Transcript Pointers and Animations

Lec 23 Memory Maps
Primitive variables and
Reference (object) variables
Reminder
• Primitive variables
– of type int, double, char, boolean
– the variable contains the data
• Object variables
– of type String, Point, Pad, Dairy, int [] (i.e. array)
– refer to an object (or contain null)
– actual data (the object) is elsewhere in memory
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
Expected Output??:
3
2
java.awt.Point[x=3,y=3]
java.awt.Point[x=2,y=2]
java.awt.Point[x=3,y=3]
A primitive/object example
int i = 2; 
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
2
A primitive/object example
int i = 2;
int j = i; 
i = 3;
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
2
j
2
A primitive/object example
int i = 2;
int j = i;
i = 3; 
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
3
j
2
A primitive/object example
i 3
int i = 2;
int j = i;
i = 3;
System.out.println(i);  print 3
System.out.println(j);
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
j
2
A primitive/object example
i 3
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);  print 2
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
j
2
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
3
p
j
2
point
x 2
y 2
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p; 
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);
3
p
q
j
2
point
x 2
y 2
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
Point p = new Point(2,2);
Point q = p;
p.setLocation(3,3); 
System.out.println(p);
System.out.println(q);
3
p
q
j
2
point
x 3
y 3
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
3
p
Point p = new Point(2,2);
Point q = p;
q
p.setLocation(3,3);
System.out.println(p); print 3,3
System.out.println(q);
j
2
point
x 3
y 3
A primitive/object example
int i = 2;
int j = i;
i = 3;
System.out.println(i);
System.out.println(j);
i
3
p
Point p = new Point(2,2);
Point q = p;
q
p.setLocation(3,3);
System.out.println(p);
System.out.println(q);  print 3,3
j
2
point
x 3
y 3
Why is this?
• This exemplifies a key difference between primitive
variables and reference (i.e. object) variables: For
primitives types, we have primitive variables, which
actually store a value
– Assignment statements for primitive variables actually
copy the value over
• For class types, we have reference variables, which
only store a reference to an object
– Assignment statements for reference variables only copy
a pointer over
Memory diagrams
• Memory diagrams are a very useful tool for
understanding what's going on inside memory
• We will use the following conventions:
– Represent a variable with an oval containing its type
(optional), name, and contents (which goes inside a box
within the oval)
• For primitive variables, put the value right in the box
• For reference (class type) variables, draw an arrow from that box
to the actual object which is elsewhere on the diagram
– Represent objects as boxes; the type goes on top and is
underlined, any variables within the object are listed inside
the box (the ovals can be omitted in this case)
Memory Map -- Assignment
• When an assignment statement happens:
– If the receiving variable is a primitive type, just
copy the value into its box
– If the receiving variable is a class type, make its
arrow point to whatever object it is getting
assigned to
– In other words, always copy what's in the box - it
will be either a pointer or a primitive value!
More practice on primitive versus
reference variables
• Challenge: How could we make it so that we actually
make a true copy of the Point in the last example?
– One way is to make a new Point and then copy the desired
x and y coordinates into it
– Another way is to make a new Point using the desired x
and y coordinates directly
– This example makes a true copy of a Point and does some
pointer manipulation.
• Here is a sequence of memory diagrams for that code
• When an object no longer has references to it, you can
no longer get to it and it eventually gets erased by the
garbage collector; this frees up the memory it was
using
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
point
x 2
y 2
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 2
y 2
point
x 0
y 0
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 2
y 2
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 3
y 3
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3) 
System.out.println(q); //prints a point at (2,2)
point
x 3
y 3
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2) 
point
x 3
y 3
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 3
y 3
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy 
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 3
y 3
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2) 
System.out.println(q); //prints a point at (2,2)
How to make a true copy of a Point
Point p = new Point(2,2);
p
Point q = new Point();
q.setLocation(p);
q
p.setLocation(3,3);
System.out.println(p); //prints a point at (3,3)
System.out.println(q); //prints a point at (2,2)
point
x 2
y 2
p = q; //Makes p point to q's object; this is NOT a true copy
System.out.println(p); //prints a point at (2,2)
System.out.println(q); //prints a point at (2,2) 
Some technical jargon for methods:
• Mutator methods: methods that change the state of the object
– For the Particle class: passTime and reset are mutator methods
• Accessor methods: methods that return info about the state of the
object
– For the Particle class: getXPosition, getYPosition, getXVelocity, and getYVelocity
are accessor methods
• Predicate methods: methods that return a boolean (true/false value)
– For the Particle class: isFalling is a predicate method
• Invoking a method with dot syntax is called calling a method or
sending a message to that object
• When calling a method, the values given between the parentheses
are called arguments
• If you get something back as a result of calling a method, it is called a
return value
Edit-complile-test cycle
• You may have noticed that when programming, we
usually:
– 1) Edit the source code a little bit
– 2) Compile to make sure we have the syntax right
(automatic in Eclipse)
– 3) Test our program to see if it behaves the way we
expected so far
– 4) Go back to 1) until we are done writing the program
• This process is known as the edit-compile-test cycle
• It is often better to make small changes and check your
work so you can spot and correct errors quickly and
before they become confounding