Data update

Task/Gotchas/Java/gotchas.java

@@ -0,0 +1,47 @@
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public final class Gotchas {
+
+	public static void main(String[] aArgs) {
+		// Gotcha 1: An integer argument to a Collection, such as a List, sets the capacity of the Collection,
+		//           but does not fill the Collection with elements.
+		List<Integer> numbers = new ArrayList<Integer>(100);
+		// The above list has the capacity to hold 100 elements, but is currently empty.
+		
+		// Setting the element with index 3 to a value of 42 will create a runtime exception,
+		// because the list has length 0, and this element does not yet exist.		
+		numbers.set(3, 42);
+		// The gotcha is only revealed when a runtime exception is thrown.
+		System.out.println(numbers);
+		// java.lang.IndexOutOfBoundsException: Index 3 out of bounds for length 0
+		
+		// Gotcha 2: Copying a Collection in a simple manner works,
+		//           but means that changes to the original Collection are reflected in the copy,
+		//           which is not normally the desired outcome.		
+		Set<String> letters = new HashSet<String>();
+		letters.add("a"); letters.add("b"); letters.add("c");
+		
+		// Create a copy of the set.
+		Set<String> copy = letters;	
+		// The two sets are identical.
+		System.out.println(letters + " :: " + copy);
+		
+		// Add an element to the set 'letters'.
+		letters.add("d");
+		// The same letter has been added to the set 'copy'.
+		// Both sets now contain the same 4 letters.
+		System.out.println(letters + " :: " + copy);		
+		// In a program this can cause mysterious results which can be difficult to debug.
+		
+		// The correct way to copy a Collection is to use a copy constructor as shown below.
+		Set<String> correctCopy = new HashSet<String>(letters);
+		letters.add("e");
+		
+		// The set 'correctCopy' only contains its original 4 letters.
+		System.out.println(letters + " :: " + correctCopy);
+	}
+
+}