CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2002 Strategy
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Doc 8, Strategy Slide # 2

Strategy

Intent

Doc 8, Strategy Slide # 3

import java.awt.*;
class  FlowExample  extends Frame  {
   
   public FlowExample( int  width, int height ) {
      setTitle( "Flow Example" );
      setSize( width, height );
      setLayout( new FlowLayout( FlowLayout.LEFT) );
      
      for ( int label = 1; label < 10; label++ )
         add( new Button( String.valueOf( label ) ) );
      show();
   }
   
   public  static  void  main( String  args[] ) {
      new  FlowExample( 175, 100 );
      new  FlowExample( 175, 100 );
   }
}
Doc 8, Strategy Slide # 4

• 20 different Layout classes
• At least 39 subclasses of Component using layouts

Doc 8, Strategy Slide # 5

import java.util. Comparator;
import java.util.*;
   
class Student  {
   String name;
   
   public Student( String newName) { name = newName;}
   
   public String toString() { return name; }
   }
   
final class StudentNameComparator implements Comparator {
   
   public int compare( Object leftOp, Object rightOp ) {
      String leftName = ((Student) leftOp).name;
      String rightName = ((Student) rightOp).name;
      return leftName.compareTo( rightName );
   }
   
   public boolean equals( Object comparator ) {
      return comparator instanceof StudentNameComparator;
   }
}
   
public class Test  {
   public static void main(String args[])  {
      Student[] cs596 = { new Student( "Li" ), new Student( "Swen" ), 
          new Student( "Chan" ) };
      //Sort the array
      Arrays.sort( cs596, new StudentNameComparator() );
   }
}
Doc 8, Strategy Slide # 6

| list |
list := #( 1 6 2 3 9 5 ) asSortedCollection.
Transcript 
   print: list;
   cr.
list sortBlock: [:x :y | x > y].
Transcript 
   print: list;
   cr;
   flush.
Doc 8, Strategy Slide # 7

• Subclasses of SortedCollection or
• Comparator classes (blocks)

• Combine different comparators
• Sort the same list with different comparators

Doc 8, Strategy Slide # 8

Applicability

• You need different variants of an algorithm

• An algorithm uses data that clients shouldn't know about

• A class defines many behaviors, and these appear as multiple switch statement in the classes operations

• Many related classes differ only in their behavior

Doc 8, Strategy Slide # 9

Consequences

• Families of related algorithms

• Alternative to subclassing of Context

What is the big deal? You still subclass Strategy!

• Eliminates conditional statements

Replace in Context code like:

      switch  ( flag ) {
         case A: doA(); break;
         case B: doB(); break;
         case C: doC(); break;
      }

With code like:

      strategy.do();

• Gives a choice of implementations

• Clients must be aware of different Strategies

   SortedList studentRecords = new SortedList(new ShellSort());

• Communication overhead between Strategy and Context

• Increase number of objects

Doc 8, Strategy Slide # 10

Implementation

• Defining the Strategy and Context interfaces

How does data flow between them

Context pass data to Strategy

Strategy has point to Context, gets data from Context

In Java use inner classes

• Strategies as template parameters

Can be used if Strategy can be selected at compile-time and does not change at runtime

   SortedList<ShellSort> studentRecords;

• Making Strategy objects optional

Give Context default behavior

If default used no need to create Strategy object

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