Java Coding OOP_3 David Davenport Computer Eng. Dept., Bilkent University Ankara - Turkey. Some important Java interfaces + Inner classes

IMPORTANT… Students… This presentation is designed to be used in class as part of a guided discovery sequence. It is not self- explanatory! Please use it only for revision purposes after having taken the class. Simply flicking through the slides will teach you nothing. You must be actively thinking, doing and questioning to learn! Instructors… You are free to use this presentation in your classes and to make any modifications to it that you wish. All I ask is an saying where and when it is/was used. I would also appreciate any suggestions you may have for improving it. thank you, David. David

SOME JAVA INTERFACES…

Java Interfaces Design by interface gives flexibility The Java API has a lot of interfaces Iterator Comparable Serializable & a multitude for GUI event-handling!

Java’s Iterator Interface boolean hasNext() Object next() void remove() // Scanner class implements Iterator tokens = new Scanner( “To be or not to be”); while ( tokens.hasNext() ) System.out.println( tokens.next() ); To iterate (to go) through a collection, processing each element once and once only

Java’s Iterator Interface boolean hasNext() Object next() void remove() // given an ArrayList, list Iterator x = list.iterator(); while ( x.hasNext() ) System.out.println( x.next() ); Surprisingly, ArrayList does not implement Iterator. It implements Iterable! Iterator iterator()

The Enumeration Interface boolean hasNextElement() E nextElement() // StringTokenizer tokens = new StringTokenizer( “To be or not to be”); while ( tokens.hasMoreElements() ) System.out.println( tokens.nextElement() ); Similar to, but older than Iterator. Has different names & does not have remove method. Examples include StringTokenizer & Vector (which also has Iterator!)

Java’s Comparable Interface int compareTo( Object o) compare this object with o and return negative, zero, positive to indicate respectively! // Assuming x implements Comparable // e.g. String if ( x.compareTo( y) > 0 ) exchange( x, y);

Java’s Serialization Interface Allows Java objects to be converted to/from a stream of bytes! Rather special since you do not need to write any methods, merely Make class implement Serializable Have a default constructor Ensure all properties are Serializable Check the Java API documentation for details and example code.

Interface notes Can’t create objects of interface type, only a type that implements it Interfaces can extend interfaces (but not classes) to form a hierarchy separate from class one Start design with interfaces! Java provides “instanceof” operator to test object type (but use very sparingly) Cloneable is another common interface.

INNER CLASSES…

Inner Classes Nested or Inner classes are defined inside other classes have direct access to outer class can be named or anonymous Named Inner classes Simply define one class inside another! (generates outer$inner.class files)

Inner Classes (cont.) Anonymous Inner classes Create in-line (on the fly!) Useful when only a single object needed & class not otherwise reusable. variation of new statement… className x = new superClass_or_interface() { // the prop’s & methods }