Added abstract classes

java.html.markdown

@@ -451,6 +451,74 @@ public class ExampleClass extends ExampleClassParent implements InterfaceOne,
 	public void InterfaceTwoMethod() {
 	}
 }
+
+
+// Abstract Classes 
+// Abstract Class declaration syntax
+// <access-level> abstract <abstract-class-name> extends <super-abstract-classes> {
+//     // Constants and variables
+//     // Method declarations
+// }
+
+// Methods can't have bodies in an interface, unless the method is
+// static. Also variables are NOT final by default, unlike an interface.
+// Also abstract classes CAN have the "main" method.
+// Abstract classes solve these problems.
+
+public abstract class Animal 
+{
+	public abstract void makeSound();
+
+	// Method can have a body
+	public void eat()
+	{
+		System.out.println("I am an animal and I am Eating.");  
+		// Note: We can access private variable here.
+		age = 30;
+	}
+
+	// No need to initialise, however in an interface 
+	// a variable is implicitly final and hence has
+	// to be initialised.
+	private int age;
+
+	public void printAge()
+	{
+		System.out.println(age);  
+	}
+
+	// Abstract classes can have main function.
+	public static void main(String[] args)
+	{
+		System.out.println("I am abstract");
+	}
+}
+
+class Dog extends Animal
+{
+	// Note still have to override the abstract methods in the
+	// abstract class.
+	@Override
+	public void makeSound()
+	{
+		System.out.println("Bark");
+		// age = 30;	==> ERROR!	age is private to Animal
+	}
+
+	// NOTE: You will get an error if you used the 
+	// @Override annotation here, since java doesn't allow
+	// overriding of static methods.
+	// What is happening here is called METHOD HIDING.
+	// Check out this awesome SO post: (http://stackoverflow.com/questions/16313649/)
+	public static void main(String[] args)
+	{
+		Dog pluto = new Dog();
+		pluto.makeSound();
+		pluto.eat();
+		pluto.printAge();
+	}
+}
+
 ```
 
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