Reference Details copying, comparing

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Reference Details(copying, comparing)
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References

• You were introduced to the concept of pointers
  earlier this term.
• The pointer capabilities in pseudocode are
  relatively tame as compared to languages such as
  C which was originally developed for system
  programming
• In fact, pointers (and their misuse) are
  considered by some experts to be a very dangerous
  aspect of programming.
• The designers of Java decided to implement their
  language and eliminate many of the "dangerous"
  aspects of pointers. They even decided to call
  them something else references

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Whats all the fuss?

• Pointers (and references) are used for a very
  simple reasons efficiency
• Pointers save time
• Pointers save space
• Example
• Procedure add(cur iot in/out Ptr toa Node,
• dataIn iot in SomeBigRecType)
• ...
• Function get returnsa SomeBigRecType
• (cur iot in Ptr toa Node)
• ...

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Efficient?

• The add procedure asks us to pass in a big record
  of some type. This (as we have defined it) would
  require us to make a local copy inside the
  module.
• Then with a line of code like
• temp.data lt- datain
• We would make another copy!
• The get module would reverse the process. It
  might again copy the record one or more times.
• All this space...all this time to copy!

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The Real World

• In the real world code would be much more likely
  to simply make some space for the data once and
  from then on just pass pointers to the data.
• So the linked list Node wouldnt really contain a
  data record and a next pointer.
• It would contain a pointer to a data record AND a
  pointer to the next Node.

Big record
data
Big record
data
next
next
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Java References

• Java takes advantage of this concept by
  manipulating all objects (which are actually
  chunks of memory on the heap) using references.
• This may cause some confusion!
• Lets make a small class to use as an
  illustration.
• This class will hold info about boxes.

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class Box
len

• class Box
• private int len
• private int wid
• public Box(int len, int wid)
• setLen(len)
• setWid(wid)
• public void setLen(int len) this.len len
• public void setWid(int wid) this.wid wid
• public int getLen() return len
• public int getWid() return wid
• public int getArea() return getLen()getWid()
  
• public String toString()
• return ("Rec l "getLen()", w "getWid())
• // Box

wid
OK?
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Confusion?

• Box b1 new Box(3, 4)
• Box b2 new Box(3, 4)
• if(b1 b2)
• System.out.println("The boxes are obviously
  equal!")
• else
• System.out.println("I'm sooooo confused!")
• Your final answer?

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Solution

• We need a method that compares two boxes!
• (And it has to be in the Box class!)
• public boolean equals(Box b)
• if(getLen() b.getLen() getWid()
  b.getWid())
• return true
• else
• return false

class Box private int len private int
wid public Box(int len, int wid)
setLen(len) setWid(wid) public void
setLen(int len) this.len len public void
setWid(int wid) this.wid wid public int
getLen() return len public int getWid()
return wid public int getArea() return
getLen()getWid() public String toString()
return "(Rec l "getLen()", w
"getWid()
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Solution

• Box b1 new Box(3, 4)
• Box b2 new Box(3, 4)
• if(b1.equals(b2))
• System.out.println("The boxes are obviously
  equal!")
• else
• System.out.println("Im sooooo confused!")
• Your final answer?

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Copying?
box(3, 4)
b1

• Box b1 new Box(3, 4)
• Box b2 new Box(5, 7)
• b2 b1
• Where did the other box go?

box(5, 7)
b2
box(3, 4)
b1
b2
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Duplicating?

• What if I want to make another identical box?
• One way
• Box b1 new Box(3, 4)
• Box b2 new Box(b1.getLen(), b1.getWid())
• Another way...add a duplicate method into the Box
  class!
• public Box duplicate()
• return new Box(getLen(), getWid())
• Now we can say Box b2 b1.duplicate()

cloning?
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Copying?
box(3, 4)
b1

• Box b1 new Box(3, 4)
• Box b2 b1.duplicate()
• Box b3 b2
• b2.setWid(8)
• System.out.println(b3.getWid())
• b2 null

box(3, 4)
b1
box(3, 8)
b2
b3
box(3, 4)
b1
box(3, 8)
b2
b3
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Reference Details
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Complete Box Class

• class Box
• private int len
• private int wid
• public Box(int len, int wid)
• setLen(len)
• setWid(wid)
• public void setLen(int len) this.len len
• public void setWid(int wid) this.wid wid
• public int getLen() return len
• public int getWid() return wid
• public int getArea() return getLen()getWid()
  
• public String toString()
• return "(Rec l "getLen()", w "getWid()
• public boolean equals(Box b)
• if(getLen() b.getLen() getWid()
  b.getWid())
• return true

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