Chapter 2 ADTs and Java Classes

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Chapter 2ADTs and Java Classes

• CS 260 Data Structures
• Indiana University Purdue University Fort Wayne

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Abstract Data Type (ADT)

• An Abstract Data Type (ADT) consists of private
  data together with methods that act on that data
• A Java class can be used to represent an ADT
• The class definition for an ADT should have
• private instance variables
• public instance methods and constructor(s)
• Any internal helper method should be private
• The instance variables define the current state
• The instance methods give the behavior

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Example
public class Throttle private int
top // the max throttle position private
int pos // the current throttle
position private Gizmo giz // something
like cruise control public Throttle( int
size) public void shift( int amount
) // end class Throttle
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Reference variables

• Application code
• Throttle t1
• Throttle t2
• t1 new Throttle( 10 )
• t1.shift( 3 )
• t2 new Throttle( 7 )
• t2 t1
• t1 and t2 are aliases
• Aliases are poor practice
• t2.shift( 6 )
• Now t1.pos is 9
• t2 null
• Eliminates the alias
• t2.shift( 3 )
• causes NullPointerException

t1
t2
top pos giz
top pos giz
one instance
another instance
Each instance has a separate copy of each
instance variable
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Clones and the test for equality

• A clone is an exact copy of an instance
• t2 t1 does not create a clone
• This just gives an alias
• A special clone( ) method must be implemented
• Test for equality
• t2 t1 is true when t1 and t2 refer to the same
  instance
• What is wanted is a test for whether different
  instances contain exactly the same data
• A special equals( ) method must be implemented

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Inheritance

• Recall that classes form a hierarchy
• Class Object is the ancestor of all other classes
• A class inherits all the state and behavior from
  its parent class
• All instance variables
• All methods
• A class can add new instance variables to the
  instance variables it inherits
• A class may add new methods
• A class may redefine any method

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Inheritance
Object

Throttle
Vector
String

Stack
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A clone( ) method for class Throttle
public class Throttle implements Cloneable
public Object clone( ) Throttle
ans try ans ( Throttle )
super.clone( ) // clone( ) inherited from
class Object ans.giz ( Gizmo )
giz.clone( ) // clone( ) defined in class
Gizmo catch ( CloneNotSupportedException
) throw new RuntimeException( message )
return ans // end clone
// end class Throttle
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Clone( ) method

• t2 ( Throttle ) t1.clone( )

t1
ans
First step ans ( Throttle ) super.clone( )
top pos giz
Second step ans.giz ( Gizmo ) giz.clone( )
top pos giz
clone of t1
aGizmo
clone of aGizmo
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Using the clone( ) method

• To make a clone of Throttle t1 . . .
• Casting to class Throttle lets the compiler know
  that the result is a specific Object of class
  Throttle

Throttle t2 t2 ( Throttle )
t1.clone( )
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A equals( ) method for class Throttle

• The equals method should work even when you
  compare a Throttle with some other kind of object
• In that case it should return false

public boolean equals( Object obj )
Throttle t if ( obj instanceof Throttle
) t ( Throttle ) obj return (
t.top top ) ( t.pos pos )
t.giz.equals( giz )
else return false // end equals
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Clone( ) and equals( ) methods

• Note that there are special requirements when
  instance variables contain objects as well as
  primitive values in instance variables
• Method clone( ) requires that class Gizmo
  implements its own clone( ) method
• Method equals( ) requires that class Gizmo
  implements its own equals( ) method
• If this is not so, then the needed methods must
  be added to the Gizmo class

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Arrays

• All array objects have clone( ) and equals( )
  methods

int a new int 100 int b b
( int ) a.clone( ) if ( a.equals( b ) )

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Static methods

• Static methods are not applied to objects as in .
  . .
• t.shift( 3 )
• Instead, static methods are called as in . . .
• Throttle.staticMethod( t2 )
• Integer.toString( n )
• There are no instance variables (like top, pos,
  giz) directly available to a static method
• When there is a Throttle parameter t2, then
  instance variables are available in the form
  t2.top, t2.pos, and t2.giz
• Static methods are not appropriate for OOP
• Static method are needed in Java since . . .
• Java is not a pure object-oriented language
• Primitive variables are not objects