets initialized like this 
+        // 1) are immutable 
+        // 2) can't contain null elements (even check for null elements fails)!
+        public static void main(String[] args) {
+            COUNTRIES.add("FINLAND"); // throws UnsupportedOperationException
+            COUNTRIES.remove("NORWAY"); // throws UnsupportedOperationException 
+            COUNTRIES.contains(null); // throws NullPointerException
+        }
+        private static final Set<String> COUNTRIES_WITH_NULL = 
+                    Set.of("SWEDEN", null, "NORWAY"); // throws NullPointerException
+
+    } // End TestJava11Initialization class
 } // End LearnJava class
 
 // You can include other, non-public outer-level classes in a .java file,