Let us at first remind what is inheritance (samples in Java):

public class Pet {
  public void say() { }
}

public class  Dog extends Pet {
  public void say() { System.out.println("I am a dog."); }
}

public class  Cat extends Pet {
  public void say() { System.out.println("I am a cat."); }
}

public class Test{
  public static void main(String[] args) {
    Pet pet1 = new Dog();
    Pet pet2 = new Cat();
    pet1.say();
    pet2.say();
  }
}

This  programm will output:

I am a dog.

I am a cat.

The idea is very simple: despite pet1 and pet2 are of type Pet, pet1 is pointing to object of class Dog and pet is pointing  to object of class Cat.

The common question which is usually raised is: why we don’t have just  Dog pet1 = new Dog(); and Cat pet2 = new Cat(); ? Whe do we need to access it via Pet?

Let me give you some real-world examples where do we need it:

1. Servlets. When we create a servlet we inherit base servlet class and override method  doGet() or doPost()to add our functionality to the servlet. The server (for example Tomcat) have list of our servlet classes deployed, and as soon as it gets request for our servlet it loads our class, create an object and call doGet() or doPost() on it. As soon as server have no idea what classes do we have it address object of our class via variable of type HttpServlet.

2. The similar idea was used in early versions of Struts library.

3. In .NET as well as in Java you override Exception class or one of it’s successors to create exception specific to your application. The system (Java or .NET)  knows only how to work with Exception (and RuntimeException specificly in Java) and works with all your exceptions uniformaly.

public class Test {
	private static void test() throws Exception {
		for (int i = 0; i < 3; i++) {
			try {
				throw new Exception("Exception text");
			} finally {
				System.out.println("Finally exception " + i);
				continue;
			}
		}
		System.out.println("Impossible");
	}

	public static void main(String[] args) throws Exception {
		test();
		System.out.println("Impossible 2");
	}
}

will produce output:

Finally exception 0
Finally exception 1
Finally exception 2
Impossible
Impossible 2


So, be extremely careful with your breaks!

public class Test2 {
  static class TestClass1 {
      int x = 1;
      String test() {
          return "1";
      }
  }

  static class TestClass2 extends TestClass1 {
      int x = 2;
      String test() {
          return "2";
      }
  }

  static class TestClass3 extends TestClass2 {
      int x = 3;
      int testX() {
          return ((TestClass1)this).x;
      }
      String test() {
          return super.test();
      }
  }

  public static void main(String[] args) {
      TestClass3 obj = new TestClass3();
      System.out.println(obj.testX());
      System.out.println(obj.test());
  }
}

It produces expected output

1
2


The point is that there is no way to call test() method of the TestClass1 class!

super.super.test()

won’t compile but

((TestClass1)this).test()

will produce java.lang.StackOverflowError.

Despite that we can access superclass members of any level by casting this to appropriate class (see The Java Language Specification at http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.11.2), polymorphism allows us to access immediate superclass only, not overridden methods of higher levels (see The Java Language Specification at http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.12.4.9).

public class Test3 {
   static class SomeEntity {
       private Integer id;

       private String data;

       public SomeEntity() {
           super();
       }

       public SomeEntity(Integer id, String data) {
           super();
           this.id = id;
           this.data = data;
       }

       public Integer getId() {
           return id;
       }

       public void setId(Integer id) {
           this.id = id;
       }

       public String getData() {
           return data;
       }

       public void setData(String data) {
           this.data = data;
       }

       public class SomeEntityId {
           private SomeEntity link = null;

           public SomeEntityId(SomeEntity link) {
               super();
               this.link = link;
           }

           @Override
           public boolean equals(Object obj) {
  if (this == obj)
   return true;
  if ((obj == null) || (obj.getClass() != this.getClass()))
   return false;
  if (this.link.getClass() != ((SomeEntityId) obj).getLink().getClass())
   return false;

               Integer thisId = link.getId();
               Integer otherId = ((SomeEntityId) obj).getLink().getId();
               if ((thisId == null) || (otherId == null))
                   throw new RuntimeException("Id not defined!");

               return thisId.equals(otherId);
           }

           @Override
           public int hashCode() {
               Integer thisId = link.getId();
               if (thisId == null)
                   throw new RuntimeException("Id not defined!");

               return thisId.hashCode();
           }

           public SomeEntity getLink() {
               return link;
           }
       };

       public SomeEntityId getDTO() {
           return new SomeEntityId(this);
       }
   }

   public static void main(String[] args) {
       SomeEntity e0 = new SomeEntity(null, "no id");
       SomeEntity e1 = new SomeEntity(1, "e1");
       SomeEntity e2 = new SomeEntity(2, "e2");
       SomeEntity e3 = new SomeEntity(3, "e3");
       SomeEntity e33 = new SomeEntity(3, "e33");

       Set aSet = new HashSet();
       // aSet.add(e0.getDTO());
       aSet.add(e1.getDTO());
       aSet.add(e2.getDTO());
       aSet.add(e3.getDTO());
       aSet.add(e33.getDTO());

       System.out.println("set size: " + aSet.size());
       System.out.println("set:");
       for (SomeEntity.SomeEntityId seId : aSet) {
           SomeEntity se = seId.getLink();
           System.out.println("entity: " + se.getData());
       }
   }
}

I even could suggest you to add it to entities code generation in such tools as Hibernate Tools.
I would also like to mention also that SomeEntityId object could be stored in transient field of an entity.
Comments are welcome.

switch

operators with strings, doubles, your custom objects, etc. But anyway sometimes it is possible to get rid of your nested ifs if you will use static maps. Let us consider an example: here we need to set JavaBean properties of certain types:

 protected static Object fillTestBean(Object bean) throws InvocationTargetException, IllegalArgumentException, IllegalAccessException {
  Method[] methods = bean.getClass().getMethods();

  for (Method method : methods) {
   if (RTTIUtils.isSetter(method)) {
    Class type = RTTIUtils.getSetterType(method);
    if (type == String.class)
     RTTIUtils.invokeSetter(bean, method, testStr);
    else if (type == boolean.class)
     RTTIUtils.invokeSetter(bean, method, testBoolean);
    else if (type == byte.class)
     RTTIUtils.invokeSetter(bean, method, testByte);
    else if (type == char.class)
     RTTIUtils.invokeSetter(bean, method, testChar);
    else if (type == short.class)
     RTTIUtils.invokeSetter(bean, method, testShort);
    else if (type == int.class)
     RTTIUtils.invokeSetter(bean, method, testInt);
    else if (type == long.class)
     RTTIUtils.invokeSetter(bean, method, testLong);
    else if (type == float.class)
     RTTIUtils.invokeSetter(bean, method, testFloat);
    else if (type == double.class)
     RTTIUtils.invokeSetter(bean, method, testDouble);
    else if (type == Boolean.class)
     RTTIUtils.invokeSetter(bean, method, testBooleanObj);
    else if (type == Byte.class)
     RTTIUtils.invokeSetter(bean, method, testByteObj);
    else if (type == Character.class)
     RTTIUtils.invokeSetter(bean, method, testCharacterObj);
    else if (type == Short.class)
     RTTIUtils.invokeSetter(bean, method, testShortObj);
    else if (type == Integer.class)
     RTTIUtils.invokeSetter(bean, method, testIntegerObj);
    else if (type == Long.class)
     RTTIUtils.invokeSetter(bean, method, testLongObj);
    else if (type == Float.class)
     RTTIUtils.invokeSetter(bean, method, testFloatObj);
    else if (type == Double.class)
     RTTIUtils.invokeSetter(bean, method, testDoubleObj);
    else if (type == Date.class)
     RTTIUtils.invokeSetter(bean, method, testDate);
   }
  }

  return bean;
 }

(here all variables starting with test are constants)
Seems really annoying to write all this ifs and, also what if we will need to check values? Another function will also need this nested ifs!
In this case I usually do the following (factory method -like solution):

 private static final Map initObjects = new HashMap();
 private static boolean isCosideredType(Class type) {
  return initObjects.containsKey(type);
 }
 static {
  initObjects.put(String.class, testStr);
  initObjects.put(boolean.class, testBoolean);
  initObjects.put(byte.class, testByte);
  initObjects.put(char.class, testChar);
  initObjects.put(short.class, testShort);
  initObjects.put(int.class, testInt);
  initObjects.put(long.class, testLong);
  initObjects.put(float.class, testFloat);
  initObjects.put(double.class, testDouble);
  initObjects.put(Boolean.class, testBooleanObj);
  initObjects.put(Byte.class, testByteObj);
  initObjects.put(Character.class, testCharacterObj);
  initObjects.put(Short.class, testShortObj);
  initObjects.put(Integer.class, testIntegerObj);
  initObjects.put(Long.class, testLongObj);
  initObjects.put(Float.class, testFloatObj);
  initObjects.put(Double.class, testDoubleObj);
  initObjects.put(Date.class, testDate);
 }

 protected static Object fillTestBean(Object bean) throws InvocationTargetException, IllegalArgumentException, IllegalAccessException {
  Method[] methods = bean.getClass().getMethods();

  for (Method method : methods) {
   if (RTTIUtils.isSetter(method)) {
    Class type = RTTIUtils.getSetterType(method);
    if (isCosideredType(type)) {
     RTTIUtils.invokeSetter(bean, method, initObjects.get(type));
    }
   }
  }

  return bean;
 }