Abstract and Nested Classes Abstract classes: Incomplete abstract classes. Complete abstract classes. Comparison with interfaces. Nested Classes: Static nested classes. Inner classes. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Abstract Classes Class has abstract prefix. Instances may not be constructed. Incomplete methods: just a header. Method has abstract prefix. Implicit in interface definitions. Objects have no meaningful existence. Class elements used by defining sub classes. OOP with Java, David J. Barnes Abstract and Nested Classes

The Abstract Number Class public abstract class Number implements Serializable { public abstract int intValue(); public abstract long longValue(); public abstract double doubleValue(); public abstract float floatValue(); public byte byteValue(){ ... } public short shortValue(){ OOP with Java, David J. Barnes Abstract and Nested Classes

Incomplete Abstract Classes Define a common interface for multiple sub classes. Sub class implementations vary. Implementation of floatValue, in Number sub classes, depends on type of attribute. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes The Double Sub Class public class Double extends Number { public Double(double value){ this.value = value; } ... // This probably involves loss of information. public float floatValue(){ return (float) doubleValue(); // The value being wrapped. private final double value; OOP with Java, David J. Barnes Abstract and Nested Classes

Complete Abstract Classes No abstract methods. Independent instantiations have no meaning. ChessPiece super class of Rook, Knight, etc. Sub classes provide additional or overriding functionality. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract Class or Interface? Interface methods are abstract. Interfaces do not have constructors. Abstract classes avoid the need to define duplicate method bodies. Multiple inheritance. Extending an abstract class prevents extension of a second class. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Object Cloning All classes inherit a clone method. Returns an identical copy. A shallow copy, by default. A deep copy is often preferable. The (empty) Cloneable interface must be implemented. CloneNotSupportedException OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes A Cloneable Point class Point implements Cloneable { ... public Object clone() { try{ // A shallow copy. return super.clone(); } catch(CloneNotSupportedException e){ // This should never happen! throw new Error("Cloning error: "+e); OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Preventing Cloning Prevention might be necessary. A unique attribute - database lock or open file reference. Not sufficient to omit Cloneable. Sub classes might implement it. clone should throw an exception: CloneNotSupportedException. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes An Unclonable Class class NotCloneable { ... // Explicitly prevent sub class cloning. public Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException("..."); } OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Static Nested Classes (aka Nested Top-Level Classes!?). Used to show strong coupling between two classes. VariableController and HeaterLevelException. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes class VariableController extends HeaterController { // Class constants for basic temperatures. public static final int DefaultLevel = 16, MinLevel = 5, MaxLevel = 30; // An illegal temperature level has been set. public static class HeaterLevelException extends RuntimeException { public HeaterLevelException(String message){ super(message); } // The attached heater's current level. public int getLevel(){ ... OOP with Java, David J. Barnes Abstract and Nested Classes

Static Nested Class Names Nested class has a qualified name. VariableController.HeaterLevelException Separate .class file created. VariableController$HeaterLevelException.class Qualified name used outside the enclosing class. catch(VariableController.HeaterLevelException e) OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Inner Classes Non-static nested classes. Instances only exist within the context of an enclosing object. May not have static members. May access private members of the enclosing class. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Inner Class Roles Used to fulfill distinct roles in support of an enclosing object. Subordinate inner class. Interpretational inner class. Local inner class. Anonymous inner class. Very common in supporting GUIs. OOP with Java, David J. Barnes Abstract and Nested Classes

Subordinate Inner Classes Used to break up a single large class. Implement subordinate sub-tasks. E.g., logging a ship's movements. Keeps enclosing class less cluttered. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes public class LoggingShip extends SimpleShip { ... public void move(){ super.move(); getLogger().log(); } protected class ShipLog { protected void log(){ Position p = new Position(getX(),getY()); getLog().add(p); protected LinkedList getLog(){ return positionLog; private LinkedList positionLog = new LinkedList(); private final ShipLog logger = new ShipLog(); OOP with Java, David J. Barnes Abstract and Nested Classes

Interpretational Inner Classes Interpreting an object’s data to outside objects. Independently of the internal representation. E.g. Iterator and Enumeration implementations. DataValues - a collection of numbers. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes public class DataValues { public void add(double d){ getNumbers().add(new Double(d)); } public double get(int index) throws IndexOutOfBoundsException { Double d = (Double) getNumbers().get(index); return d.doubleValue(); public int numItems(){ return getNumbers().size(); ... protected LinkedList getNumbers(){ return numbers; private final LinkedList numbers = new LinkedList(); OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes public class DataValues { public Iterator getPositives(){ return new PositiveSelector(); } protected class PositiveSelector implements Iterator { public boolean hasNext(){ return nextPosition() >= 0; public Object next() throws NoSuchElementException { int i = nextPosition(); if(i >= 0){ Object o = getNumbers().get(i); setIndex(i+1); return o; else{ throw new NoSuchElementException(); ... OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Local Inner Classes A class defined within a method. Permitted within any block. Instances may only be created from within that method. No visibility modifier is used. Instances may access the method’s final variables and arguments. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes public class DataValues { public Iterator selectAbove(final double limit){ class AboveSelector implements Iterator { ... protected int nextPosition(){ int i = getIndex(); final int howMany = numItems(); while(i < howMany){ if(get(i) > limit){ return i; } i++; return -1; return new AboveSelector(); OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Anonymous Classes Local classes often do not require a name. When they implement an interface or extend a super class. When definition and creation are closely linked. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes public class DataValues { ... public Iterator selectAbove(final double limit){ return new Iterator() { public boolean hasNext(){ } public Object next(){ }; OOP with Java, David J. Barnes Abstract and Nested Classes

Anonymous Classes in GUIs. import javax.swing.*; import java.awt.*; import java.awt.event.*; public class Picture extends JFrame { Container contents = getContentPane(); // Add a button to quit the application. JButton quit = new JButton("Quit"); quit.addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent e){ System.exit(0); } }); contents.add(quit,"South"); ... OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Inner Classes and this class Outer { // 'this' accessible from here. ... class Middle { // 'this' and 'Outer.this' accessible from here. class Innermost { // 'this', 'Middle.this' and 'Outer.this' // accessible from here. } private Innermost i = new Innermost(); private Middle m = new Middle(); OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Review Abstract classes are incomplete and must be sub classed to be useful. Abstract classes offer an alternative to interfaces where partial implementation is useful. Classes implement cloning by implementing the Cloneable interface. OOP with Java, David J. Barnes Abstract and Nested Classes

Abstract and Nested Classes Review (cont.) Nested classes may be static nested classes or inner classes. Inner class instances always exist within the context of an enclosing object. Inner classes have access to the private members of their enclosing class. Inner classes are useful for fulfilling a range of roles within their enclosing class. OOP with Java, David J. Barnes Abstract and Nested Classes