Overview of Interface in Java

1. Java Interface(สรุป) from K.P. Chow University of Hong Kong (some slides from S.M. Yiu)

2. Data (including implementation) + Operations implementation Still remember? Users A set of interface Interface Interface A collection of method definitions (without implementation) Defines a set of methods without implementing them

3. Interface Declaration IsEmpty( ) Set Exists(x) Data:{a, b, c, …..} Operation Implementation Details Methods Declaration Users Interface body Add(x) Constant Declaration Delete(x) Note: there is a Set interface in Java public interface Set_interface { boolean IsEmpty(); boolean Exists(Object x); void Add(Object, x); void Delete(Object, x); int nonsense = 0; } (public, abstract) (public, static, final)

4. Implement all methods declared in the interface (& its superinterfaces) Can implement > 1 interfaces Implement all these methods A class “implements” an interface Inherits the constants public class example implements Set_interface { …… boolean IsEmpty() {…….}; boolean Exists(Object x){…….}; void Add(Object, x){……..}; void Delete(Object, x){……..}; }

5. Interface vs Abstract Class A subclass can only inherit from ONE superclass One class can implement >1 interfaces Conceptually, interface wants the classes to guarantee a set of behavior (services) while inheritance implies an “is-a” relationship

6. Usages of Interfaces • To simulate multiple inheritance without the problem of name clash from different parents • Specifies the exact signature of the methods that must be provided by the implementation, e.g. public interface DataInput { int skipBytes(int n); void readFully(byte b[]); byte readByte(byte b[], int off, int len);} • Use the interface type as a parameter for a method: inside the method, you can invoke any of the methods promised by the interface parameter, e.g. void howBigIsIt(Comparable c) { … Object somethongElse = … if ( c.compareTo(somethingElse) ) … }

7. Why Multiple Inheritance? Java Applet Java Thread Java Socket Your Applet:a multithread applet talking to the server

8. Some examples public interface URLdir { IP url_to_ip (Url x); Url ip_to_url (IP y); } public interface TelDir { TelNo name_to_tel (String name); String tel_to_name (TelNo x); } public class Agent extends Some_class implements TelDir, URLdir { TelNo name_to_tel (String name) {…..}; String tel_no_name (TelNo x) {……}; IP url_to_ip (Url x) {…..}; Url ip_to_url (IP y) {….}; ……. }

9. I want to write a general sorting method! public interface Comparable { public int compareTo(Object o); } Compare this object with o. Return –1, 0, +1 if this object is <, =, > o respectively // not general // public void my_sorting (int [] temp){….}; // too general // public void my_sorting (Object [] temp){….};

10. …… Type_A x[] = new Type_A[5]; Type_B y[] = new Type_B[5]; ……. my_sorting(x); my_sorting(y); public void my_sorting (Comparable [] temp){….}; // OK // public class Type_A implements Comparable{ ……. }; Note: defining an interface is defining a new reference data type. So, interface can be used anywhere that a data type can be used. public class Type_B implements Comparable{ ……. };

11. my_sorting public void my_sorting(Comparable[] temp) { int i, j; Object t; for ( i=0; i<temp.length-1; i++ ) for ( j=i+1; j<temp.length; j++ ) { if ( temp[i].compareTo(temp[j]) > 0 ) { t = temp[i]; temp[i] = temp[j]; temp[j] = t; } } } Interface Comparable is implemented by classes Integer, Float, Long, ….