Working with multiple objects

1
Working with multiple objects

• A variable can refer to a single object
• IBehavior cc new ColorChanging()
• IBehavior br new Breathing()
• A variable referring to many objects? No, but we
  saw how to build a composite object.
• IBehavior cc new CompositeBehavior(cc,br)
• But its not too easy to add/remove items from a
  composite. Are there options?

2
Collections

• interface java.util.CollectionltEgt
• classes
• java.util.HashSetltEgt
• java.util.ArrayListltEgt
• E is the type of element contained in the
  collection

3
Using a Collection (HashSetltEgt)

• HashSetltStringgt names new HashSetltStringgt()
• names.add(Amy) names.remove(Bob)
• names.add(Bob)
• names.add(Cindy)
• names.add(Dave)
• names.add(Emma)

4
for-each loop (w/HashSetltEgt)

• for (String name names)
• System.out.println(name)
• This prints out the following in the console
  window
• Amy
• Cindy
• Dave
• Emma

5
Collections.shuffle

• Collections.shuffle(ListltEgt)
• The shuffle method randomizes the order of
  elements in the collection it is passed.
• The collection passed to shuffle must be a List
  a collection which maintains items in some order.

6
Using a Collection (ArrayListltEgt)

• ArrayListltStringgt names2 new ArrayListltStringgt()
  
• names2.add(Amy) names2.remove(Bob)
• names2.add(Bob)
• names2.add(Cindy)
• names2.add(Dave)
• names2.add(Emma)

7
for-each loop (w/ArrayListltEgt)

• for (String name names2)
• System.out.println(name)
• This prints out the following in the console
  window
• Amy
• Cindy
• Dave
• Emma

8
shuffling

• Collections.shuffle(names2)
• for (String name names2)
• System.out.println(name)
• When I ran it once and again and again
• Emma Dave Cindy
• Dave Cindy Amy
• Amy Emma Emma
• Cindy Amy Dave

9
Returning tothe boolean operators

• and
• or
• ! not

10
(x y) is true only ifx is true and y is true

• truth tables
• both convey same information, but in different
  ways

11
(x y) is true only ifx is true or y is true

• truth tables
• both convey same information, but in different
  ways

12
!x is true only ifx is false

• truth tables
• both convey same information, but in different
  ways

13
Short circuiting

• and are so-called "short circuit" operators
• They only evaluate as much as is needed
• In an expression such as xy, if x is false
  there is no need to evaluate y (since xy must
  be false)
• In an expression such as xy, if x is true there
  is no need to evaluate y (since xy must be
  true)

14
Use of short-circuiting

• short-circuiting can be used to effect a form on
  conditional evaluation
• public void shortCircuitTest(int x)
• int y5
• int z2
• if ( x!0 (y/x)ltz )
• System.out.println("Division was safe")
• else
• System.out.println("Avoided divide-by-zero")
• shortCircuitTest(3) prints Division was safe
• shortCircuitTest(0) prints Avoided divide-by-zero