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Lec 23 Memory Maps Primitive variables and Reference (object) variables

Lec 23 Memory Maps Primitive variables and Reference (object) variables

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Lec 23 Memory Maps Primitive variables and Reference (object) variables

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  1. Lec 23 Memory MapsPrimitive variables and Reference (object) variables

  2. 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

  3. 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]

  4. A primitive/object example i 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); 2

  5. A primitive/object example i j 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); 2 2

  6. A primitive/object example i j 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); 3 2

  7. A primitive/object example i j 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); 3 2

  8. A primitive/object example i j 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); 3 2

  9. A primitive/object example i j 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); 3 2 point x y 2 p 2

  10. A primitive/object example i j 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); 3 2 point x y 2 p 2 q

  11. A primitive/object example i j 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); 3 2 point x y 3 p 3 q

  12. A primitive/object example i j 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); print 3,3System.out.println(q); 3 2 point x y 3 p 3 q

  13. A primitive/object example i j 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);  print 3,3 3 2 point x y 3 p 3 q

  14. 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

  15. 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)

  16. 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!

  17. 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

  18. How to make a true copy of a Point point x y 2         Point p = new Point(2,2);        Point q = new Point();        q.setLocation(p);        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) p 2

  19. How to make a true copy of a Point point x y 2         Point p = new Point(2,2);        Point q = new Point();        q.setLocation(p);        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) p 2 q point x y 0 0

  20. How to make a true copy of a Point point x y 2         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);        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) p 2 q point x y 2 2

  21. How to make a true copy of a Point point x y 3         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p 3 q point x y 2 2

  22. How to make a true copy of a Point point x y 3         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p 3 q point x y 2 2

  23. How to make a true copy of a Point point x y 3         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p 3 q point x y 2 2

  24. How to make a true copy of a Point point x y 3         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p 3 q point x y 2 2

  25. How to make a true copy of a Point point x y 3         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p 3 q point x y 2 2

  26. How to make a true copy of a Point         Point p = new Point(2,2);        Point q = new Point();q.setLocation(p);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) p q point x y 2 2

  27. 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

  28. 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