CS121/IS223 Week 12, Slide 1
CS121/IS223
Object Reference Variables
Dr Olly Gotel
ogotel@pace.edu
http://csis.pace.edu/~ogotel
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Understanding Parameter Passing, Objects and Object Reference Variables | CS 121, Lab Reports of Computer Science
Material Type: Lab; Professor: Gotel; Class: Computer Programming I; Subject: Computer Science; University: Pace University-New York; Term: Unknown 1989;
Typology: Lab Reports
Pre 2010
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CS121/IS
Object Reference Variables
Dr Olly Gotel
ogotel@pace.edu
http://csis.pace.edu/~ogotel
Having problems?
-- Come see me or call me in my office hours
-- Use the CSIS programming tutors
Agenda
- Make sure you have completed lab 1 and the
shopping questions in lab 2
- Objects & object reference variables - what are these
object things anyway?
- Understanding parameter passing – passing objects is
not the same as passing primitives
Finish Lab 2 after this week
Object Declaration, Creation, Assignment
Car myCar = new Car();
1 - Declare a reference variable , of class type Car, called myCar
2 - Create a new Car object
3 - Link the new object to the named reference variable
What is going on here? 3 distinct things…
Need to step back a moment…
(Called an object reference variable)
(1)
(3) (2)
What is Going On?
- Tells the JVM to allocate space for a reference variable on the
stack; reference variable is called myCar; it is of type Car
- Stack – smallish area of memory; holds an address in
memory (where to find an object); we know how much
memory we need to hold an address
- Tells the JVM to allocate space for a new Car object on the
heap
- Heap - big area of memory; holds the object because we do
not know how big it will be
- Assigns the new Car object to the reference variable myCar;
think of the reference variable as a locator for the object
Variables & Values in Java
- Variables implemented as memory locations
- Values implemented as 0’s & 1’s
- Values placed in memory locations
- The Java compiler needs to know the type of every
variable - to know the values it can take and
permissible operations
- In Java, every variable must be declared before it can
be used - so it can allocate space in memory
Types in Java
- Primitive types:
- directly represented by typical processors – the
most efficient
- begin with lowercase letter
- Class types:
- a type for objects
- begin with UPPERCASE
letter
8 primitive types in
Java, others are
represented using
objects – sometimes
called object reference
types
Primitive Types in Java
- byte – 8-bit integer
- short – 16-bit integer
- int – 32-bit integer
- long – 64-bit integer
- float – 32-bit floating point
- double – 64-bit floating point
- char – 16-bit unsigned Unicode character code
- boolean – 1 bit true or false
Values are actual integers, floats, characters, etc
Integer data types – no fractions
Floating point data
types –fractions
A Way to Think of Primitives
- We can predict these!
- Think Predict/Primitive
Class Types in Java
- Non primitive types exist & are important
- Class types are abstractions created from primitive
types (like a bank account, an address, etc)
- A class type that is widely used in java is String –
to represent a sequence of characters (char)
- There is a class String that you use to implement
strings, so strings are actually objects, which requires
a bit of extra code to use
- They are user-defined types
“This is a string”
- strings are literal values
Variables in Java
- A container of sorts
- Must have a type & a name (identifier)
- Can hold primitives or class types
- When they hold primitive types they hold the values
directly in the container
- When they hold class types, they hold references to
objects in the container (these are called object
reference variables), the objects are elsewhere
Pointers
In Java, we are VERY
concerned with references
Your object reference variable
actually points to the object!
Dog myDog = new Dog();
- Declare a reference variable:
Dog myDog = new Dog();
- Create an object:
Dog myDog = new Dog();
- Link the object & the reference:
Dog myDog = new Dog();
Object Declaration, Creation & Assignment
Example from [Sierra & Bates 2003] - best ever!
Dog type
Dog type
myDog
myDog
Dog object
Dog object
Index cards
Type Safety
Can’t do:
Dog myDog = new Cat();
myDog can only point to a Dog object!
But we could do:
Animal myDog = new Dog();
If Dog is defined as a type of Animal. Later!
Ouch!
CS121/IS223 Week 12, Slide 20
Assigning Ref Variables to Ref Variables
Dog myDog = new Dog();
Dog yourDog = myDog;
Dog type
myDog
Dog object
Heap – area of
memory where
you can request
big variable-sized
boxes
Dog type
yourDog
Stack - area of
memory for
storing
variables
(small boxes of
known size)
An alias
myDog.equals(yourDog)
1 object can be referenced by lots of variables