C has abstract methods if any of the following is true: C explicitly contains a declaration of an abstract method. Any of C's superclasses has an abstract method and C neither declares nor inherits a method that implements it. A direct superinterface of C declares or inherits a method (which is therefore necessarily abstract) and C neither declares nor inherits a method that implements it. A subclass of an abstract class that is not itself abstract may be instantiated, resulting in the execution of a constructor for the abstract class and, therefore, the execution of the field initializers for instance variables of that class. package org.dwwwp.test; /** * @author jcrypto */ public class AbstractClass { private static final String hello; static { System.out.println(AbstractClass.class.getName() + ": static block runtime"); hello = "hello from " + AbstractClass.class.getName(); } { System.out.println(AbstractClass.class.getName() + ": instance block runtime"); } public AbstractClass() { System.out.println(AbstractClass.class.getName() + ": constructor runtime"); } public static void hello() { System.out.println(hello); } } package org.dwwwp.test; /** * @author jcrypto */ public class CustomClass extends AbstractClass { static { System.out.println(CustomClass.class.getName() + ": static block runtime"); } { System.out.println(CustomClass.class.getName() + ": instance block runtime"); } public CustomClass() { System.out.println(CustomClass.class.getName() + ": constructor runtime"); } public static void main(String[] args) { CustomClass nc = new CustomClass(); hello(); //AbstractClass.hello();//also valid } } Output: org.dwwwp.test.AbstractClass: static block runtime org.dwwwp.test.CustomClass: static block runtime org.dwwwp.test.AbstractClass: instance block runtime org.dwwwp.test.AbstractClass: constructor runtime org.dwwwp.test.CustomClass: instance block runtime org.dwwwp.test.CustomClass: constructor runtime hello from org.dwwwp.test.AbstractClass Enumerations This language feature was introduced in J2SE 5.0. Technically enumerations are a kind of class containing enum constants in its body. Each enum constant defines an instance of the enum type. Enumeration classes cannot be instantiated anywhere except in the enumeration class itself. enum Season { WINTER, SPRING, SUMMER, AUTUMN } Enum constants are allowed to have constructors, which are called when the class is loaded: public enum Season { WINTER("Cold"), SPRING("Warmer"), SUMMER("Hot"), AUTUMN("Cooler"); Season(String description) { this.description = description; } private final String description; public String getDescription() { return description; } } Enumerations can have class bodies, in which case they are treated like anonymous classes extending the enum class: public enum Season { WINTER { String getDescription() {return "cold";} }, SPRING { String getDescription() {return "warmer";} }, SUMMER { String getDescription() {return "hot";} }, FALL { String getDescription() {return "cooler";} }; } Interfaces Interfaces are types which contain no fields and usually define a number