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Java Forum / General / March 20074 Questions on Java interface class
Hello Please help Q 1) Is it possible for an interface class to have no method? Example public interface One{ } Q 2) Is it possible for an interface class to declare variables only no method? Example public interface Two { Final int books = 50; } Q 3) What is the main difference between interface and abstract class? Q 4) Is it possible to have only abstract class without interface class? Thank you -sem > Hello Please help > [quoted text clipped - 4 lines] > Q 4) Is it possible to have only abstract class without interface > class? This sounds remarkably like a homework assignment. Here's some documentation you might want to consult: http://java.sun.com/docs/books/jls/third_edition/html/interfaces.html http://java.sun.com/docs/books/jls/third_edition/html/classes.html#8.1.1.1 > > Hello Please help > [quoted text clipped - 10 lines] > > http://java.sun.com/docs/books/jls/third_edition/html/interfaces.htmlhttp://java .sun.com/docs/books/jls/third_edition/html/classes.html#8.... Thank you. But it is not a homework question. I did my own thought and want to know others opinion. Here are myown respond are they right?? > >Q 1) Is it possible for an interface class to have no method? my answer is NO > > Q 2) Is it possible for an interface class to declare variables only no method? my answer is NO > > Q 3) What is the main difference between interface and abstract class? If you extends more than one interface and did not implement some methtod then for these classes you have to name them abstract > > Q 4) Is it possible to have only abstract class without interface Based on the above definition my answer is NO Am I right? or wrong? -sem > > class? > But it is not a homework question. I did my own thought and want to > know others opinion. > Here are myown respond are they right?? >>> Q 1) Is it possible for an interface class to have no method? > my answer is NO The correct answer is yes. >>> Q 2) Is it possible for an interface class to declare variables only no method? > my answer is NO Interfaces may not have instance variables at all. >>> Q 3) What is the main difference between interface and abstract class? > If you extends more than one interface and did not > implement some methtod then for > these classes you have to name them abstract Not correct. If you /implement/ an interface and don't want to implement all its methods, the class must be abstract. But that doesn't explain the difference between interfaces and abstract classes. The difference between an abstract class and an interface is the difference between any class and an interface. An interface is a contract; it defines a type to which implementing classes must conform, but has no implementation. Method bodies are forbidden. Instance variables are forbidden. An abstract class, like any class, is an implementation, possibly of an interface's contract. It may, and usually does, contain method bodies. >>> Q 4) Is it possible to have only abstract class without interface > Based on the above definition my answer is NO The correct answer is yes. -- Lew > But it is not a homework question. I did my own thought and want to > know others opinion. > Here are myown respond are they right?? >>> Q 1) Is it possible for an interface class to have no method? > my answer is NO The correct answer is yes. >>> Q 2) Is it possible for an interface class to declare variables only no method? > my answer is NO The correct answer is yes. >>> Q 3) What is the main difference between interface and abstract class? > If you extends more than one interface and did not > implement some methtod then for > these classes you have to name them abstract Not correct. If you /implement/ an interface and don't want to implement all its methods, the class must be abstract. But that doesn't explain the difference between interfaces and abstract classes. The difference between an abstract class and an interface is the difference between any class and an interface. An interface is a contract; it defines a type to which implementing classes must conform, but has no implementation. Method bodies are forbidden. Instance variables are forbidden. An abstract class, like any class, is an implementation, possibly of an interface's contract. It may, and usually does, contain method bodies. >>> Q 4) Is it possible to have only abstract class without interface > Based on the above definition my answer is NO The correct answer is yes. -- Lew Sem wrote: > Hello Please help > Q 2) Is it possible for an interface class to declare variables only > no method? [quoted text clipped - 4 lines] > Final int books = 50; > } "Final" is not a keyword and will cause you grief used in a declaration that way, even in places where such a declaration would otherwise be permissible. > Q 4) Is it possible to have only abstract class without interface > class? A true devoté if Inversion of Control ("IOC", see the Swing project) might think of such a class as ronin. -- Lew > Hello Please help > [quoted text clipped - 4 lines] > public interface One{ > } Yes. > Q 2) Is it possible for an interface class to declare variables only > no method? [quoted text clipped - 4 lines] > Final int books = 50; > } No instance variables allowed in an interface. "Final" is not a keyword and will cause you grief used that way. > Q 3) What is the main difference between interface and abstract class? An interface is a contract: it has no implementation code. Method bodies are forbidden. An abstract class, like any kind of class, is an implementation: it can (and usually does) have implementation code (method bodies). > Q 4) Is it possible to have only abstract class without interface > class? Yes. A true devoté if Inversion of Control (IOC, see the Swing project) might think of such a class as ronin. -- Lew Hi, Sem > Hello Please help > > Q 1) Is it possible for an interface class to have no method? Yes, you can declare a interface without any method. > Example > [quoted text clipped - 11 lines] > > } Indeed, It works. > Q 3) What is the main difference between interface and abstract class? All Java books will tell you the difference, some key points: 1. Apparently, a class can implement several interfaces, while it is not permitted to derive several abstract classes. 2. Usually, interface stands for some kind of attributes, such as "Serializable", "Accessible". So we could say the object has such attribute or ability But abstract class indicates the essential property of the class, such as "Vehicle", "Computer". So we usually say the object belongs to .......... 3. Abstract classes can have their concrete methods, but the methods declared in a interface are all abstract. > Q 4) Is it possible to have only abstract class without interface > class? I don't known what exactly the question is. > Thank you > > -sem > Hi, Sem > On Mar 14, 12:35 pm, "Sem" <semj2e...@gmail.com> wrote:> Hello Please help > [quoted text clipped - 43 lines] > > > -sem For interfaces, Variable values declared will be "final" by default. private/protected parameters are not allowed. But (public, package scope) static final variables can be defined. public interface Two { final int books = 50; } == public interface Two { int books = 50; } Q 1) Is it possible for an interface class to have no method? The correct answer is yes: please refer to java.io.Serializable, Cloneable interface which are marker interfaces Q 2) Is it possible for an interface class to declare variables only no method? Yes: Sometimes, application wide configuration information can be stored in interfaces and just implement the interface wherever the information is needed. Q 3) What is the main difference between interface and abstract class? Interfaces, as defined by everyone, is a contract that implementing classes need to adhere to. As for difference between Abstract Class/Interface, Abstract Class forms part of Object Hierarchy (Super Class-sub class), Abstract class attributes can be instance specific. Please refer to java.util.AbstractCollection, java.util.AbstractList... for examples. Q 4) Is it possible to have only abstract class without interface Yes. You don't want to mix abstract class concepts with interfaces. Interfaces are contract to implementing that they adhere to the method signature, scoping, and exception handling. > Yes: Sometimes, application wide configuration information can be > stored in interfaces and just implement the interface wherever the > information is needed. Joshua Bloch and others have gone into some detail excoriating the practice of putting application constants in an interface. The better practice is to use a class. An interface should define a type. The use as a container for variables is not consistent with that design philosophy. The addition of static imports to the language was motivated in part by the desire to stop programmers from using interfaces as constants bags. -- Lew > Hello Please help > [quoted text clipped - 4 lines] > public interface One{ > } I know this is a really wild idea but... Did you consider a) trying to run such code through a java compiler? b) reading the Java API docs/language specification? lex Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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