More Containers

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More Containers
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Method Locate
/ ... _at_param obj target to be found _at_return
Object reference if found and null
otherwise / public Object locate( Object obj
) int pos find( obj ) if ( pos gt 0
) return listpos return null
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Method Locate
Bag b new Bag( 10 ) ... // Assume we have
Person class Person p1 new Person() // Does
it work? Person p2 b.locate ( p1 ) NO! Method
locate() returns an Object reference! public
Object locate( Object obj ) We can assign a
child to parent. We CANT assign a parent to a
child. Need a CAST.
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Method Locate
Bag b new Bag( 10 ) ... Person p1 new
Person() Object obj b.locate ( p1 ) if ( obj
! null obj instanceof Person ) Person p2
(Person)obj System.out.println(
p2.getAddress() ) ... Cast obj to Person
when it points to a Person. public Object locate(
Object obj )
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C Template
In C , youll have templates. You can write a
general container, but instantiate it to only
take specific types. Java now has generics, and
we will use generic Java containers later on in
the semester.
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SortedList Class
The data declaration can be the same as that for
the Bag. private Object list private
int num 0 private final int GROWBY 3
All the Bag methods we wrote would also work as
SortedList methods, except the add and the
delete. Both add and delete must keep list in
sorted order.
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Method add
index
0 1 2
num-1 length-1
New item
Find the position to be added Move items
down one position Add the new item
Update num
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Method delete
index
0 1 2
num-1 length-1
Find the item (index) to be deleted Move
items up one position Update num
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Methods add and delete
Both add and delete must keep list in sorted
order. Could use the simple implementation for
Bag Call a private sort method after adding and
removing Can use either Linear Search or Binary
Search
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SortedList Class
New method public Object itemAt ( int index
) if ( index lt 0 index gt num )
return null return listindex Not
applicable to Bag class
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Usage Example
SortedList l new SortedList( 10 ) // add and
remove items // get index Object obj l.itemAt
( index ) if ( obj null ) ... else if (
obj instanceof Person ) ... else if ( onj
instanceof Date ) ...
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Set Class

• An item can appear in a set at most once.
• The data declaration can be the same as that for
  the Bag.
• private Object list
• private int num 0
• private final int GROWBY 3
• All methods can be the same as Bag except
• add - dont add if already in set
• count (number of times an item in the container)
• not needed (just use contains)
• deleteAll not needed (just use delete)

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New Methods for Set Class

• intersection
• union
• setDifference
• isSubset

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Method add
public boolean add( Object obj ) if (
contains( obj ) ) // in the set already
return false if ( num gt list.length ) //
Its full grow() listnum obj //
store the reference return true
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Method interesection
public Set intersection( Set s ) Set temp
new Set( num ) for (int i 0 i lt num i
) if ( s.contains( listi ) )
temp.add( listi ) return temp Other
methods are similar. Returns a set. Does not
change the current object.
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Usage Example
Set s1, s2, s3 s1 new Set( 5 ) s2 new Set
( 10 ) // add and remove items if
(s1.isSubset( S2 ) ) ... s3
s1.intersection( s2 )
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Next Week

• Monday
• Quiz2
• Tuesday
• Lab/Prog3
• Wednesday
• Prog3
• Thursday
• Prog3 Work Plan due
• Friday
• Test 1 (70 points)

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Program 3