抽象类型

//By default, all methods in all classes are concrete, unless the abstract keyword is used. abstract class Demo {     // An abstract class may include abstract methods, which have no implementation.     abstract public int sum(int x, int y);      // An abstract class may also include concrete methods.     public int product(int x, int y) { return x*y; } }  //By default, all methods in all interfaces are abstract, unless the default keyword is used. interface DemoInterface {     [abstract] int getLength(); //Abstract can be used here, though is completely useless          //The default keyword can be used in this context to specify a concrete method in an interface     default int product(int x, int y) {         return x * y;     } }