Pointers and References

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Pointers and References
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Machine addresses

• Computer memory consists of one long list of
  addressable bytes

• A pointer is a data item that contains an address

A reference is a data item that contains an
address
C has pointers, but Java has references
So whats the difference?
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C (and C) vs. Java

• In C you can dereference (follow) a pointer
• In Java you can dereference (follow) a reference
• In C you can assign one pointer variable to
  another
• In Java you can assign one reference variable to
  another
• In C you can determine whether one pointer is
  larger than, equal to, or smaller than another
  pointer
• In Java you can determine whether one reference
  is equal to another reference
• In C you can create a pointer to anything
• In Java you can have references to objects
• In C you can do integer arithmetic on pointers

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References

• Recall that an Abstract Data Type (ADT) has a set
  of values and a set of operations on those values
• Pointers and references have the same set of
  values (memory addresses)
• Pointers have more defined operations than
  references
• Pointers are more flexible and more general than
  references
• References are safer than pointers (from error or
  malicious misuse)
• References allow automatic garbage collection
  (pointers dont)
• A (non-abstract) Data Type also has an
  implementation
• The implementations of pointers and references
  are similar
• Java references carry information about the thing
  referenced in C, its up to the compiler to
  figure out what it can

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Data structures

• Basically, pointers and references are the same
  thing they point to (refer to) something else in
  memory
• A Data Structure is a description of how data is
  organized in memory
• Many (not all) data structures are built from
  objects pointing/referring to one another
• Understanding pointers (references) is
  fundamental to this course
• If this course were taught in C or C instead of
  Java, all the nuts and bolts would be the same
• This course is in Java, but its not about Java
• You need to know how to create your own data
  structures
• I will also teach some Java-specific packages
• In real life, its stupid to redo work thats
  already been done for you

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A trivial example

• We use a lot of references in Java
• class Person String name Person
  spouse Person (String n) name
  n Person john new Person("John")Per
  son mary new Person("Mary")john.spouse
  marymary.spouse john

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A more serious example

• A binary tree is a data structure in which every
  node (object) has zero, one, or two children
  (references to other nodes)
• Arithmetic expressions can be represented as
  binary trees

• To evaluate an arithmeticexpression
• If it is a leaf, return its value
• Otherwise, evaluate its two subtrees, and perform
  the indicated operation

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Binary trees in Java

• public class BinaryTree public Object
  value // the information in this node
  private BinaryTree leftChild private
  BinaryTree rightChild // Constructors and
  methods...
• To make binary trees as useful as possible, we
  make the value in a node an Object
• As implementers of the BinaryTree class, our job
  is to make sure that the structure of the binary
  tree is always valid
• We dont really care what the user puts in the
  value field

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Size of objects

• Objects in Java have, generally speaking, a fixed
  size
• The size of all primitives is known
• Objects dont actually contain other
  objectsthey just have references to other
  objects, and a reference is 4 bytes
• So what about Vectors?
• A Vector is like an array, but it gets bigger as
  you put things into it
• A Vector actually has two sizesits capacity,
  which is how many references it can hold, and its
  size, which is how many references are in it
  right now
• When you exceed the capacity of a Vector, Java
  creates a new Vector for you

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The magic of Vectors

• Suppose you have two references to a Vector, and
  you use one of them to add elements to the Vector
• What happens if Java decides to replace this
  Vector with a bigger one?

• It looks like the second reference is a dangling
  pointer, referring to nothing
• This doesnt happen! Java protects you from this
  error
• But how?

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Vectors secret trick

• A reference to a Vector is actually a reference
  to a reference to a Vector

• In this way, the real reference has to be
  changed in only one place, and all the other,
  indirect references automatically work
• Its clever, but it isnt magic

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The End