CS110 Lecture 14
Tuesday, March 23, 2004
• Announcements
– hw6 due Thursday
– pass/fail, withdraw deadline April 8
• Agenda
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Questions
Java 1.5
Inheritance (Chapter 5)
Shapes
JFile, TextFile and Directory
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Java version 1.5
• Beta release available from Sun
– download now for your PC if you wish
– no midsemester change to lab computers
• Improved collection management
– Casts no longer required
– Automatic wrapping of primitive types
– New for loop syntax replaces Iterator idiom
• See java 1.5 link on course home page
• TreeMap15Demo.java available there
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Backward compatibility
• Important software design issue
• Java 1.5 has no new keywords
• To compile java 1.4 source code with 1.5
– javac Whatever.java
• To compile java 1.5 features
– javac –source 1.5 Whatever.java
– java 1.4 code still compiles (but with warnings)
• What should JOI do?
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TreeMap (1.5) summary
• declaration:
TreeMap<KeyType, ValueType> mapName;
• creation:
new TreeMap<KeyType, ValueType> ( );
• put:
mapName.put(KeyType key, ValueType val)
• get:
mapName.get( KeyType key)
no cast to ValueType needed
• length: mapName.size( )
read as “in”
• looping: for( KeyType key : mapName.keySet( ))
{ // do whatever with key }
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New for syntax works with arrays too
• zamples.com/JspExplorer/samples/
samplesJDK1_5.html
int array[] = {1, 2, 3, 4};
int sum = 0;
for (int e : array)
sum += e;
System.out.println(sum);
• But note how this code violates our conventions
– we require int[] array
– braces should surround one line for loop body
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Design problem
• Bigbank and Littlebank are similar but different
• Both have fields
– name, balance, transaction count (same meaning)
– accountList (different meanings)
• Both have methods
– visit, processTransaction, getBalance, … (same code)
– whichAccount, report (different code)
• Can we write big and little Bank classes
without copying code?
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Design problem
• Directory should store TextFiles and Directories
• Directory and TextFile both have
– owner, create/mod date (same meaning)
– size, contents (different meanings)
• Directory has methods to add to, get from and
loop on its contents (the TreeMap of files in it)
• TextFile has methods to manipulate its text
• Can we write these classes without copying code?
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Design problem
• HLine and VLine each have length, paintChar,
with setters and getters
• Each kind of line has a paintOn method to paint
itself on a Screen in its own particular way
• When you wrote VLine you started with a copy of
HLine, and changed a small fraction of the code
• Is there a better way?
• Yes
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Why do we need a better way?
• Copy code and change a little bit
 design problem
– can’t see similarities and differences
– bugs must be fixed in two places
• Inheritance solves this problem
• Inheritance is the second Big Thing in OOP
(sending messages is the first Big Thing)
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Inheritance
class Line
abstract class Line: fields and
methods for lines (length,
getLength, setLength)
class HLine:
paintOn(Screen,int,int)
class VLine:
paintOn(Screen,int,int)
parent class,
superclass
class HLine extends Line
child classes,
subclasses
class VLine extends Line
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How inheritance works
(Hline.java main)
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Line hline = new HLine( 10, ‘x’);
• Variable hline declared of type Line can have a value
that’s a reference to an instance of some child of Line
• Line foo = new Line() not legal (since Line is abstract)
70 hline.setPaintChar(‘*’);
• Send setPaintChar message to Line hline,
which happens to be an HLine
– Java looks for setPaintChar method in class Hline
– Not there! So looks in parent class Line
– Found it! So search stops
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How inheritance works
65 hline.paintOn(screen);
• Send paintOn message to Line hline,
which happens to be an HLine
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No paintOn(Screen) in class HLine
Java finds paintOn(Screen) in parent class Line
paintOn(Screen) delegates to paintOn(Screen, int, int)
paintOn(Screen, int, int) is abstract in class Line
Java finds implementation in class HLine
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abstract
• New Java keyword
• abstract class can’t be instantiated
(only its children can)
• abstract method is a signature with no body
(just a promise)
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Constructors in a subclass
64
Line hline = new HLine( 10, ‘x’);
invokes HLine constructor
20 public HLine( int length, char paintChar )
21 {
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super( length, paintChar );
23 }
line 22: invokes parent class (Line) constructor
(java keyword super is “my parent”)
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Shapes in hw7
class Object
abstract class Shape: fields and methods needed by all child
classes (paintChar, getter and setter, paintOn(Screen))
abstract class Line: fields and
methods for lines (length,
getLength, setLength)
abstract class Rectangle fields and methods for
rectangles (length, width)
class HLine:
paintOn(Screen,int,int)
class Box
class VLine:
paintOn(Screen,int,int)
class Frame
class Shape // extends Object
class XLine extends Shape
class HLine extends XLine
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Trees
• Common in computer science:
– Java class hierarchy (shows inheritance)
– Windows tree for files and directories (folders)
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Vocabulary: Tree, hierarchy
Root (often drawn at the top!)
Child, parent, branch, leaf, node
Draw with arrows,
or in outline (indented) form
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