Within a source file or class we find the typical layout of file MyClass.java : [/* * header comment, for CVS, IP issues, etc. */] [package foo;]

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1 Producing Production Quality Software Lecture 9: System Design in Java Prof. Arthur P. Goldberg Fall, 2005 Small scale Within a source file or class we find the typical layout of file MyClass.java : [/* * header comment, for CVS, IP issues, etc. ] [package foo;] [import blah.blech.one;] // grouped by package [import blah.blech.two;] // be specific where appropriate [import blah.blech.three;] [import blah.yuck.*;] * javadoc for MyClass * include definitions class MyClass extends SuperClass implements A, [B, C ] /* * Implementation Notes // Static Constants (AKA class variables) javadoc e.g., resize all columns. public static final int AUTO_RESIZE_ALL_COLUMNS = 4; // instance variables The <code>size</code> of the table. protected int datasize; // private state private boolean noneofyourbusiness; // Constructors // static Methods // public methods // protected Methods Similar organization applies to abstract classes, and interfaces.

2 Composition and forwarding Since Java only supports single inheritance, what should you do when you want to use multiple types in a class? Extend one and implement the others. Forward the interfaces implemented to actual implementations. Example from Arnold, Gosling, Holmes, pp 116-7: Body.java: * an astronomical body class Body private static long nextid = 0; public long idnum; public String name ="<unnamed>"; public Body orbits = null; Body() idnum = nextid++; Body ( String bodyname, Body orbitsaround ) this(); name = bodyname; orbits = orbitsaround; Attr.java: * An attribute class that stores name-value pairs. * Names are strings; any type of object may be a value. * TODO add error checking, documentation. Arthur Goldberg * Oct 15, 2003 class Attr private final String name; private Object value = null; public Attr( String name ) this.name = name; public Attr( String name, Object value ) this.name = name; this.value = value; public String getname() return name; public Object getvalue() return value; public Object setvalue( Object newvalue ) Object oldval = value;

3 value = newvalue; return oldval; public String tostring() return name + "='" + value + "'"; Attributed.java: public interface Attributed void add( Attr newattr ); Attr find( String attrname ); Attr remove( String attrname ); java.util.iterator attrs(); AttributedImpl.java: import java.util.hashmap; import java.util.iterator; class AttributedImpl implements Attributed protected HashMap attrtable = new HashMap(); public void add( Attr newattr ) attrtable.put( newattr.getname(), newattr ); public Attr find( String name ) return (Attr) attrtable.get( name ); public Attr remove ( String name ) return (Attr) attrtable.remove( name ); public Iterator attrs( ) return attrtable.values().iterator( ); AttributedBody.java: import java.util.iterator; * AttributedBody uses 'composition and forwarding' to implement the * interface Attributed. Arthur Goldberg * Oct 15, 2003 class AttributedBody extends Body implements Attributed private AttributedImpl attrimpl = new AttributedImpl (); public AttributedBody() super(); public AttributedBody( String name, Body orbits) super(name, orbits);

4 // Forward all Attributed methods to the attrimpl object public void add( Attr netattr ) attrimpl.add( netattr ); public Attr find( String name ) return attrimpl.find( name ); public Attr remove ( String name ) return attrimpl.remove( name ); public Iterator attrs( ) return attrimpl.attrs( );

5 Accessibility and Overriding Methods Only accessible methods can be overridden. The determination is made at run-time. For example: package P1; public abstract class AbstractBase private void pri() System.out.println( "AbstractBase.pri()" ); void pac() System.out.println( "AbstractBase.pac()" ); protected void pro() System.out.println( "AbstractBase.pro()" ); public void pub() System.out.println( "AbstractBase.pub()" ); public final void show() pri(); pac(); pro(); pub(); Now lets make some subclasses. First, in another package: package P2; import P1.AbstractBase; public class Concrete1 extends AbstractBase public void pri() System.out.println( "Concrete1.pri()" ); public void pac() System.out.println( "Concrete1.pac()" ); public void pro() System.out.println( "Concrete1.pro()" ); public void pub() System.out.println( "Concrete1.pub()" ); // what does "new Concrete1().show();" output? // why? Next, in the same package: package P1; import P2.Concrete1; public class Concrete2 extends Concrete1 public void pri() System.out.println( "Concrete2.pri()" ); public void pac() System.out.println( "Concrete2.pac()" ); public void pro() System.out.println( "Concrete2.pro()" ); public void pub() System.out.println( "Concrete2.pub()" ); // An inheritance hierarchy that weaves in and out of a package should usually // be avoided, because it confuses people. We're doing it just for illustration. // What does "new Concrete2().show();" output? // why? Finally, extend Concrete2 in a different package: package P3; import P1.Concrete2; public class Concrete3 extends Concrete2

6 public void pri() System.out.println( "Concrete3.pri()" ); public void pac() System.out.println( "Concrete3.pac()" ); public void pro() System.out.println( "Concrete3.pro()" ); public void pub() System.out.println( "Concrete3.pub()" ); // what does "new Concrete3().show();" output? // why?