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

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)

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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

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